Sheep Breeding and Reproduction

Dr. Randy Harp

Sheep Genetics

• 27 pairs of Genes• Two genes that form a gene pair may be the same

(Homozygous)• If they are different = Heterozygous• If Heterozygous, then one allele of the gene pair

may express itself over another = dominance• This dominance can vary from complete to co-

dominance• A hidden gene expression is called recessive

Sheep Genetics

• Some are sex linked

• Examples

• Genotype vs Phenotype

• Estimated phenotypic variance of the flock (pg BRD 19)

• Heritability

Heritability

• Proportion of the total phenotypic variation that is due to the variation in additive gene effects

• In other words, the proportion of differences due to genetic effects and is important in the prediction of response rates from selection.

• The square root of the variance is the standard deviation, which is the ave. deviation of each indiv. in the pop. from the pop. Average

• Heritability est. – handout BRD 33

Correlation

• Quantifies a relationship between two variables

• Measured between zero and one

• Positive versus negative correlations

• Multiple effects (Regression models)

Animal Identification

• Accurate identification

• Written records

• Accurate measurements of economic importance

• ??? Registered or not

• Two ID nos. , one for flock or indiv. ID and the other for registration ID (if reg.)

Record Keeping

• Determine clear goals

• Allow for records such as pedigree, birth weight, singles or twins, weaning weight, etc.

• Five categories of records:– REPRODUCTION, MATERNAL , GROWTH,

WOOL AND CARCASS

Estimated Breeding Value (EBV)

• EBV = b (P- ave.P), where b is the heritability of a particular trait

• Example; Ram A has a grease fleece weight of 15 # and the ave. flock grease fleece weight is 11 #, then 15-11 = selection differential [4]

• 4 x .4 (h) = 1.6 # of grease fleece advantage for breeding Ram A

EPD’s (Expected Progeny Differences)

• Simply ½ of the EBV

• The ave. EPD in a population is + or – from the average of the population. A comparison

• Ex. If a ewe has a +.3 for no. of lambs born, then one would expect the progeny to produce .3 more lambs per lambing than the progeny of average ewes.

EPD’s

• Another ex. is two rams A and B

• +.5 and +1.1 for fleece weight, respectively

• Ram B progeny would be expected to have .6 pounds heavier fleeces than Ram A

• Example 3; Ram has +1.0 and Ewe has +.5 for weaning weight, then the progeny will be 1.5 # more than the pop. ave. for WW

EPD’s

• Accuracy• Gives an idea of reliability of the estimat• Measured from 0-1• An accuracy of .45 is not very reliable,

whereas > .9 is considered reliable• Low accuracies are a result of limited

information known about the parents or low numbers of progeny occur

Generation Interval

• The time lapse between birth of an animal and the birth of its replacement – usually 3-4 years for sheep

• A measurement of progress or rate of improvement

• Genetic Improvement per year = heritability x selection differential divided by the generation interval

Methods of Selection for Single Traits

• Individual selection-selection on their own performance

• Family selection-selection based on bloodlines; useful when (h) is low

• Pedigree selection-similar to family and is dependent upon how closely related the ancestors. Useful when considering same sex

Methods of Selection for Single Traits

• Progeny Test- observing the performance of the offspring. Must be mated to several ewes and then look at the offspring. Best when looking at carcass traits

• Combined Selection- uses more than one of the above mentioned methods

Methods of Selection for Multiple Traits

• Tandem Selection- focuses on multiple traits, yet one at a time. After the performance of one is achieved, then move to the next trait

• Independent Culling- Set minimum standards for more than one trait at a time for the indiv. Cull any that does not meet the min. stds for any trait

Methods of Selection for Multiple Traits

• Selection Index- rank indiv. animals for two or more traits based on a combination effect

• One the farm testing:

• Primary focus is on the ewe flock

• Therefore, traits such as prolificacy, weaning weight and fleece weight

National Sheep Improvement Program

• Oversees the promotion, funding, development and implementation of a national genetic evaluation program for sheep

• Minimum criteria for record keeping• Lamb records: ID #, sire and dam ID, date of

birth, sex, type of birth, & type of rearing.• Additional records are reproductive, growth and

fleece traits

NSIP

• Focuses on:• Number of lambs born per ewe lambing• Body weight at 30,60,90,120 & 240 days• Grease &/or clean fleece wt.• Staple length• Fleece grade• Pounds of lamb weaned per ewe exposed per year

Adj. Factors for No. of lambs born per lambing to a common ewe

Age of Dam Adj. Factor

1 1.45

2 1.15

3 1.05

4 1.00

5, 6, & 7 .96, .96 & .95

8 .98

9+ 1.00

Major Economically Important Traits

• Reproductive Efficiency• Carcass Merit• Milk Production• Total Ewe Production- # lamb/ewe exposed• Mature Size• Hardiness & Adaptability• Wool Production• Growth Rate

Selection for Growth

• Growth is a very important trait, esp. for meat breeds

• Adjusted 90 day weaning weight• If birth weight is known:• [Actual wt. - birth wt. x adj. Days (90)/

actual age ]+ birth wt.• If birth wt. is not known:• Actual wt. X adj. Age in days/age

Selection for Fleece Traits

• Grease and Clean Fleece Wts• Yield• Staple length- at least 3”/yr.• Fiber diameter• Crimp• Color• Density• Belly Wool-undesirable on sides

Importance of Genetic Improvement in Seedstock

Flocks• Most produce their own replacements• Therefore, genetic improvement from outside is

by the Ram purchased• Theoretically, Genetic merit increases at the same

rate as the genetic merit of the rams, yet because of generation intervals and replacement production usually improvement lags two generations behind

Mating Systems

• Purebreeding- common genetic group• Outbreeding- unrelated within a breed• Inbreeding-closely related with one or more

common ancestors (more than 50% related)• Linebreeding-common ancesters, but not 50%

related• Crossbreeding- mating of different breeds• Grading Up-enhanced concentration of

crossbreeding to rams of a single breed

Inbreeding Coefficients

• Full brother mated to sister = .25

• Sire on daughter = .25

• Half brother to half sister = .125

• Sire on Mother = .5

• Therefore; > .5 has to be compounded over time and added generation to generation

Crossbreeding

• Heterosis or Hybrid Vigor

• Superiority of the crossbred indiv. Relative to the average performance of the parents

• Maximized when parents have no breed ancestry in common (3 breed terminal X)

Crossbreeding Systems

• Two Breed Terminal(50% Heterosis)• Three Breed Terminal (100%)• Three Breed Rotational (86 %)• Four Breed Rotational (93 %)• Two Breed Rotational• Roto-Terminal- combination of rotational and

terminal systems. The poor ewes would still be used in a terminal crossing manner

Inherited Defects

• Many defects are recdessive in nature• Selection pressure is exercised against certain

traits, esp. lethals and fleece defects• Dwarfism, Spider Syndrome, Jaw Defects,

Rectal Prolapse, Inverted Eyelids, Cryptorchidism, Horns or Scurs, Face covering, Color, Skin Folds, Silky, Britch fibers, grey color, paralyzed limbs, earless, etc.

SheepBreeding and Reproduction

• Improved lamb production

• More lambs per lambing

• More frequent lambing

• Increased percent of total sheep nos.

• Reducing death losses

Sheep Breeding and Reproduction

• breed dependent - seasonal breeding• Puberty-weight and age (70-100 # or 5-9 months)• Breed to lamb as 2 year olds, yet can breed as a

yearling, but with less efficiency• Actually, greater production per lifetime from

yearlings; Ramb. is later maturing and less effic.• gestation - 144-152 days (med wool less)• heat periods - 20 to 42 hours (no signs)

SheepBreeding and Reproduction

• ovulation occurs late in heat cycle

• 14- 20 day cycle ave. 16-17 days

• Prolifacy - > 100% lamb crop

• flushing ewes works in sheep, too

• Factors affecting reprod.– daylight (<14 hrs/da), temp. ( <74 F or > 100),

nutrition

Hormonal Control

• Progesterone (Progestins)- produced by the corpus luteum

• Prostaglandins- induces the regression of the corpus luteum (ineffective in normally cycling ewes); lutalyse

• Estrogen• Gonadotropins- GnRH, HCG, FSH & LH• Melatonin- regulatory of the seasonal breeder;

produced by the pineal gland• ACTH

SheepBreeding and Reproduction

• Lambing– > 4 sq. ft. pen for ewes, clean & dry– presentation of front legs– orphan lambs is not uncommon– 25% death loss is common– Feed ewes small amounts of water at first with

oats, wheat bran and hay (small amounts)

SheepBreeding and Reproduction

• Ram management

• mating guide– ram lamb hand (20-25) pasture (25-35)– yearling or > hand (50-75) pasture (35-60)– use marker rams to detect bred ewes both at

breeding and 60 days after breeding

Replacement Ewe SelectionItem 140 % Prolif 170 % Prolif

No. ewes 100 100

Ewe lambs 70 85

Twin lambs 40 70

Tw lambs > ave. in WW

20 35

Ewe lambs exposed

20 30

Preg. Ewes 20

Physiological changes of reproduction in the ewes

• REPRO – 4 chart

• Ewes in the south have longer breeding patterns

• Sheep near the equator are less likely to have seasonal breeding

Factors affecting reproduction in the ewe

• Heredity• Age• Photoperiod (seasonal)• Temperature and humidity• Nutrition and Exercise• Parturition and lactation• Disease and parasites• Fertility of & assoc. with the ram

Pregnancy Testing

• Breeding Marks• Ultrasonic Scanning- best detected between

70 & 120 days• Bagging or Udder Palpation• Blood Progesterone- at the time of the next

expected heat• PSPB- a protein called pregnancy-specific

protein B after day 21 of breeding.

Factors affecting the reproduction in the Ram

• Breeding soundness exam

• Palpation of the testicles, epididymis, and penis and visual appraisal of feet, legs, eyes and jaws.

• Semen evaluation

• Disease prevention

• Heat stress

Semen Collection

• Components- seminal fluid and sperm• Quality of sperm – morphology and

viability (percent live)• Methods- artificial vagina and

electroejaculation• Semen handling-dilution characteristics:

glucose or fructose, egg yolk, citrate or phosphate, antibitotics, glycerol

Insemination

• Natural- 3-5 billion sperm inseminated• AI – vaginal approach – 200 million “• AI – cervical approach- 100 million “• AI - Intruterine insemination via lapraroscopic

surgery- 20 million• Time of insemination – vaginal or cervical = 12 to

18 hrs after onset of estrus• Synchronizing with progesterone sponge should

be 48-58 hrs after removal

Accelerated and Out-of-Season Lambing

• Day length control (natural or artificial)

• Considerations

• Produce lambs when feeding conditions are favorable (growth rate and cost of feed)

• Market lambs when lamb supply is low

• Fertility and prolificacy

Desirable traits for accelerated lambing

• Ewes can breed year round

• Ewes that can mate while lactacting

• Ewes that have a good lambing rate (ie twinning)

• Sires that produce a desirable market lamb and have the libido and fertility for conception year round