Module - Woolwiseminerals, vitamins and water. There is considerable confusion when the terms ‘energy’ and ‘protein’ are used in discussing feeds and their nutritive value

Applied Animal Nutrition: The Theory and Practice of Animal Nutrition ANUT300/500 –1 - 1

©2009 The Australian Wool Education Trust licensee for educational activities University of New England

Applied Animal Nutrition 300/500

Module 1

The theory and practice of animal nutrition




Topic 1

1. What are the Major Nutrients

1.1 What constitutes a major nutrient?

1.2 The major nutrients and their roles

1.3 Anti–nutritional factors



1. What are the Major Nutrients

Learning Objectives

On completion of this topic you should be able to:

• Describe the major components of diets for livestock: energy and nutrients

• Explain the importance of ‘anti-nutrients’

Key Terms and Concepts

Major nutrients; Energy; Protein; Amino acids; Minerals; Vitamins; Water; Anti-nutritional factors.

Introduction to the Topic All living animals, and indeed the cells they are made of, have the same basic requirements for energy, protein (as amino acids), minerals, vitamins and water. There is considerable confusion

when the terms ‘energy’ and ‘protein’ are used in discussing feeds and their nutritive value. For all animals, and especially ruminants, feeds cannot provide purely ‘energy’ or ‘protein’. All sources of

protein can contribute in some way to the energy metabolism of the animal. In ruminants, even sources of purified carbohydrate, containing no protein as such, provide a source of energy for

rumen microbes to grow and the microbes then supply additional protein and amino acids to the host animal. We have lectures that focus on ‘energy’ and ‘protein’ and, from what has just been said,

this may seem inappropriate. However, there are some feeds that do provide more energy relative to protein than other feeds. Conversely, there are feeds that provide more protein relative to

energy. Such sources are referred to as ‘energy’ and ‘protein’ concentrates. It is important not to think in terms of absolutes when it comes to defining feeds as sources of energy or protein.

Most protein and energy concentrates will also provide vitamins, minerals and small amounts of water.

We refer to feeds that are ‘energy concentrates’, and ‘protein

concentrates’ when these are rich in energy or protein. Most feeds,

however, provide at least small amounts of most nutrients.



1.1 What constitutes a major nutrient?

In considering major nutrients it is also important to have a ‘feel’ for the quantities of feedstuffs and nutrients required to make a practical difference to the animal. For example there are many

advertisers’ claims in relation to the benefits of lick–blocks—that they provide additional ‘energy’ and/or ‘protein’. However, the quantity of block ingested by an animal is not likely to make any

significant contribution to these major categories of nutrients. For example if a sheep consumes 20 g/d of a ‘high energy’ block containing 11 MJ ME/kg, this intake contributes only around 2% of

its daily maintenance energy requirement. Similar calculations can be made for the intake of protein. In order to maintain constant live weight, sheep need to consume around 1 kg dry matter per

day and around 100 g crude protein/d. Cattle have to consume around 6–10 times these quantities to maintain weight. Obviously these are very rough estimates and will vary tremendously

depending on the quality of the feed and the requirements of the animal. They do, however, provide useful figures against which to check whether any supplement is providing nutritionally

significant amounts of the major dietary components, ‘energy’ and ‘protein’.

A summary of the major nutrients produced by digestion of feeds

is presented in Table 1–1. Volatile fatty acids (VFA), lipids (fats and oils) and glucose (or starch) can be considered as the main ‘energy’ nutrients. Proteins are degraded to their basic units,

peptides and amino acids, before leaving the gut.



Table 1–1 The major nutrients for ruminants — their source and major roles (UNE Animal Science database)

Nutrient

group Source Main use

Volatile

fatty acids

Rumen and hind gut

fermentation

(acetic, propionic, and

butyric acids)

Energy

(propionate/glucose)

Lipids/fats Dietary and microbial

fermentation

Energy, essential

fatty acid

Glucose

Glucose is an essential

nutrient

Dietary starch which

escapes fermentation

Energy

Building unit for

glycogen

Protein

Dietary (escape

protein)

Rumen microbial

protein

Amino acids for

tissue protein

synthesis

Vitamins Diet and rumen

microbes Essential nutrients

Minerals Diet Essential nutrients

1.2 The major nutrients and their roles

Energy Strictly speaking, energy is not a nutrient. It is defined as ‘the

ability to do work’. It can be changed from chemical forms into light, sound and heat, but cannot be destroyed. Energy is essential

to maintain life and is provided to animals by various feed substrates and by the products of microbial digestion of feeds. Energy is stored in the chemical bonds of materials such as fats

and carbohydrates and is released when these bonds are broken during cellular metabolism. The volatile fatty acids absorbed from the rumen and hind gut can be converted to a number of

Urea blocks are sometimes referred to as “protein supplements”. However, urea

is not protein—in fact it is an end–product of protein breakdown in mammals.

Rumen microbes can, however, use urea from the diet, or blood that is recycled

via saliva, as a source of N from which they can re–build true protein. This is

sometimes referred to as the ‘protein conservation’ cycle.



functional and storage nutrients (including fat) as well as being substrates that can be directly metabolised to provide energy (as

ATP) for maintenance and metabolism of the animal tissues. VFA are much less important as energy sources in pigs and poultry although a small amount of VFA energy may come from hind gut

fermentation in these species.

Glucose is an essential nutrient for tissue use in both ruminants

and non–ruminants—it is needed for energy metabolism by the brain, kidneys, red blood cells and for the foetus. It is also required in large amounts as the precursor of lactose, the main

sugar in milk. It is also possible that it has a role in meat quality as it may influence muscle glycogen and marbling.

Glucose, the building sugar released upon digestion of dietary starch, is absorbed from the small intestine of monogastric animals. There is very little glucose absorption in ruminants,

except in situations when starch passes through the rumen without being fermented. Most of the glucose available in tissues of ruminant animals is produced in the liver from propionate and

amino acids (this is a process called gluconeogenesis). Non–ruminants can also use some amino acids to make glucose if not enough is supplied by their diet.

The lipids (oils and fats) come from dietary sources and from intestinal digestion of rumen microbes. Lipids are absorbed from

the small intestine and most can be synthesised by tissues from fatty acids. They are either be used in metabolic processes or stored in fat depots. Several fatty acids cannot be synthesised by

tissues and therefore must be obtained from outside sources. They are referred to as ‘essential fatty acids’.

Protein

Proteins are all made up of 20 different amino acids. All 20 amino

acids are used in growth of the animal as essential building blocks of muscle and other tissues. About 10 of these cannot be synthesised in tissues of livestock and must be absorbed from the

small intestine. They are referred to as “essential amino acids”. (The other amino acids can be made in the tissues). In ruminants, essential amino acids and proteins are produced by the rumen

microbes as well as being supplied directly by the diet. A fraction of the dietary proteins can pass to the small intestine without fermentation in the rumen (‘bypass’, ‘protected’ or ‘escape’

protein). Amino acids are also essential for milk and wool protein production.

Minerals



Minerals are present in the ash remaining after the organic component

of feeds or tissues are combusted. Minerals are essential components of numerous tissues of the body and the

‘macro–minerals’ Ca and P are major components of bone and milk. Many of the ‘micro–minerals’ such as Co and Se

are ‘activators’ of enzymes and are essential for maintenance, growth and production of animals. Deficiencies of

vitamins can cause specific symptoms of disease or just impair production without obvious clinical symptoms.

Intensively fed animals are normally given mineral and vitamin pre–mixes to remove the possibility of deficiencies.

For grazing ruminants, after N, sulphur is the next most important mineral (needed for microbial protein synthesis).

Supplements containing both N and S are often needed, particularly when feeding grains or in animals on dry

pastures.

Calcium and phosphorus are macro

minerals that must be present in relatively large amounts in the diets of all livestock. They are particularly

important in laying hens and requirements are affected by interactions with Vitamin D. Sub–clinical

deficiency of phosphorus is widespread in the Northern Australian beef industry. Reduced fertility and growth are

examples of sub–clinical phosphorus deficiency. A clinical sign of phosphorus deficiency is ‘peg–leg’. ‘Peg–leg’ is seen

as a stiff gait and brittle bones in cattle (Ternouth 1990).

Trace minerals such as zinc and manganese can be deficient in diets for poultry if mineral pre–mixes are not

used.

There is a ‘zone of tolerance’ for most minerals between

‘deficient’ and ‘toxic’ amounts in diets, but there are differences in tolerance between species. Copper at relatively low

Some rules of thumb for mineral

and vitamin nutrition

In terms of practical ruminant

nutrition, the only vitamins required

are vitamins A and E. These are only

needed when animals are in feedlots

for long periods of time and/or when

no green feed is available for extended

periods. Vitamin D can be needed by

animals kept indoors for long periods

(>6months).

Of the minerals, calcium is the mineral

most commonly needed when animals

are on high grain diets without access

to green feed. Most grains contain Ca:P

in the ratio of around 1:1 and for these

minerals to be balanced in the diet, the

ratio needs to be closer to 2:1. The

need to add Ca in order to balance the

Ca:P ratio in grain feeding occurs

mainly in feedlots and supplementary

feeding. The potential deficiency is

usually overcome by including

approximately 1% of limestone or

gypsum with the grain component of

the diet.

The ratio of N:S in rumen microbes is

around 12:1. This means that under

most conditions where additional N is

required for optimal microbial protein

synthesis, there will also be a

requirement for additional S. This can

be supplied by mixing urea and

ammonia sulphate (9 units of urea to 1

unit ammonia sulphate, by weight) or

else by using gypsum (around 1% by

weight of grain) instead of limestone to

provide both Ca and S when feeding

cereal grains. Some amino acids and

fatty acids cannot be synthesised by

the cells of farm animals and must also

be provided in the diet (or, in the case

of ruminants, absorbed from the gut).



concentrations may be toxic for sheep: poultry and pigs can exist quite happily on a diet containing five times the concentration that

is fatal for sheep! Heavy metals (mercury, lead) can be toxic in all species and can accumulate in animal products.

Vitamins

Vitamins are organic molecules that are essential and are required in small amounts for the normal functioning of animal cells.

Vitamin nutrition is a large subject in its own right particularly as, in recent times, the use of large doses of purified vitamins has been considered by some to have medicinal value in preventing

and/or treating disease. Vitamins are present in varying concentrations in many feeds and the supplementation of diets with purified vitamin premixes is only required when animals are

housed indoors with no sunlight (vitamin D) or when they have no access to green feed. Vitamin E is important as an anti– oxidant and is used for this role to prevent oxidation of feed ingredients.

It is also an essential nutrient.

Aspects of mineral and vitamin nutrition and the numerous

interactions that occur between minerals and vitamins are complex. We will deal elsewhere with several examples of mineral and vitamin nutrition where there are important ‘rules of thumb’

or issues of practical significance. Mineral and vitamin nutrition are areas where it is advisable to seek expert local information. Advice is often district–specific because soils, and the minerals

they yield to plants, can vary significantly from area to area and even from paddock to paddock.

Water

Water is also arguably not a nutrient. However, it is essential for

all animals. In fact animals can live for much longer without food than without water. Water represents 50–75% of body weight and a considerable part of any weight gain is water, especially in young

animals. In most animals a loss of 10% of the body water is potentially life–threatening, although merino sheep and camels are exceptional in being able to tolerate up to 25% depletion of

their body water. Water acts as a solvent for metabolites and minerals and allows these materials to be carried around the body in the blood stream. It is also the vehicle of excretion of unwanted

metabolites. Evaporation of water from the lung or skin surfaces causes cooling of animals in hot conditions.

The vitamins designated A, D, E and K are fat–soluble: vitamin

C (ascorbic acid) and the B complex are water–soluble



1.3 Anti–nutritional factors

When nutritionists formulate diets for livestock, they tend to think of the feed ingredients as being providers of energy, protein, minerals and vitamins as discussed already. It is important,

however, to recognise that many ingredients also contain chemicals that have effects on digestion and metabolism. Detrimental compounds are referred to as ‘anti–nutritional

factors’, ‘antinutrients’ or ‘antifeedant compounds’. We do not cover this topic in detail in this Unit, but some of the issues are noted below. Non–starch polysaccharides (NSPs), which are

relatively high in some cereal grains, increase digesta viscosity, especially in poultry, and affect gut function and production. Uncooked soyabeans contain an anti–trypsin factor and can

interfere with assimilation of vitamin K and blood clotting. Other anti–nutritional chemicals include alkaloids, saponins, condensed tannins, haemagglutins and lectins. Phytic acid that is found in

relatively high concentrations in grains and vegetable protein meals reduces mineral absorption, (especially phosphorus, zinc, calcium and magnesium). Buffel grass (Cenchrus ciliaris) which

grows in Queensland has high concentrations of oxalic acid which affects absorption of calcium and can produce the condition referred to as ‘Big Head’ in horses (Hungerford 1998).

Many chemicals that are produced by edible plants are thought to have evolved as part of the plant’s defences against being eaten

(by livestock or insects) or to give the plants competitive advantages over other plants. In turn, many animals have evolved ways of coping with these toxic chemicals (by learning to avoid

eating such plants or by using detoxicification mechanisms in the liver). For example, some Western Australian mammals have developed an efficient chemical detoxication method in the liver

than allows them to eat many fluoroacetate–containing plants (Gastrolobium species) in the south–western part of the state. Brush–tailed possums, bush rats and western grey kangaroos from

this area are capable of safely eating these plants: the same plants are rapidly fatal for livestock, but also for red kangaroos and eastern grey kangaroos from eastern Australia (Figure 1–1).



Figure 1–1 Gastrolobium grandiflorum and Acacia georginae, (Gigyea) which grow in Queensland, contain fluoracetate which

is the toxic component of 1080 poison. At times, cattle may die after eating these trees (Source: McCosker and Winks 1991).

Soybean has oestrogen–like compounds, isoflavones, that are

touted as a natural alternatives to hormone replacement therapy (HRT). Oestrogenic clovers can affect reproduction in sheep.

Livestock are not only subject to toxic plant secondary metabolites, but also to plant–associated toxins produced by fungi and bacteria that are growing on or within a plant species. Annual

rye grass (Lolium rigidum) can become infected with a corynetoxin–producing bacterial species: ingestion of infected grass can cause convulsions and death, and reproductive

abnormalities in all animal species. Endophytes (fungi) growing on fescue and rye grass produce alkaloids that adversely affect animal production.



Readings The following readings are available on CD:

• Egan AR (1974) Protein-energy

relationships in the digestion products of sheep fed herbage diets differing in digestibility and nitrogen concentration.

Australian Journal of Agricultural Research 25, 613-630.

• Hogan JP, Weston RH (1967) The digestion of two diets of differing protein content but with similar

capacities to sustain wool growth. Australian Journal of Agricultural Research 18, 973.

• Poppi DP, McLennan SR (1995) Protein and energy utilisation by ruminants at

pasture. Journal of Animal Science 73, 278-290.

!

Self Assessment Questions

1. List the major dietary ingredient

categories.

2. Name three chemicals in dietary ingredients that could have anti–

nutritional properties.

3. What is the important product of starch digestion?

4. Do pigs and poultry require fat in their diets? Why?

5. What are the end–products of rumen

microbial digestion of dietary organic matter that provide the majority of the host’s energy requirements?

6. What is the most important non–dietary constituent in digesta flowing from the rumen to the lower digestive tract?

7. How do ruminants obtain protein for intestinal digestion?

8. When feeding cattle on a grain–based

diet, what mineral would you consider to

!



be potentially limiting?

9. When using urea supplements for grazing

cattle, would you consider also offering a mineral supplement? Why?

10. How do ruminants obtain vitamins? When,

if ever, would you consider giving a vitamin supplement to sheep?

References

Hungerford TG (1998) 'Hungerford's Diseases of

Livestock.' (Mcgraw-Hill Book Company: Sydney)

McCosker T, Winks L (1994) 'Phosporus nutrition

of beef cattle in northern Australia.' (Department of Primary Industries, Queensland)

Ternouth JH (1990) Phosphorus and beef production in northern Australia. 3. Phosphorus in cattle - a review. Tropical

Grasslands 24, 159-169.




Topic 2

2. Energy Requirements

2.1 Energy requirements for maintenance

2.2 Energy requirements for production

2.3 Total energy requirements from birth to slaughter



2. Energy Requirements

Learning Objectives On completion of this topic you should be able to:

• Describe the requirements for energy by livestock to

support their survival and various forms of production.

Key Terms and Concepts Energy requirements for maintenance; energy requirements for

production; efficiency of energy use by tissues; total energy requirements from birth to slaughter as body composition changes; manipulating carcass composition through nutritional

management.

Introduction to the Topic The feed energy that an animal has available for tissue

metabolism determines how much of the various animal products it can produce. The feed energy available to tissues is expressed as metabolisable energy (ME). In practice, the first call on the ME is

to supply maintenance energy (MEm). This is the energy required just to keep the animal alive but not growing or making any products. ME intake must exceed MEm if the animal is to be able to

partition some energy into productive functions such as growth (MEg), lactation or reproduction. We must also be aware that energy is never transformed with 100% efficiency. Furthermore, it

is now recognised that the efficiency of use of ME for both maintenance and production varies for different types of production. It also varies between animals: efficiency is partly

heritable and subject to breeding.

MEm is always considered to include the energy for basal

metabolism (cell survival) plus that needed to combat adverse climatic conditions (energy to maintain the animal’s core temperature when the weather is cold, or energy to dissipate heat

by panting in hot conditions). The energy used for walking and grazing is also often included in MEm. If these components are included, MEm is substantially more than fasting heat production

(basal metabolic rate).



Because some ME is always required for the animal to survive, if ME intake is low, the animal will mobilise enough energy from

body stores to ensure its survival. It will survive while it has sufficient reserves, but it will lose weight.

The ME available for productive functions (the ME in excess of maintenance needs) has the opportunity to be partitioned into growth, or milk production, or reproductive purposes or to be

used for draught power.

It is important to be able to predict to which productive function,

and how efficiently ME will be partitioned. Young growing animals, for example, partition much of the energy available for gain into deposition of protein, whereas nearer to maturity they partition

more into fat. In early lactation, a high–producing dairy cow will obtain energy for milk production from body stores and will lose live weight and body condition, i.e. she directs all energy above

maintenance into production of milk. Towards the end of lactation, ME available for production will be partitioned more towards energy gain in the carcass and less towards milk.

Requirements for energy include that used for:

• maintenance (usually includes walking, eating). The energy required is the amount used for these processes divided by efficiency of ME use for maintenance, km

• to combat environmental extremes. Additional energy is required for animals outside their thermo–neutral range.

• growth and deposition of tissue net energy. This can be calculated from the energy costs of deposition of protein and fat

(water does not have an energy cost), but the composition of the tissue gain must be known. The energy required is the net energy stored divided by the kg.

• Energy required for pregnancy, lactation, draught.

ME (total requirement) =

ME (maintenance)

+ ME (growth)

+ ME (lactation)

+ ME (pregnancy)

The chemical energy in protein and fat is the net energy storage.

This energy comes from ME and is usually stored at about 50%

efficiency.



Dietary requirements

Broiler production provides a convenient example of how the

above concepts are embodied in equations for predicting the energy requirements for growing birds. The equation given below

indicates that birds require ME to meet their needs for basal maintenance, energy to combat climatic challenge, and energy to match that retained in the growing tissues (gain).

ME (kJ/day) = 6.78W0.653 [1.0 + 0.0125(21–T) + 13.1G]

where W = current live–weight

T = mean daily temperature (°C)

G = live–weight gain (g/day)

The equation implies that 21°C is a critical temperature above which ME requirements increase (energy required for heat

dissipation), and that gain costs 13.1 kJ/g. (A more sophisticated equation might recognise that the ‘energy cost of live–weight gain’ will also increase with age because the animal will deposit

proportionally more fat as it ages.) Figure 2–1 shows how the ME requirement for maintenance increases above and below a temperature of 25°C. However, feed and ME intake are higher at

lower ambient temperatures.

Figure 2–1 The maintenance ME cost and voluntary ME intake

of chickens as environmental temperature rises. The shaded area at any temperature shows the ME available for growth at that temperature which is maximal at about 20°C (MAFF 1975).



Efficiency of ME use in ruminants

ME supplied in excess of the maintenance requirement is, as

discussed above, partitioned first into ‘maintenance’ and then ‘production’. ME is used with different efficiencies for each

process (the efficiency coefficients are referred to as km and kp). These efficiencies may vary between breeds and genotypes. They also vary with the type of production, and the type of feed—in

particular depending on its fermentability in the rumen (Table 2–1) which affects the type of VFA pattern that develops in the rumen and hence the ratio of glucogenic and non-glucogenic VFA

absorbed from the gut.

Table 2–1 Efficiencies of use of ME for maintenance and

production in ruminants (MAFF 1975)

Hay M/D=9 Barley

M/D=13

Maintenance km 0.68 0.76

Pregnancy kp 0.13 0.13

Lactation kl 0.58 0.66

Weight gain kg 0.30 0.56

The model used to estimate the ME requirement for a particular level of production is as follows. First, calculate the net energy requirements for maintenance and deposition of tissues, and then

increase both amounts to allow for km and kp. Although this conceptual model has stood the test of time, it is not easy to comprehend why efficiency of use of ME for gain should

necessarily be lower than efficiency of energy use for maintenance. An alternative view is that maintenance itself has a variable efficiency for animals of a given weight depending, for

example, on their previous nutritional history. We will, however, continue to discuss the conventional model.

2.1 Energy requirements for maintenance Maintenance energy is considered to be net energy used to ‘maintain’ body functions (protein turnover in tissues, synthesis and breakdown of enzymes with the obligatory losses of energy as



heat during metabolism, temperature control, essential excretory functions etc). The concept arises from the notion of basal

metabolic rate—the energy required to maintain an ‘idle’ animal in energy equilibrium—and basal heat production. A pioneer scientist, Brody gave a general equation for predicting basal

energy requirement across a wide range of species of animals from mice to elephants, viz.

Basal net energy requirement = 300 W0.75 kJ/day

A coefficient of 250 is now considered more appropriate for sheep, and 350 more appropriate for cattle, 400 for poultry and 460 for pigs. During transfer of ME to NE there are losses of

energy as heat. Brody found that basal net energy requirement was about 75% of the true maintenance ME requirements for non– herbivorous species and about 50% for cattle and sheep, i.e. a km

for cattle and sheep of 0.5. These days, we consider that km for ruminants is variable, depending on the quality of the feed, i.e.

km = 0.02 (M/D) + 0.5

where M/D is the ‘energy density’ of the feed in MJ/kg.

The maintenance ME requirement of a 40 kg sheep (W0.75 = 15.9) consuming hay with a M/D of 9 is therefore given by:

(250 x 15.9)/ (0.02 x 9 +0.5) = 5.85 MJ/day

In the Australian Feeding Standards, more precise predictions for

ruminants of maintenance energy requirements have been recommended that take account of species, sex, climate and work associated with grazing. Such equations are easily handled in

computer models such as Grazfeed (see later). Precise predictions of maintenance energy requirements are clearly less important for intensively fed animals which usually have ME intakes that are

several times that required for maintenance.

Ruminant feeds with relatively low digestible energy density are those with a

high proportion of indigestible fibre and lignin. These demand high energy for

chewing, swallowing and processing in the gut. Thus, efficiency of energy use

from such feeds is relatively low.



2.2 Energy requirements for production

Energy requirements for growth and weight gain

Animal products such as meat, milk and wool, and the foetus and associated maternal tissues during pregnancy are made up of

polymers such as protein and fat. These macro–molecules are stores of chemical energy. During their synthesis, energy is required for the formation of the bonds between the monomers

(building units) and ME is incorporated to enable these macro–molecules to be synthesised. Because many different polymers are produced by growing cells, the net energy costs associated with

growth depend on the composition of the tissues laid down. This composition changes as the animal matures and deposits more fat in the tissues relative to protein and water (Figure 2–2). Water

content of the carcass decreases as the fat content increases. (Water itself does not contain energy, but more energy is stored in 1 g of fat than in 1 g of protein plus water.)

Figure 2–2 The body composition of cattle as they grow towards maturity. The net energy of the wet tissues varies

from about 10 MJ/kg at birth to 27 MJ/kg at maturity (Butterfield 1988).

In short, the net energy and thus the ME required per kg gain increase as the proportion of fat in the weight gain increases. This

is because fat is a ‘denser’ store of energy than protein (more gross energy/kg) and contains virtually no water.



Feed conversion efficiency (gain/unit feed). FCE will be greatest in animals laying down lean tissues (predominantly protein and

water) and lowest in those in the fattening phase of growth. At higher feeding levels, the amounts of grain required to produce 1 kg gain (referred to as FCR) vary from about 3 kg in young animals

depositing mainly lean tissue to about 10 kg in older ruminants laying down mainly fat. Note that FCR is commonly used in the poultry industry. It is the reciprocal of FCE.

Tables giving the net energy cost of gain in animals growing at

different rates are available (e.g. MAFF, 1993) (see ANUT 221, p.16–7). ME requirements for gain are obtained by taking the net energy values and dividing by kg the efficiency of ME deposition in

new tissues.

Note that the efficiency of use of ME for gain is also highly

dependent on the ME content of the feed (expressed as MJ/kg feed DM or ‘M/D’ for short). kg is low in animals on low digestibility roughages probably because ME is expended in the processes of

ingestion, chewing, digesting and eliminating undigested materials in faeces. kg is also affected by the ratio of legume:grass for pasture– fed animals, and is affected by season, being higher

in Spring—perhaps because there is more soluble carbohydrate in the plant material grown in Spring than in Autumn.

Energy costs for pregnancy

The daily rate of growth of the foetus increases throughout

pregnancy becoming rapid in the last trimester and is especially high in ewes with two or more foetuses. Energy is required for

gain of foetal tissues and conceptus and also to maintain the foetus(es), so the efficiency of ME use for foetal growth appears quite low.

FCR = 1/FCE

The inefficiency (1–k) of ME use results in energy release

as heat.

Fat represents an energy store that animals can draw

upon when ME intake is insufficient for current needs.



Energy costs for milk production

Energy is needed to synthesise the macro–molecules in milk such

as lactose (milk sugar) and casein (milk protein) and an indication of the net energy needed to synthesise these constituents can be

obtained by combusting dried milk in a Bomb Calorimeter. The chemical energy in the milk components must be available from the ME supplied to the mammary gland after the maintenance ME

requirements of the animal have been met. As we have seen for other forms of production, the chemical energy (net energy) is not stored in milk with 100% efficiency. The actual ME requirement for

lactation exceeds the net energy deposition and the efficiency coefficient is referred to as kl.

Because the energy requirement for milk production varies with the fat content of the milk, the energy requirement is usually determined for ‘fat corrected milk’ using 4% as a standard. (If the

fat content is greater than 4% then the ‘fat corrected’ yield is

higher than actual.)

Consideration of the energy required for the lactating animal is

more complex than for growth. The cow has requirements for maintenance, milk production and growth but the latter can be negative (weight loss in early lactation) or positive (weight gain in

late lactation). High producing cows often loose weight in early lactation as they mobilise fat to provide energy for milk production, especially if they are supplemented with protein

concentrates. As the ratio of protein:energy in the tissues mobilised at this stage is relatively low, extra dietary or microbial protein is required. The energy provided from 1 kg of weight loss

declines as the fat content of the mobilised tissues declines. The potential for this weight loss to supply energy and nutrients for milk production depends on the body condition of the cow at the

time of calving.

In late lactation, there is usually an additional energy cost of a new

pregnancy to be considered.

Energy costs for work (draught)

Energy used for work is mainly chemical energy being converted

into heat (and the occasional sound of a grunt!). Glycogen (a

There is a trend in some countries that have used tractor power and trucks for

transport to return to animal power (e.g. Cuba, Sri Lanka). (A pair of bullocks

can plough 1 ha land in about 40 h and develop 40 horsepower whereas a 35

hp tractor could do the same job in about 5 h — Sriastava, 1989: ACIAR).



polymer, and a form of animal starch) is stored in muscle and can be degraded to the monomer, glucose.

Glucose tends to be the major energy source in resting muscle, but fatty acids are used during heavy work. Fatty acids include,

acetate that is absorbed from the rumen, or is mobilised from fat stores in the body. Thus draught animals are normally fed roughage to promote rumen microbial populations that produce

high acetate ratios in their VFAs. Roughages are normally also low in protein. Conflicts for nutrients may occur in pregnant or lactating females used for draught: nevertheless the use of

females as draught animals is quite common in some countries (as is the use of women to do heavy work).

Workloads are classified, for convenience, into light, medium and heavy, but such classifications have been a cause for confusion. There are very few good estimates of energy expenditure (and

thus requirements) of working animals.

Estimates made by Lawrence (1991) of energy expenditure of

working animals and for pregnancy and lactation from MAFF (1975) are given in Figure 2–3.

2.3 Total energy requirements from birth to slaughter

Managing feeding to produce a leaner carcass

We have already seen that the relative fractions of protein and fat

alter as the animal approaches its mature live–weight—with fat deposition increasing relative to protein/water as the animal

matures. However, it is possible to alter the final fat: protein ration in the carcass by nutritional management. A period of nutritional restriction in steers in the early growth phase results in fatter

carcass at maturity than in steers not restricted (Figure 2–4). Nutritional restriction in later life has a somewhat different effect, also resulting in a fatter carcass at maturity. However, protein

deposition ‘catches up’ after later life restriction so that there is more lean at the same mature weight than in cattle restricted earlier (Figure 2–5).

When the time taken for animals to reach a given weight is

longer, the total amount of feed eaten is greater and this lowers

feed conversion efficiency.



Figure 2–3 Resting metabolic rate appears to be higher, by perhaps 10%, in animals ‘in training’. * *(Training helps people

lose weight because they burn energy when they train, but also because their metabolic rate is higher at other times.) (adapted from Lawrence 1991 and MAFF 1975).

Figure 2–4 The mass of protein and fat in the empty carcass of animals grown to maturity on a relatively good and constant plane of nutrition (dotted lines) compared with the

composition of steers subjected to a period of nutritional restriction in the early growth period (solid lines) (Oddy et al. 1997).



Figure 2–5 The mass of protein and fat in the empty carcass of animals grown to maturity on a relatively good and constant

plane of nutrition (dotted lines) compared with the composition of steers subjected to a period of nutritional restriction in the fattening phase (solid lines) (Oddy et al.

1997).

The actual composition at slaughter weight is also affected by the overall rate of growth. In general, animals that grow more slowly end up leaner. Unfortunately, however, there is a downside to this

method of obtaining a leaner carcass.

Growth rate and feed efficiency

Animals that are feed restricted require more time, and therefore

use up more ME (more feed) to reach a given live weight and this reduces lifetime FCE. An example is given for restricted vs ad

libitum fed broilers (Figure 2–6).

Figure 2–6 When the time taken for animals to reach a given weight is longer, the total amount of feed eaten is greater and

this lowers feed conversion efficiency. This is because animals require feed every day just to survive, even if not growing or producing, i.e. they require feed for ‘maintenance’ (Ball et al.

1997).




• Ryan, Williams and Moir (1993)

Compensatory growth in sheep and cattle. 1. Growth pattern and feed intake. Australian Journal of Agricultural

Research. 44: 1609-1621.

!


1. In what forms do energy losses occur between gross energy in feed and net

energy deposited in tissues?

2. Give a simple equation for predicting the ME requirements of humans and livestock.

3. Write an equation for predicting the ME requirement for 1 kg tissue gain in a lamb, given that the net energy stored in

that gain is 13 MJ.

4. Energy cannot be destroyed, so what happens to the energy dissipated in cell

maintenance?

5. Draw a figure showing the changes in water, protein and fat content of live–

weight gain as animals mature.

6. How can feeding management be manipulated to achieve leaner carcasses

in lambs?

7. How could you estimate the net energy costs to a cow associated with production

of 1 litre of milk? How would this estimate differ from the ME requirement for production of 1 litre of milk?

!

References

Ball, Oddy and Thompson (1997) Nutritional manipulation of body composition and efficiency in ruminants. Recent Advances in Animal

Nutrition in Australia. 11: 192-208.

Butterfield (1988) New concepts of sheep growth.



Griffin Press Limited: South Australia.

SCA (1990) 'Feeding Standards for Australian

Livestock - Ruminants.' (CSIRO Publications Melbourne)

Lawrence PR, Pearson RA, Dijkman JT (1991)

Techniques for measuring whole body energy expenditure of working animals: a critical review (IAEA-SM-318/18). In 'Isotope and related

Techniques in animal Production and Health: Proceedings of an international Symposium on the Use of nuclear and related Techniques'.

Vienna, Austria. (Ed. SP Flitton) pp. 211-232 (International Atomic Energy Agency)

MAFF (1975) 'Energy allowances and feeding

systems for ruminants.' Ministry of Agriculture, Fisheries and Food, London, UK.

MAFF (1993) 'Prediction of the energy values of

compound feeding stuffs for farm animals: summary of the recommendations of a working party sponsored by the Ministry of Agriculture,

Fisheries and Food.'

Oddy, Ball and Pleasants (1997) Understanding body composition and efficiency in ruminants – a

non-linear approach. Recent Advances in Animal Nutrition in Australia. 11: 209-222.




Topic 3

3. Digestible and Metabolisable Energy

3.1 Feed digestibility and availability of energy

3.2 Prediction of digestible energy intake

3.3 Feeding systems for energy



3. Digestible and Metabolisable Energy


• Describe the concept of digestible energy.

• Describe the concept of metabolisable energy.

• Discuss the nutritional factors affecting DE and ME


Feed Digestibilty; Availability of energy; Gross Energy; Digestible Energy; Metabolisiable Energy; Net Energy; Factors affecting

digestible energy intake; Feeding systems for energy.

Introduction to the Topic The commonly used unit of energy is the megajoule (MJ) although

the ‘calorie’ is also used. One calorie is the amount of energy required to raise the temperature of 1 ml of water 1 degree celsius and 1 MJ equals 4128 calories or 4.182 kcal. All living cells require

energy to maintain their integrity and to grow and produce secretions and so on. Only chemical energy can fulfil cellular needs and this is provided by the organic materials an animal

ingests. Energy cannot be created or destroyed – so the energy ingested can be accounted for by adding that excreted to that retained in tissues and that converted to heat. The energy stored

chemically in organic materials (the gross energy) is released as heat if the material is completely oxidised in cells or combusted in an oxygenated atmosphere.

3.1 Feed digestibility and availability of energy

The energy that different feeds provide to the animal’s tissues is

determined by the extent of digestion and absorption of the nutrients contained in the feed and the energy associated with those nutrients. The ways that energy is ‘processed’ by animals in

the gut and in the tissues of animals is summarised in Figure 3–1.



Figure 3–1 Partitioning the gross energy of feed into fractions of increasing usefulness to the animal.

It is important to understand these components and to realise that all of the gross energy is accounted for in the subsequent categories (energy conservation applies): energy is either

conserved in chemical forms or released as heat.

Gross energy—(GE) is the total energy released through oxidation

when a sample of feed is ignited in an atmosphere of pure oxygen. This is effectively the amount of energy that is derived when the feed is completely burnt in an atmosphere of oxygen

(i.e. oxidised). The different feed groups and their gross energy values are summarised in Table 3–1.

Digestible energy—(DE) is the difference between gross energy intake and the amount of energy excreted in the faeces.

Metabolisable energy—(ME) is the difference between the digestible energy and the loss of energy in the form of urine and methane gas released by rumen and hind–gut microbes. ME is

approximately 81% of DE in ruminants, which means that approximately 19% of DE is lost as urine and methane energy.

Net energy—(NE) is the amount of energy available for use by the animal from ME after accounting for the heat that is generated during the processes of digestion and metabolism. This heat is, in

effect, an indicator of the inefficiency of ME use for synthesis of



macro–molecules by ruminant tissues. NE can be further divided into energy used for maintenance and for production.

When a sample of feed is combusted in a chamber containing pure oxygen, the ignition produces energy in the form of heat. In

analytical laboratories, this method of determining the gross energy content of a feed is done in an instrument known as a Calorimeter Bomb. The amount of heat released from any sample

depends on its composition as shown in Table 3–1. In tables produced by the Qld DPI, the values for a sample of meatmeal used for feeding adult birds are given as 12.1 and 10.0 MJ/ kg for

DE and ME content. The tabulated values reflect the gross energy values of the fat, protein and carbohydrate in meatmeal and the respective digestibilities of these components, and the further

effect of an ash content of 32% in reducing the ‘energy density’ of the meatmeal.

Table 3–1 Gross energy of major chemical constituents of feed as determined by Bomb Calorimetry (Source: UNE animal science database).

Gross energy (MJ/kg DM)

Constituent

18 Carbohydrate

24 Protein

39 Fat

Gross Energy

When a feed sample is combusted in a Bomb Calorimeter, the heat energy released is referred to as the gross energy of the feed (GE).

However, as we have seen already, the gross energy of a feed is not all digested and absorbed (i.e. not all of it is ‘digestible’). Some feed energy passes through the gut and is lost in

undegraded materials in faeces. The amount that is absorbed from the gut depends on the types of carbohydrates and lipids present and is much lower when there are high concentrations of

The first law of thermodynamics states the principle of conservation of

energy, i.e. that energy cannot be created or destroyed, but only

changed from one form to another. Thus, chemical energy may be

converted in cells into heat (or perhaps light or sound energy).



indigestible fibre and lignin present. Straw, for example, has a lower digestibility than starch. This means that less energy is

extracted by the animal from straw as it passes through the gut than from the same amount of starch.

Digestible Energy

The digestible energy intake (DEI) of an animal is the gross energy intake in feed multiplied by the digestibility coefficient of the feed.

The apparent digestibility coefficient (or more simply ‘the digestibility) of the feed material is calculated as

[the amount of gross energy ingested minus the gross energy in the faeces]

[gross energy in feed ingested]

Not all of the digestible energy (DE) is actually available for use

within the animal. Some is released as methane by eructation (burping) and in flatus, having been produced by anaerobic gut microbes, and some is excreted in energy–rich compounds in the

urine. In ruminants, the DE lost by these two routes may be up to 19 % of the GE. The remaining energy (about 81 % of DE) is referred to as the metabolisable energy (ME). This is the energy

available to cells in the body for metabolism—for maintenance (enabling cells to stay alive and to function effectively), and for deposition in cells, albeit somewhat inefficiently, in products such

as carcass gain or milk (referred to as production). The ME content of a feed is usually tabulated as ME/kg ‘as fed’, or ME/kg DM (often abbreviated to M/D). A point of practical significance is that

the ME value of feeds may be higher when expressed on a ‘dry matter’ basis in contrast to an ‘as fed’ basis (and this difference can greatly affect their cost effectiveness).

3.2 Prediction of digestible energy intake

In vivo. Estimates of digestibility made in vivo ( i.e. ‘in the living

animal’) are derived by measuring the amount of feed ingested and the amount of faeces excreted by animals housed in specially designed crates in which the faeces and urine excreted can be

separated. Estimates of the digestibility of dry matter, or any other component of the dry matter, e.g. protein, energy or an individual mineral, can be made in a similar way. Digestible DM intake is

given by feed DM intake multiplied by feed DM digestibility. Digestible energy intake can be similarly calculated if the gross energy of feed and faeces DM are known. Such experiments give a

‘real’ estimate of the digestibility of the feed sample under the conditions existing when the evaluation was made, but are time–



consuming and expensive to carry out. Thus other simpler procedures are often used.

In vitro (meaning, literally, ‘in glass’). The processes of digestion are simulated in test tubes in the laboratory. Synthetic digestive

enzymes or rumen fluid, with living microbes to secrete digestive enzymes, can be used.

In sacco digestibility (ruminant). Feed samples (ground to simulate chewed material) are placed in a porous bag (40 mm pores) that enables fluid and microbes to enter from the outside,

but prevents feed particles from being lost unless they are first digested. The rate and extent of feed disappearance is estimated over time. This method is very dependent on how finely the feed is

milled before being placed in the sac.

Both the in vitro and in sacco techniques give quite good

predictions of in vivo digestibility values, and are convenient and relatively inexpensive to perform.

Digestibility is also sometimes predicted from measurements of the chemical composition of feeds. NSW Agriculture has used the

following equation to predict dry method digestibility of roughages, viz.

Digestibility of DM = 83.6 – 0.82ADF% + 2.62N%

For ruminants, ME is often predicted from DE as follows:

ME intake = 0.81 x DE intake

The 19% loss of DE implied by this equation is an approximation of the energy losses from a ruminant via methane and urinary compounds. Percentage methane losses from non–ruminants are

relatively low, and differences between DE and ME are therefore are much smaller.

For ruminant feeds, ME content is usually about 80%

of DE content.



Feed digestibility in ruminants

The digestibility of a feed is largely determined by its intrinsic

chemical and physical properties. However, in the case of ruminants, feeds of intrinsically low digestibility will be even less

well digested in the rumen if there are deficiencies of nitrogen or other minerals that restrict the ability of the microbes to grow and ferment feed constituents efficiently. Thus, the efficiency of

digestion of low–quality feed may be increased by supplementing ruminants with urea or sulphur when the diet is low in protein (other minerals are usually adequate for rumen microbes). This is

the basis for supplementing cattle with urea–molasses blocks when they are grazing on dry standing roughage. The cattle have the potential to digest more of the feed, but the lack of protein

building monomers for the microbial cells limits their rate of growth which in turn reduces the rate of digestion of feed and lowers digestibility in the rumen.

Acids and alkalis are often used to treat hay, straw and other agricultural by– products to increase their digestibility. (These

chemicals are more effective than the enzymes of microbes in breaking the chemical bonds in complex carbohydrates such as cellulose and releasing their constituent sugars, but some of these

chemicals are corrosive and dangerous to use.) In this situation it is important to recognise that the increased potential digestibility can only be achieved if the rumen microbes are given even more

building monomers to allow them to take advantage of the extra available energy.

• Low digestibility in the rumen means feeds must be retained for prolonged periods in the rumen to enable them to be reduced in size (comminuted) sufficiently, by rumination and microbial

digestion, to pass out of the rumen. Slow rumen emptying causes the rumen to become distended and this causes the animal to reduce its feed intake.

• Low digestibility and low intake leads to low digestible DM intake and low ME intake. This is often made worse by nutrient (N and

S), deficiency in the rumen, and imbalance in the ratio of protein to energy (P:E ratio) in the materials available for absorption from the gut.

• Fine grinding increases the surface area available for microbial attachment and digestion and may increase feed intake. But it

also decreases feed retention time in the rumen which tends to decrease digestibility in the rumen. As a consequence, there may be increased fermentation in the large intestine.



• Rate of passage of digesta through the rumen (referred to as

dilution rate) increases when animals are in cold environments and decreases in heat stressed animals. Lower retention times of digesta (and microbes), i.e. higher dilution rates, tend to

increase the efficiency of microbial growth in the rumen which improves microbial supply to the host.

3.3 Feeding systems for energy

A number of feeding systems are based on the use of DE or ME to describe the requirements of the animal and the amount of

useable energy that various feeds can provide. Table 3–2 summarises the ME content and the concentration of fibre in various sources of feed grain and roughage for ruminants.

Table 3–2 Metabolisable energy and acid detergent fibre (ADF) content of ruminant feeds (Source: Feed Evaluation Service,

NSW Agriculture).

Feed

ME

(MJ/kg DM)

ADF (%)

Description

Wheat grain

13 3.9 Concentrate

Barley

grain 12.2 8.8 Concentrate

Oat grain

12.0 19.9 Concentrate

Grazing oats

10.4 26.0 Forage/roughage

Lucerne 9.1 36.6 Forage/roughage

Oaten hay

8.0 39.6 Roughage

Wheat

straw 6.0 54.1 Roughage



It is clear from the information in Table 3–2 that the ME content of the diet decreases with increasing amounts of indigestible fibre

(ADF, acid detergent fibre) in the form of roughage. Among the grains, oats provide a high level of ME even though it has considerable fibre in the hull. This is because oat grain contains

around 7 times more oil than wheat or barley (approximately 7%). If all dietary factors are well balanced, and provided there is a normal and efficient pattern of rumen fermentation, then the

amount of energy that the animal can ingest and its growth rate are closely related to the DE or ME concentration of the diet. Although this is a good general rule, it should be applied with

great caution because there are four major areas where the relationship between ME concentration in the diet and performance of the animal can break down. These are listed

below:

(a) a deficiency of nutrients for rumen microbes (normally this

means a deficiency of nitrogen or sulphur on low quality feeds);

(b) too much lipid for microbial activity and for efficient fibre

degradation;

(c) too much readily fermentable carbohydrate in the form of sugars

or starch leading to acidic conditions in the rumen, poor feed utilisation and a low intake; or

(d) an imbalance in nutrients absorbed by the animal and/or toxic factors in the feed which can reduce feed intake irrespective of the ME concentration of the diet.

3.4 Factors affecting digestible energy intake

Based on the simple principle of the digestive tract having a finite capacity to hold and process feed, it is logical to conclude that animals should be able to ingest greater quantities of feeds that

ME values found in tables of feed constituents have

normally been determined in animals given ‘well–

balanced’ diets—they may overestimate the ME

available to animals if the feeds are given in diets that

are imbalanced.



are more digestible. The more fermentable or digestible feeds are more quickly broken down into small particles and cleared from

the digestive tract and this makes space for new feed to be added. There are however some important exceptions to this basic principle and these have already been outlined above.

Nutrients for rumen microbes

Table 3–3 shows the response in terms of dry matter intake and

live–weight gain when cattle with access to tropical grass hay were given different supplements. The first supplement considered was urea. Simply by providing additional nitrogen for the rumen

microbes, the intake of grass was increased by around 50% from 2.26 kg/d to 3.01 kg/d. In this example it was not the digestibility of the basic feed limiting feed intake but rather the amount of

nitrogen available to the rumen microbes, responsible for fermenting the roughage, that was the primary factor limiting the amount of feed the animals could eat.

Table 3–3 Dry matter intake and live–weight change in cattle (initially 170 kg live– weight) fed a tropical grass hay when

supplemented with urea or urea plus an escape protein supplement (cottonseed meal) (Data from D. Hennessy, NSW Agriculture).

Dry Matter Intake (kg/day)

Live-weight Change (kg)

Native tropical grass hay

2.25 -0.41

Hay + urea 3.01 -0.32

Hay + cottonseed meal

3.72 0.11

Hay + cottonseed meal + urea

4.43 0.22

Too much lipid slows fermentation and limits intake

When the level of lipid in the diet exceeds around 5%, the lipid reduces the ability of rumen microbes to degrade fibre and has a

negative effect on feed intake of ruminant animals. This negative effect on fibre degradation, in turn, further reduces the amount of roughage that the animal can ingest, and so reduces feed intake.



Problems associated with high levels of fat inclusion in the diet

have only been of practical significance in recent years since oil and fat have become unwanted by–products in many Western diets in countries where obesity and heart disease have become major

problems. The reduced demand for fat in the human diet means it can be fed to animals as a by–product.

In addition to using cottonseed as a supplement the issue of high levels of fat can become important in certain diets for dairy cattle where lard is used to increase the DE density of the ration.

Acidosis associated with grain feeding

Cereal grains can be included in the diet to increase the DE

content of the diet. However, there are potential problems when grain is a high proportion of the diet. The rapid fermentation of

starch leads to a decrease in pH and this, in turn, reduces the digestion of fibre and leads to a reduced feed intake. Severe acidosis also has a direct toxic effect on the animal that reduces

feed intake independent of the effect on fibre digestion. This adverse effect of cereal grain on the feed intake can cause a dramatic reduction in the total amount of DE available to the

animal. In many situations a change in diet designed to increase DE intake, through supplementation with grain, can actually decrease the DE available to the animal.

Figure 3–2 shows the adverse effects on live–weight gain of increasing the amount of grain fed at any one time. With

infrequent feeding of grain supplements under grazing conditions, the amount of grain presented to animals on each occasion quickly reaches the stage were it is likely to lead to acid build–up

through the rapid fermentation of large quantities of readily fermentable carbohydrate. This example shows responses of sheep to supplements of lupin and barley grain. Both grains

contain similar DE contents and when fed in small amounts each day, both produce similar levels of live–weight gain. However, when fed at weekly intervals, the value of barley as a supplement

is significantly lower than that of lupins. This is due to the adverse effects of acid build–up in the rumen and hind gut. If these effects of acidosis are prevented, using virginiamycin, animal

performance on barley is similar to that on lupins.



Figure 3–2 The effect of feeding barley or lupin grain daily, twice weekly, weekly or fortnightly at levels equivalent to 200

g/d to animals with free access to hay containing 1.5% urea. The barley was fed with or without virginiamycin (Vm). The bars represent the standard error of the difference between

treatments (s.e.d.) (Godfrey et al., 1993).

An imbalance of nutrients absorbed by the animal can limit intake

If nutrients are not provided to the animal in the balance with which they are required for growth or production then intake can

be limited by factors other than the concentration of DE in the diet, i.e. animal factors. Table 3–3 shows data for the supplementary feeding of cattle given tropical grass and

supplemented with urea and/or cottonseed meal. DM intake was higher when animals were fed a supplement of cottonseed meal and urea than when fed urea on its own. Cottonseed meal

provides protein directly to the animal which is over and above that available from the rumen microbes, i.e. ‘escape protein’. The increased intake of DM in response to the additional protein from

cottonseed meal suggests that the animals’ requirements for protein were not fully provided for just by microbial protein. When the additional protein was available then DE intake was further

increased in order to achieve a balance between protein and energy available to the tissues. There is good evidence for the high producing dairy cow, that has a very high demand for protein

at peak lactation, for an increase in feed intake in response to supplements that supply ‘escape’ protein.




• Krehbiel, Ferrell and Freetly (1998) Effects

of frequency of supplementation on dry matter intake and net portal and hepatic influx of nutrients in mature ewes that

consume low-quality roughage. Journal of Animal Science. 76:2464-2473.

• Soto-Navaro, Krehbiel, Duff, Galyean, Brown and Steiner (2000) Influence of feed intake fluctuation and frequency on

nutrient digestion, digesta kinetics and ruminal fermentation profiles in limit-fed steers. Journal of Animal Science. 78:2215-

2222.

!


1. What is meant by the ‘gross energy content’ of a feed and how can it be estimated?

2. What is meant by ‘metabolisable energy’ and how does ME content of a feed differ from its digestible energy content?

3. Indicate, briefly, how the digestibility of a feedstuff can be estimated ‘in vivo’ and ‘in vitro’.

4. If the digestibility of hay is 75%, is this ‘good’ or ‘bad’ quality hay?

5. In what ways do losses of energy occur

between the total energy that an animal ingests (energy intake) and the energy stored in growth or products (energy

retention)? Do these losses add to 100%?

6. What are two important roles of metabolisable energy in growing animals?

7. Since energy used for maintenance, like all forms of energy, cannot be destroyed, were does this energy end up?

8. Why does addition of Virginiamycin to grain diets reduce the risk of acidosis in animals given high-grain diets?

!



References

Godfrey, S. I., Rowe, J. B., Speijers, E. J. and Toon, W. (1993). Lupins, barley, or barley

plus Virginiamycin as supplements for sheep at different feeding intervals. Australian Journal of Experimental Agriculture 33, 135–

140.




Topic 4

4. The Interactions between Energy and Protein

4.1 Protein and energy for non–ruminants

4.2 Protein and energy for ruminants



4. The Interactions between Energy and Protein


• Describe the importance of energy on protein metabolism

• Describe why the provision of energy and protein substrates effect the efficiency microbial protein production

• Describe why urea supplementation can be more effective with the provision of an energy source


Fermentation, rumen bacteria, efficient microbial production, sulphur, non-protein nitrogen, ATP – adenosine triphosphate

Introduction to the Topic

All animals require both energy and protein for their survival

(maintenance). They need additional amounts of both to grow and to deposit tissue or produce milk and wool. Growth of cells and tissues involves the synthesis of compounds such as protein (from

amino acids), carbohydrates (from simple sugars) and fats (also from sugars). Thus animals need the building monomers in relatively constant proportions to grow, and they cannot grow and

reproduce unless all the building monomers are available in their cells and tissues along with energy substrates to supply the chemical energy (ATP) needed to create the chemical bonds. By

analogy, it is like building a brick wall (polymers) using bricks as the building material (monomers) and mortar (chemical bond energy) to hold the bricks together.

Thus animals will not grow well unless energy and protein are supplied in the diet in the right proportions. We formulate diets

for laying hens with, say, 17% protein (170 g protein/kg DM) in the dry matter. The dry matter may contain, for example, 10 MJ/kg DM in which case the diet provides 17 g protein/MJ of ME

(megajoules of metabolisable energy). At times, we may need to be even more specific and describe the diet in terms of the



amounts of individual essential amino acids per MJ of ME. Similar considerations apply to ruminant animals at the tissue level, but

the situation is made more complex because the rumen microorganisms modify the ingested protein and energy substrates.

4.1 Protein and energy for non–ruminants

Although we often refer to ‘protein requirements’ for pigs and

poultry, it is more appropriate to consider their requirements for important essential amino acids that cannot be synthesised in their tissues. The requirements for lysine, for example, can

therefore be expressed in relation to digestible energy (DE) or metabolisable energy (ME) intake.

More mature animals will require lower ratios than younger animals as the latter will be depositing a higher ratio of fat:protein as they age.

4.2 Protein and energy for ruminants

The positioning of the rumen so that it intercepts the food upon entering the gut means that the food components are markedly changed by the action of the rumen microbes before gastric and

intestinal digestion. Sugars such as glucose are largely converted to VFA, and high quality proteins are often almost completely degraded to simpler molecules such as amino acids and ammonia.

A ‘healthy’ and efficient fermentation process in the rumen is critical to the nutrition of ruminant animals and central to the supply of both energy and protein. The efficient fermentation of

carbohydrate by rumen microbes is closely linked with (and provides the energy for) their production of microbial protein. This interaction between the fermentation of carbohydrate and the

utilisation of the building blocks for microbial protein synthesis is depicted in Figure 4–1.

Rumen bacteria are actually tiny plants and have the same

requirements for energy, protein, minerals and vitamins as other living cells. They can however make all 10 or so essential amino acids from simple nitrogen sources such as ammonia. They also

have enzymes capable of degrading cellulose (fibre) to glucose whereas the host does not produce these enzymes. If conditions are not favourable for microbial growth, there will be fewer

microbes, resulting in reduced protein synthesis and fibre



degradation. The most important factors to ensure efficient rumen fermentation are:

• Provision of nutrients for the microbes of fermentable substrates, nitrogen sources and sulphur; and

• Maintaining a stable pH in the rumen (avoiding acidic

conditions).

Figure 4–1 The link between fermentation of carbohydrate to

produce energy and supply of essential building blocks for microbial cells (from Ørskov, 1992).

The rumen can become too acidic due to rapid fermentation of

starches and sugars which can lead to the accumulation of acids. This is the problem of ‘acidosis’ or ‘grain poisoning’.

Figure 4–2 shows the importance of a sufficient supply of nitrogen

to support an efficient microbial production. It also indicates that non–protein nitrogen sources (NPN) such as urea or ammonium sulphate can be used by the microbes to synthesise amino acids

and proteins. The level of nitrogen (N) in the diet for efficient microbial synthesis is normally 9–12% crude protein (N x 6.25),

Crude protein (CP) = Nitrogen (N) x

6.25.

Thus, 12% CP = N x 6.25,

and 12% crude protein contains

1.92% nitrogen.



but this level also depends on the ME content of the feed, so the requirement can also be defined as about 1.4 gN/MJ dietary ME

(this is discussed in more detail later). Nitrogen supplied in excess of requirements for microbial protein synthesis is absorbed from the rumen as ammonia and converted to urea in the liver. From

the liver, it enters the bloodstream and is mainly excreted in the urine. However, some urea can be recycled to the rumen via saliva or by diffusion through the gut wall and reused to make microbial

protein. This reduces the dietary N requirement.

Figure 4–2 There is increasing microbial protein production

(tungstic aid precipitable N) with increasing dietary N (crude protein) until it is fermentable energy which limits protein growth (from Satter and Roffler, 1977).

Low rumen pH associated with presence of starch in the diet, in addition to possibly producing acidosis, can reduce the rate and

extent of rumen fibre degradation and this is shown in Figures 4–3a and 4–3b. This is of significant practical importance when considering supplementary feeding, since grains which contain

high levels of fermentable starch can have a negative effect on the efficiency of fibre utilisation. (Dietary lipid above 5% in the feed DM can also limit fibre degradation, but for different reasons.)



Figure 4–3 (a) and (b) The inclusion of starch in the ruminant diet reduces fibre digestion mainly through rapid fermentation

leading to acid build up and low pH (adapted from Mulholland et al. (1976).

A significant amount of protein is produced by microbes as they grow and multiply in the rumen and microbial protein, when digested in the small intestine, is normally the principal source of

amino acids for ruminants. Additional protein is supplied to the small intestine when dietary protein ‘escapes’ fermentation in the rumen.

Provided the rumen is functioning in an efficient way, then the amount of protein produced by the microbes is sufficient for

moderate levels of animal production. Exceptions to this are the requirements for milk production in early lactation in high producing dairy cows and for very rapid growth in young animals.

For moderate levels of growth and for maintenance, rumen microbial protein synthesis is likely to meet the animals’ requirements in practically all situations. Nevertheless, many

workers have shown significant responses to supplementation with protein meals such as cottonseed meal or lupins. The reason for this is likely to be that these supplements, in addition to

providing ‘escape’ protein, also supply significant amounts of energy to the animal, and potentially glucogenic amino acids. The additional energy may also stimulate rumen microbial protein

growth (provided adequate amounts of N and S and other nutrients are also available). The effect is an increased supply of VFAs and microbial protein as well as additional ‘escape’ protein

and other substrates such as lipids becoming available in the small intestine.



The data in Table 4–1 illustrates how, when barley (and other cereal grains) are given as energy concentrates, the amount of protein available to the animal can exceed that provided in the

feed because some of the energy stimulates microbial growth and protein production in the rumen. On the other hand, with a relatively degradable protein such as occurs in lupin grain, the

amount of protein available to the animal can actually be less than that provided in the diet. This is a good example of how ‘protein’ and ‘energy’ interact and how their individual effects cannot easily

be separated.

Table 4–1 Flow of crude protein from the abomasum of sheep fed chaff ad libitum and 500 g/d of a supplement consisting of lupin or barley grain. [Note: feeding was at regular intervals

throughout the day] (Adapted from Lindsay et al. (1980))

Diet Intake

DOMI

Intake

of CP

Abomasal flow

of CP

Chaff + barley*

651 54 132

Chaff + lupins*

787 184 156

Ruminant tissues are supplied with the

so–called essential amino acids when

they digest rumen microbial protein,

even if the diet contains only NPN.

‘Escape protein’, ‘by–pass protein’ and

‘protected protein’ all refer to protein

that undergoes digestion after passing

through the rumen.




Poppi DP, McLennan SR (1995) Protein

and energy utilization by ruminants at pasture. J Anim Sci 73, 278-290.

Wallace (1994) Ruminal microbiology, biotechnology and ruminant nutrition:

progress and problems. Journal of Animal Science. 72: 2292-3003.

Weimer (1998) Manipulating ruminal fermentation: A microbial ecological perspective. Journal of Animal Science.

76: 3114-3122.

!


1. Why are microbial fermentation and microbial growth in the rumen said to be

‘coupled’?

2. Why is the ratio of protein:energy in absorbed nutrients available to the host

animal an important consideration when formulating diets for livestock?

3. What is the fate of NPN supplied to the

rumen in excess of requirements for microbial growth when the latter is limited by availability of energy?

4. What do you expect will happen to rumen ammonia concentration if extra energy is ingested in the form of dietary starch?

(starch is N–free)

5. When might ruminants require supplementary protein in the form of

‘escape/ bypass’ protein?

!

References

Lindsay, J. R., Purser, D. B. and Hogan, J. P. (1980). Supplementation of a low quality



roughage with lupin or cereal straw. Animal Production in Australia. 13, 479.

Mulholland, J.G. et al. (1976). Workshop on Straw Feeding. WA (J.B. Rowe, pers. comm).

Ørskov, E.R. (1992) Protein Nutrition in Ruminants. 2nd Ed. Academic Press.

Satter, L. D. and Roffler, R. E. (1977). Nitrogen requirement and utilization in

dairy cattle. J Dairy Sci 58, 1219–37.

SCA (1990). Nutrient requirements of farm

animals—ruminants. Standing Committee on Agriculture, CSIRO Publishing, Melbourne.




Topic 5

5. Supplying Protein for Ruminants

5.1 Sources of protein

5.2 Matching animal requirements with microbial and dietary supply

5.3 Protected (bypass) protein



5. Supplying Protein for Ruminants


• Describe the minimal requirements for protein and amino acids

to support maintenance and production of livestock tissues.

• Describe the additional dietary requirements for protein and amino acids that are the consequence of losses of dietary protein

and amino acids during digestion and metabolism.


Sources of protein in ruminants and non-ruminants; Requirements for protein by living cells; Matching animal requirements with dietary (and microbial) supplies; ‘Escape’ or bypass protein.


In order to provide effective protein nutrition for the animal, it is

essential that we understand:

• The requirements of the animal tissues for amino acids;

• The amino acid losses during digestion, absorption and metabolism

• The amount and quality of protein provided in the feed; and

• For ruminants, the amount and quality of protein provided by rumen microbes.

Requirements for protein

Tissue maintenance—Before any production is possible, the animal has to maintain the cells of the body. This requires a

continuous supply of amino acids for normal cell function, repairs and replacement. Protein is continuously ‘turned over’ and a proportion of each amino acid is excreted rather than re–used.



Tissue growth— The protein content of tissue growth is between 14 and 20% of the ‘empty’ body weight (‘empty’ meaning ‘excluding the gastro–intestinal tract’). Growth requires a supply

of amino acids over and above the amount required for maintenance of normal body functions.

Milk production—Milk from ewes contains approximately 50 g protein/kg and between 35 and 50g/kg for dairy cows. The protein for milk production constitutes the highest demand for

protein relative to any other aspect of ruminant production.

Wool growth—The amount of wool grown can vary between 4 and

16 g/d. Wool is pure protein, with a very high content of sulphur amino acids (approximately 3 to 4 times higher than in milk and meat proteins). Therefore, while the total protein requirement for

wool growth may not be very high, wool growth is normally responsive to supplements containing the sulphur amino acids, methionine or cystine.

Gestation—The additional protein required in pregnancy only becomes important during the last trimester. Protein requirement

in pregnancy is below that required for milk production but still pregnancy does increase protein requirements.

The efficiency with which absorbed protein is converted to tissue and milk production is normally around 70% and is much higher than the efficiency of protein utilisation for wool growth which is

around 20 to 25%. There are a number of books with recommendations on protein requirements for different classes of animals. The Standing Committee on Agriculture (SCA, 1990)

commissioned a set of standards for ruminant animals in Australia.

Remember, ‘nitrogen’ and ‘crude protein’ have virtually the same

meaning. The terms tell us nothing about whether amino acids are

present.

The efficiency of use of the 20 amino acids for protein synthesis in cells

depends on the amounts of each amino acid present. A limitation of one amino

acid can make the other 19 superfluous to requirements.



5.1 Sources of protein

Figure 5–1 shows the range of forms in which nitrogen can exist from urea, through to protein. The body can breakdown protein to

peptides and amino acids, and further to urea that is mostly excreted in the urine: tissues, however, can only synthesise protein if amino acids are present. On the other hand, rumen

microbes can break down urea to ammonia and can use the ammonia to synthesise amino acids and then proteins. Rumen microbes are able to use protein and amino acid as or ammonia as

building units from which to synthesise microbial protein. Most rumen microbes can use ammonia as their only source of ammonia.

Therefore, for protein synthesis in the animal, the requirements need to be considered in terms of amino acids or protein whereas

for the rumen microbes, the total amount of nitrogen (protein and NPN) is the key consideration. This is summarised in Figure 5–2.

Figure 5–1 The range of forms in which nitrogen can exist from urea to protein (UNE Animal Science database).



Figure 5–2 Protein synthesis and requirement for primary building blocks (UNE Animal Science Database).

The amount of protein synthesised by the rumen microbes can be quite variable depending on a number of factors. There is seldom

an optimum balance of these factors—one of the following factors is usually ‘first limiting’.

• The amount of fermentable substrate;

• The availability of nitrogen and other building blocks;

• The acidity of the rumen;

• The osmotic pressure of the rumen fluid;

• The amount of lipid in the diet; and

• The presence of anti–microbial chemicals in the diet.

The protein available to the animal from dietary sources is determined by the amount eaten, the extent to which it is broken

down (or escapes break down) in the rumen, and its digestion in the small intestine. The amount of dietary protein passing through the rumen unfermented is commonly called ‘bypass’ protein but

can also be referred to as ‘escape’ protein or ‘protected’ protein.

The extent to which dietary protein is fermented and broken down

in the rumen depends on the nature of the protein and how long it remains in the rumen. Low solubility will usually increase the amount that passes through to the small intestine. Proteins with

these bypass characteristics tend to be those which have been heat–treated and/or contain some lipid which reduces the



solubility and the rate of fermentation. Extruded oil seed meals, such as cottonseed meal contain higher levels of bypass protein

than unprocessed sources of protein such as lupin grain. The processes of protein and nitrogen digestion and absorption are summarised in Figure 5–3.

Heat and solvents used in the extraction of oils often leave the proteins in the resulting meals in a denatured form that has a low

degradability in the rumen.

Figure 5–3 Protein and nitrogen digestion and absorption in

the ruminant digestive system (UNE Animals Science Database).

5.2 Matching animal requirements with microbial and dietary supply

The example discussed below was developed by Dr Simon Bird for growing steers and formed part of a paper delivered to the Australian Lot Feeders Association (May 1996, Brisbane). Table 5–

1 summarises the effect of different live–weights on the requirement for protein for growth. As the animal gets heavier and older it deposits more fat and less protein and this means that it

requires lower concentrations of dietary protein relative to digestible energy. By including an estimate of the efficiency of protein utilisation by the tissue, it is possible to determine the

amount of protein which needs to be absorbed from the digestive tract to meet requirements.



Table 5–2 shows various estimates of microbial protein synthesis and a summary of the calculation which leads to an estimate of

the requirement for true dietary bypass protein.

Table 5.1 Protein requirements for various liveweights of Bos

taurus steers (NRC 1984)

Liveweight (kg)

250 350 430

Protein required for growth 226 190 165

Protein required for maintenance 160 210 249

Total protein requirement 386 400 414

Efficiency of protein utilisation by the

tissues 0.7

Protein which must be absorbed from the digestive tract

551 571 591

Assumptions

1. Level of feed intake set at 2.8% of live–weight

2. Availability of rumen digestible nitrogen is not limiting microbial growth



Table 5–2 Calculation of yield of microbial protein, proportion of microbial protein available to the animal and the amount of

protein which must come from feed to meet the protein requirements of a Hereford steer growing at 1.6 kg/d (SCA 1990).

Live weight (kg)

250 350 430

Dry matter intake (kg/d)

7.0 9.8 12.1

Metabolisable energy intake (MJ/d)

81.0 113.0 139.0

Microbial protein

synthesised (g/d)

(Using SCA value of 8.4 g Mic p./MJ of ME)

680.0 949.0 1168.0

Microbial protein digested in small intestine

(only 70% absorbed)

476.0 664.0 818.0

Availability of absorbed microbial

protein

(only 80% of absorbed protein is available)

381.0 531.0 654.0

Animal protein requirement (Table 5–1)

551.0 571.0 591.0

less available microbial protein

381.0 531.0 654.0

Protein shortfall (g/d)

Intestinal digestion of feed protein (0.7)

170.0 40.0 none

Escape protein which

must come from feed (g/d)

243.0. 57.0 not

req



Assumptions

1. Feed intake set at 2.8% of bodyweight

2. M/D of ration set at 12 MJ of ME/kg

The feedlot diet in this example is based on barley. The protein

content of the barley has an important influence on the requirement for additional use of bypass protein (cottonseed meal) and urea. These interactions are summarised in Table 5–3.

Knowing the requirements of the particular class of animal to be fed and the production targets you can use the calculations of microbial protein synthesis and bypass protein to determine the

most efficient combination of protein and NPN supplements. In heavier, more mature animals there is a lower requirement for protein. For younger animals growing more rapidly, there is a

higher requirement for true dietary bypass protein.

Table 5–3 The effects of protein level in barley on the

requirements for cottonseed meal and urea supplements (UNE animal science, nutritional database).

Protein level in barley (%)

8.8 10.0 11.3 12.5

Cottonseed meal (g/d)

428.0

327.0

240.0

144.0

(% of ration)

6.1 4.7 3.4 2.1

Urea (g/d) 91.0 81.0 70.0 60.0

(% of ration)

1.27 1.16 1.0 0.86

The same principles, as discussed above, can be used for matching the supply of protein of dietary and microbial origin with

the requirements for milk production and for supplementary feeding of grazing animals.



Table 5–4 Examples of the degradability of different protein sources in the rumen assuming a turnover rate of 0.05

volumes/hour (from Oddy 1997).

Figure 5–4 The effect of residence time of protein concentrates in the rumen on the extent of protein degradation in the rumen (from Oddy 1997). The line indicating an average residence

time for digesta of 20 hours is equivalent to a fractional outflow rate of rumen contents (FOR) of 5%/hour.



5.3 Protected (bypass) protein

Protected protein is the protein that passes through the rumen without being fermented and broken down to ammonia. The proportion of a feed protein that is protected from rumen

fermentation is determined by the basic characteristics of the protein as well as by the length of time it is resident in the rumen. The factors that make a feed protein more protected (less

degradable) include solubility and particle size. The less soluble the protein the less likely it is to be available for microbial degradation. Similarly larger particles in the rumen have less

surface area for microbial colonisation and are therefore less likely to be degraded. Heat treatment of proteins during processes such as cooking or extrusion increase the proportion of protected

protein. Table 5–4 contains a list of feeds and the percentage of protein degraded over a 20 hour period. This shows that casein is almost entirely degraded in the rumen whereas only 55% of

cottonseed meal protein is likely to be degraded. The amount of time that the protein remains in the rumen will influence the extent of degradation. Figure 5–4 shows the relationship between

time in the rumen and the extent of degradation for barley and for cottonseed meal. The level of feed intake and the nature of other feed ingredients influence residence time of protein particles in

the rumen. Higher levels of feed intake and more fibrous feeds are likely to result in faster passage through the rumen and lower rates of protein degradation.


• Hynd and Allden (1985) Ruminal

fermentation pattern, postruminal protein flow and wool growth rate of sheep on a high barley diet. Australian Journal of

Agricultural Research. 36:451-460.

• Nolan, Norton and Leng (1973) Nitrogen cycling in sheep. Proceedings of the

Nutrition Society. 32: 93-98.

• Nolan JV, Norton BW, Leng RA (1976) Further studies of the dynamics of nitrogen

metabolism in sheep. British Journal of Nutrition 35, 127-147.

!



• Nolan JV, Kempton TJ, Leng RA (1978) In 'Proceedings of the Australian Society of

Animal Production' p. 132


1. How are proteins and ‘essential amino acids’

related?

2. Which use of amino acids by tissues takes precedence over growth and protein

deposition?

3. How does the composition of plant proteins in diets of animals differ from proteins in

tissues and products of animals?

4. Name two breakdown intermediates of dietary protein found in rumen contents.

5. Name two breakdown intermediates of dietary protein in the intestines of the pig.

6. What happens to urea supplied in the diet of

ruminants after it is ingested?

7. List three factors that reduce microbial metabolism and growth in the rumen.

8. What are the two main categories of protein in digesta flowing from the rumen to the lower digestive tract of cattle?

9. Name three processes in the body of a cow that require removal of amino acids from the bloodstream.

10. Why do you think protein entering the rumen in barley would be more rapidly degraded than protein in cottonseed meal?

!

References NRC (1984) 'Nutrient Requirements of Beef Cattle.' (National Academy of Science: Washington D.C.)

Oddy VH (1997) The impact of growth in early life on subsequent growth and body composition of cattle. In 'Proceedings of the

5th National Beef Improvement Association Conference' pp. 41-45

SCA (1990) 'Feeding Standards for Australian

Livestock - Ruminants.' (CSIRO Publications Melbourne)




Topic 6

6. From Feed Components to Meat, Milk and Wool

6.1 Monomers — Building Units

6.2 Energy — cellular energy interconversions

6.3 Metabolites



6. From Feed Components to Meat, Milk and Wool


• Understand how large molecules in feed ingredients are digested to smaller sub-units, absorbed and then synthesised into rather similar large molecules in cells and

tissues.

• Describe how energy is released during degradation of large molecules (polymers à monomers) and reused in synthesis

of large molecules.


The components of feeds and of tissues; Monomers; Polymers; Metabolism; Energy transformations; Metabolites


The discipline of nutrition depends on an understanding of the digestion and degradation of complex molecules (polymers) in

feeds to smaller, simpler molecules (monomers) in the gut and in cells, and the use of these small molecules as building units for the re–creation of complex molecules that make up animal

products. The processes of degradation (or ‘catabolism’) and synthesis of complex molecules (or ‘anabolism’) such as proteins and lipids (fats) are together referred to as ‘metabolism’.

Metabolites are chemical intermediates in these processes.

Because this Unit deals with the practical application of nutrition, our focus is on feeding management as a means of optimising

feed inputs and production outputs. This means we want to

Metabolism describes the processes of synthesis and degradation of

materials in cells. Metabolites are chemical intermediates in

metabolism.



manage the feeding of our animals to optimise saleable outputs such as egg, meat, milk, wool, and skins. We also want to feed our

breeding stock efficiently to obtain optimum reproductive performance and replacement animals for our flocks and herds. Draught animal power is also an important output from

production systems in some countries. Management of animals to achieve optimum input/output is helped by an understanding of the biochemistry in cells that is responsible for the breakdown of

polymers to simple monomers, and their re–synthesis into similar, but subtly different polymers. Energy released during breakdown reactions is conserved chemically in ATP and re–used in the re–

syntheses. These topics are integral to rumen microbiology, animal physiology and animal behaviour.

Products such as eggs, meat, milk and wool can be thought of as

being made up of chemical polymers such as protein, carbohydrate, fats. These large molecules are synthesised in the cells of body tissues from the much smaller building units, or

monomers, that are taken up from the bloodstream by tissue cells, e.g. amino acids, fatty acids and sugars. Some of these monomers come from degradation of polymers in the feed. They are

absorbed from the gut and enter the circulating blood. Some monomers (eg. glucose, amino acids) are synthesized within body tissues, others are synthesized by microorganisms living in the

gut.

Vegetable and animal proteins all contain 20 amino acids. Plants

can synthesise all 20 amino acids but 10 of these cannot be synthesized within the tissues of livestock and are referred to as ‘essential amino acids’ to distinguish them from ‘non-essential

amino acids’. Similarly, there are many different fatty acids (monomers of fats) in various types of fats or oils (lipids). However, three fatty acids (referred to as ‘essential fatty acids’)

cannot be synthesized in the animal tissues and must therefore be supplied in the diet.

ATP is adenosine triphosphate. It is found in the contents of all cells. It

stores energy from breakdown reactions and releases energy to drive

synthetic reactions.



6.1 Monomers — Building Units

The important basic building monomers are sugars, amino acids and fatty acids. It is important to have an understanding of how

the animal obtains its supplies of these basic monomers, and also to be aware of which of the major chemical groups are found in which products. Thus, in milk, we think of lactose (milk sugar), fat

and protein (casein, immunoglobulins) and water. Muscle tissue (meat) consists largely of proteins but also some fat and small amounts of other materials such as glycogen. Eggs are made up

mainly of protein (the egg white) and lipids (yolk) and minerals (egg shell is almost pure calcium carbonate). Bone has a high concentration of minerals. Wool consists almost entirely of

protein, and this protein is unusual in having a very high content of the sulphur amino acid, cysteine. This amino acid is formed in the body from methionine, one of the amino acids that the body

tissues cannot synthesize. Methionine must be absorbed from the gut and is therefore referred to as an essential amino acid. Lysine and threonine are other examples of essential amino acids that

are needed for protein synthesis. Linoleic acid is an essential fatty acid that must be supplied in the diet of all animals.

6.2 Energy — cellular energy interconversions

As well as acting as building units, fatty acids, amino acids and sugars have other roles in tissue cells. They are, for example, used in the central metabolic pathways of respiration (pathways that

degrade energy–containing intermediates in the presence of oxygen) to release the chemical energy. This energy may be conserved chemically in, for example, ATP or NADPH (molecules

used to conserve and transfer energy within cells). This energy is needed to keep cells alive and functioning normally, i.e. for ‘maintenance’. Glucose that is in excess of current requirements

can be stored as glycogen (animal starch) in the liver and in

Monomer (literally ‘a single unit’). If a brick wall is like a polymer, then

individual bricks are the monomers linked together by chemical bonds (the

mortar).

‘Essential’ amino acids cannot be synthesised by the cells of higher

animals — they must be absorbed through the gut wall.



muscle tissue, and can be mobilized later to provide glucose when this is not available from other sources. However, animals store

only enough glucose energy in glycogen to supply their needs for about a day. Glucose not needed for glycogen repletion is also stored as fat, and fat depots provide the animal with a longer–

term energy reserve. When there is an excess of amino acids, some of these can also be converted to glucose or fatty acids and then stored as glycogen or fat for use at times when the animal is

short of feed.

There is one group of substrates produced by microbes in the rumen and large intestines of animals (and humans), as yet only

briefly mentioned, known as steam volatile fatty acids (VFAs). The major VFAs are acetic, propionic and butryic acids (2, 3 and 4 carbon compounds) and they are present in the gut contents and

in the bloodstream as their salts — acetate, propionate and butyrate. (Vinegar is a pure acetic acid solution.) These acids are produced by anaerobic microorganisms by the process of

digestion known as fermentation (meaning that the digestion process occurs in the absence of oxygen). The extent of the biochemical degradation of the feed is severely restricted because

chemical oxidation is very limited in the absence of oxygen. VFA are the major end– products of rumen microbes. They contain considerable chemical energy. VFAs are absorbed from the gut

and, in the case of ruminants, are the major source of energy for cells. They also serve as building monomers for the creation of new polymers in cells, i.e. protein, lipids etc.

6.3 Metabolites The major sugar in the body is glucose. This sugar is found in the bloodstream and is an essential energy source for brain and

nervous system. It is also important for other purposes in the body. It is a major source of energy in most cells of the body. As well as being stored as glycogen when not required, it is also the

starting point for synthesis of ribose, the sugar needed for synthesis of DNA and RNA. Glucose is also a primary unit for building lactose (milk sugar) and fructose—the sugar that provides

When starch is completely degraded in the body to carbon dioxide and

water, the same amount of energy is liberated as would be produced if the

starch were combusted.

It is possible for animals to grow fat by eating too much protein



energy for the foetus. It can also be used to generate glycerol and the free fatty acids from which fats (lipids) are formed. The

pathway by which ribose is formed in cells, known as the pentose phosphate pathway also produces NADPH, a chemical used to store energy and release it to promote fat synthesis.

In monogastric animals (pigs, poultry) glucose is formed by the

digestion of starch (from grains and forages) in the intestines and is then absorbed into the blood stream. In contrast, in sheep and cattle and other ruminants, starch from cereal grains and green

forages is almost totally degraded to volatile fatty acids (VFAs) in the rumen—very little starch or glucose escapes to the sites of absorption in the intestines. As a result, ruminants must

synthesize their own requirements for glucose in tissues, mainly from propionate and glucogenic amino acids, by a process known as gluconeogenesis (Chalupa 1988).

Feed polymer breakdown and re–synthesis

In essence, livestock ingest different types of feeds, digest the

various large molecules in the feeds to much smaller monomers that can be absorbed and then circulate via the bloodstream. Cells

in the body take up these monomers and process them further through the central biochemical pathways. The biochemical pathways of glycolysis and the citric acid cycle are present in

most cells in the higher animals. Some of the intermediates in these pathways are ‘tapped off’ and re–joined together, using the chemical energy generated by respiration (e.g. ATP), to form the

large molecules that are the major constituents in the animal products of interest to us. Feed constituents are broken down to simple compounds and then re–synthesised into more complex

molecules by animals (SCA 1990).

All cells in the body require energy to stay alive. Energy is also

deposited (as chemical bond energy) as cells synthesise polymers such

as protein and fat during growth

Glucose is the sugar unit from which more complex carbohydrates

such as starch, glycogen and cellulose are synthesised.

The glycolytic pathway and the citric acid cycle are names for

central biochemical processes that occur in all cells.



Figure 6–1 represents the degradation processes in the rumen (top box), and re–synthesis of the intermediates into animal

products (using conserved energy in the form of ATP and NADPH) is represented in the lower box.

This is a fairly simple but extremely useful summary of the more complex processes by which feed inputs are converted to animal outputs (products). An understanding of the processes depicted in

the Figure 6–1 will come in handy when nutritional and metabolic issues are being considered. The diagram is intended to refer to events in ruminants but the tissue processes (in the lower box) are

essentially similar in ruminants and non–ruminants.

Figure 6–1 A summary of the processes of degradation of feed

carbohydrates, proteins and lipids in the rumen, absorption of rumen microbial end–products and processing of these metabolites to generate respired gases and animal products

(Source: UNE animal science database).




• Datta FU, Knox MR, Rowe JB, Nolan JV

(1997) Rumen-protected methionine or barley supplements for parasitised Merino lambs. In 'Recent Advances in Animal

Nutrition in Australia'. (Eds J Corbett, M Choct, JV Nolan and JB Rowe) pp. 236. (University of New England, Department of

Animal Science: Armidale, NSW)

• Pitt RE, Van Kessel JS, Fox DG, Pell AN, Barry MC, Van Soest PJ (1996) Prediction of

ruminal volatile fatty acids and pH within the net carbohydrate and protein system. J Anim Sci 74, 226-44.

!


1. Name 3 types of polymers present in the diet of livestock that are also found in livestock products, eg. steak and milk.

Would these polymers be identical in feeds and animal products?

2. Why do cells in the body require a

continual supply of energy?

3. What are the monomers of (a) proteins and (b) carbohydrates and (c) fats?

4. Glucose considered a key circulating nutrient in the body of livestock. List three important roles of glucose in

tissues?

5. What role does glucose play in the rumen?

6. How is chemical energy made available in

cells?

7. What is the fate of chemical energy during breakdown of polymers in cells and

synthesis of polymers in cells?

!

References Chalupa W (1988) Manipulation of rumen

fermentation. In 'Recent Developments In Ruminant Nutrition'. (Eds W Haresign and

DJA Cole) pp. 1-18. (Butterworths: London)

SCA (1990). Nutrient requirements of farm animals—ruminants. Standing Committee

on Agriculture, CSIRO Publishing, Melbourne.

Documents

Module - Woolwiseminerals, vitamins and water. There is considerable confusion when the terms ‘energy’ and ‘protein’ are used in discussing feeds and their nutritive value