Palatability and feeding behaviour in ruminants. A

review

R Baumont

To cite this version:

R Baumont. Palatability and feeding behaviour in ruminants. A review. Annales de zootechnie,

INRA/EDP Sciences, 1996, 45 (5), pp.385-400. <hal-00889572>

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Review article

Palatability and feeding behaviour in ruminants.A review

R Baumont

Station de recherches sur la nutrition des herbivores, Centre de recherche Inrade Clermont-Ferrand-Theix, 63122 Saint-Genès-Champanelle, France

(Received 18 January 1996; accepted 15 May 1996)

Summary &mdash; Palatability usually designates those characteristics of a feed that invoke a sensoryresponse in the animal, and is considered to be the corollary of the animal’s appetite for the feed.When only one feed is given to animals fed indoor, palatability can be evaluated by the eating rate atthe beginning of the meal. When several feeds are studied, preference tests are most often used toassess palatability. Intake measurements are critical since postingestive effects are partly confoundedwith palatability, but experimental procedures allow these two variables to be separated. Behaviouralmeasurements assess motivation for a feed rather than intake. Operant conditioning procedures showhow animals maintain their choice for a preferred feed as it becomes increasingly difficult to obtain. Phys-ical characteristics of the feed (particle size, resistance to fracture, dry matter content, height anddensity of sward, etc) contribute to the sensory response invoked by the animal. They influence easeof prehension and ease of mastication and animals generally prefer the physical form of the feedsthey can eat faster. Taste and odour are recognized as of importance in feed palatability; however, effectsof the primary tastes depend on the experimental procedures used. Nevertheless, there is some evi-dence that sheep will develop a liking for the taste of monosodium glutamate and for the odour ofbutyric acid and a dislike for acetic acid. Most of the palatability studies are short term with time scalesof minutes or hours. In the long term (several days or weeks), feed preferences seem generally to beassociated with digestive modifications. Animals use their senses to learn to associate the postinges-tive effects of the feed with its sensory characteristics. Ruminants generally develop preferences for feedsthat will provide a high satiety level rapidly. Thus, palatability measured as the sensory responseinvoked by the feed integrates its nutritive value. However, for a given nutritive value, sensory propertiesof the feed per se can stimulate or depress hedonic feeding behaviour. The role of hedonic behaviouron intake may be of particular importance in choice situations and for low producing animals. In a firstapproach, hedonic value of the feed can be assessed by the difference between the observed intakeand the predicted intake as affected by the nutritive value.

intake / palatability / feeding behaviour / forage / ruminant

Résumé &mdash; Appétibilité et comportement alimentaire chez les ruminants. L’appétibilité (ou lapalatabilité) désigne les caractéristiques de l’aliment qui provoquent la réaction des sens de l’animal.

Elle correspond au corollaire de l’appétit de l’animal pour l’aliment. Lorsqu’un seul aliment est dispo-nible, l’appétibilité peut être évaluée par la vitesse d’ingestion au début du repas. Lorsque plusieurs ali-ments sont proposés, leur appétibilité est généralement évaluée par des tests de préférence. Mesu-rer la quantité ingérée pour évaluer l’appétibilité est critiquable car alors les effets postingestifs del’aliment sont partiellement confondus avec celle-ci. Toutefois il existe des techniques expérimen-tales qui permettent de séparer les deux phénomènes. L’étude du comportement permet d’évaluer lamotivation pour l’aliment plutôt que son résultat qui est la quantité ingérée. Les procédures de condi-tionnement opérant montrent comment l’animal maintient son choix pour un aliment préféré lorsqu’ildevient de plus en plus difficile à obtenir. Les caractéristiques physiques de l’aliment (taille des parti-cules, résistance à la cassure, teneur en matière sèche, hauteur et densité du couvert végétal...) par-ticipent à la réponse des sens. Elles influencent la facilité de préhension et de mastication et les ani-maux préfèrent généralement la forme physique des aliments qu’ils peuvent ingérer rapidement. Le goûtet l’odeur sont considérés comme des déterminants importants de l’appétibilité. Mais les effets des goûtsprimaires dépendent de la technique expérimentale utilisée. Néanmoins il semble que le mouton aimele goût du monosodium glutamate et l’odeur de l’acide butyrique, mais n’aime pas l’acide acétique. Laplupart des études sur l’appétibilité sont réalisées à court terme et ne portent que sur quelques minutesou quelques heures. À long terme (plusieurs jours ou semaines) les préférences alimentaires semblentgénéralement être associées à des modifications digestives. Les animaux associent par apprentissageles effets postingestifs de l’aliment avec ses caractéristiques sensorielles. Les ruminants dévelop-pent généralement des préférences pour les aliments qui leur permettent d’atteindre rapidement un étatde satiété élevé. Ainsi l’appétibilité mesurée par la réponse des sens provoquée par l’aliment intègresa valeur nutritive. Cependant, pour une valeur nutritive donnée, les propriétés sensorielles de l’alimenten elles-mêmes peuvent stimuler plus ou moins un comportement alimentaire de nature hédonique.Le rôle de ce comportement hédonique sur la quantité ingérée peut être important dans les situa-tions de choix et pour les animaux à faibles niveaux de productions. En première approche, la valeurhédonique de l’aliment peut être assimilée à la différence entre la quantité ingérée observée et celleprévue à partir de sa valeur nutritive.

ingestion lappétibilité l palatabilité / comportement alimentaire / fourrage / ruminants

INTRODUCTION

To be consumed, a feed has first to be rec-

ognized as edible. The role of certain senses(ie, sight, smell, touch and taste) in the feed-ing behaviour of ruminants has been studiedand reviewed by a number of authors(Arnold, 1970; Goatcher and Church, 1970a;Demarquilly, 1978; Church, 1979; Grovum,1988). It is well established that the sensesare used in selective grazing but they areprobably less important when no choice isgiven to a stall-fed animal. Even in such asimple situation, however, it is impossibleto describe voluntary feed intake for a widerange of feeds solely in terms of digestiveand metabolic characteristics (Faverdin et al,1995). Scientists interested in feed intakein ruminants have probably paid less atten-

tion to the role of senses than to physicalor energy control of intake. It is remarkablethat none of the published feed intake pre-diction systems takes into account the sen-sory response to the feed.

The term palatability usually designatesthose characteristics of a feed that invoke a

sensory response in the animal (Greenhalghand Reid, 1971 This review discusses theconcept of palatability for ruminants. Aftera brief presentation of the different defini-tions found in the literature, the methodsused to evaluate palatability are reviewedand discussed. The main dietary factorsinvolved in palatability and some aspectsof the utilization of the senses by the ani-mal are then analysed. Finally, we drawsome conclusions concerning the role ofpalatability in the control of intake.

DEFINITIONS

In accordance with the definition given byGreenhalgh and Reid (1971), Church (1979)defined palatability as the &dquo;dietary charac-teristics or conditions which stimulate aselective response by the animal&dquo;, palata-bility being considered as an inherent char-acteristic of the feed (Hodgson, 1979). ForMatthews (1983), the palatability of a feed isinterchangeable with preference for the feed.It is determined by the taste, smell, appear-ance, temperature and texture of the feed.However, Forbes (1986, 1995a) claims thatpalatability cannot be considered solely asa quality of the feed since it depends on theexperience and metabolic status of the ani-mal in question; palatability of a feed is notabsolute and depends on the state of hungerof the animal (Gallouin and Le Magnen,1987). Palatability of the feed is the corollaryof the appetite of the animal, which is thestimulation to eat aroused by the feed. Eat-ing rate, especially at the beginning of themeal, is a good criterion of the animal’sappetite, and palatability of the feed isdefined as all the physical (plant bearing,spines, etc) and chemical (odour, taste, etc)characteristics of the feed that act on

appetite (Jarrige, 1988). Mertens (1994)concurs with this last definition but does notmention physical characteristics of the plant;thus, it is not clear if the physical charac-teristics that determine ease of prehensionand ease of mastication are components ofpalatability or not. At pasture, ease of har-vesting has a major effect on diet selectionas discussed by Dumont (1997). For ani-mals fed indoors, it is well known that thesame hay in long, chopped or ground formis not eaten at the same rate and in thesame amount (Jarrige et al, 1995). As it isestablished that physical characteristicssuch as particle size and water content con-tribute to the sensory response invoked bythe feed (see later) these are considered inthis review as features of palatability.

EVALUATING PALATABILITY

Palatability is obviously not a quantitativemeasure unless feed intake is measured

per unit time (Church, 1979). An ideal mea-sure of palatability will not be influenced bythe consequences of previous ingestion offeeds (Matthews, 1983) nor by the

postingestive consequences of intake(Grovum and Chapman, 1988). Severalmethods are used to evaluate palatability.They differ according to what intake orbehavioural parameters are recorded andwhether only one or more than one feed isoffered. However, none of these methodscan avoid the effects of prior learning of feedcharacteristics.

Intake measurements

Differences in voluntary intake cannot beattributed only to palatability as they resultfrom the sensory response and the diges-tive, metabolic and hormonal events fol-lowing meals. Recording intake during thefirst minutes following exposure to the feedlimits confusion of palatability with postinges-tive factors. A simple exponential modelaccurately fits intake rate during meals bothin monogastric animals and ruminants(Davis et al, 1978; Faverdin, 1985; Baumontet al, 1989). This enables us to calculateinitial eating rate at the beginning of themeal. In dairy cows fed on the same diet,the initial eating rate reflects the increasein eating motivation or &dquo;appetite&dquo; with theadvance of lactation (Faverdin, 1985). Whensheep have finished a first meal and thesame hay is offered again, initial eating ratesare similar for both meals (fig 1 althoughdigestive and metabolic status are very dif-ferent (Baumont et al, 1990). Initial eatingrate, which can double from one forage toanother, may thus be a good criterion forevaluating the sensory response invokedby a feed and thus its palatability.

Greenhalgh and Reid (1971) developeda method to isolate the effect of palatabil-ity from postingestive factors (fig 2). Con-sidering two different feeds such as strawand dried grass, voluntary intake of each ofthe two feeds is measured with the sameamount of the other feed introduced directlyinto the rumen through a fistula. In this waydiet composition and digestibility are keptconstant, and the difference in voluntaryintake between the two feeds can beattributed to palatability. However, in a laterexperiment comparing chopped and pel-leted hay, it was suggested that the rate ofdigestion of chopped hay is not the same ifit is ingested or introduced through the fistula(Van Niekerk et al, 1973). This techniquemay therefore be unsuitable for measuringthe relative palatability of feeds that differgreatly in physical structure unless inges-tive mastication can be simulated. Another

technique for avoiding postingestive effects

is sham-feeding, in which the ingested feedis diverted from the digestive tract throughan oesophageal fistula. This technique wasused on sheep for palatability trials byGrovum and Chapman (1988).

Evaluating the effect of a given chemi-cal compound on feed palatability through itstaste or flavour means separating the effectof the chemical from the substrate in whichit is incorporated. As no response to watercould be detected in the goat, sheep andcalf (Bell and Kitchell, 1966, quoted byGoatcher and Church, 1970a), water solu-tions were used to study the role of the pri-mary tastes (Goatcher and Church, 1970b,c) and of several flavours (Arnold et al,1980). In addition, the responses of normalanimals to a given taste or odour can becompared with those of animals made agus-tatory and/or anosmic by surgical proce-dures (Arnold, 1966; Michalet-Doreau, 1975;Arnold et al, 1980).

Feeding behaviour is more sensitive tothe different characteristics of feeds in achoice situation. Effects on intake of vari-ous odoriferous compounds vary in ampli-tude and sometimes even in sign accord-ing to whether the animals have a choiceor not (Arnold, 1970). Thus, palatability trialsshould be performed in choice situations.However, the result of a choice test is influ-enced by the duration of the test and by theamounts of feed offered to the animals. Ifunlimited amounts are presented over shorttest durations, animals can show an exclu-sive preference for one single feed (Kertzet al, 1982). Conversely, if limited amountsof feeds are offered over long test durationsall the feeds may be completely ingested(Hutson and Van Mourik, 1981 ). In eitherof these extreme situations intake mea-surements will not give a quantitative eval-uation of the relative palatability of the testedfeeds. Therefore, test procedures in choicesituations have to be carefully defined. Forexample, to test palatability of concentrateson goats, Morand-Fehr et al (1987) defined

the following procedure: i) feeds are testedin pairs; ii) the different combinations (sixfor four feeds to be tested) are set out in alatin square design; iii) each test lasts 30 sand is repeated four times with 200 g ofeach feed offered to the animal.

Colebrook et al (1985) developed a pro-cedure to separate the role of physical char-acteristics that influence the potential intakerate of the feed from the other sensory fac-tors. The prbcedure combines measure-ments of initial intake rate and of preferencefor the tested feed against a standard haycut at different lengths (fig 3). Another inter-esting feature of this procedure is the use ofa standard hay as a reference feed; thepalatability of a given feed is relative to thepalatability of the other feeds simultane-ously presented.

Behavioural measurements

Behavioural measurements allow the eval-uation of the motivation of the animal for

the feed rather than the result of this moti-

vation, which is intake. Two different typesof behaviour can be measured (Matthews,1983): behaviour associated with eatingfreely-accessible feeds and behaviour thatwill gain access to elected feeds.

Under grazing conditions eating time ismuch easier to measure than intake. Graz-

ing time has often been recorded at pas-ture and the differences in grazing timesinterpreted as differences in intake (Arnold,1985). The time spent grazing on differentsward types is considered to reflect the moti-vation for the different swards (Newman etai, 1992). Thus, time spent grazing is oftenused to evaluate preferences at pasture.

Monitoring grazing time allows an analysisof the temporal pattern of preference at pas-ture (Parsons et al, 1994). However, timespent grazing varies not only with the palata-bility of plants but also with the sward struc-ture (height, density, etc).

Operant conditioning procedures aredesigned to study behaviour elicited to gainaccess to feeds (Baldwin, 1978; Matthewsand Kilgour, 1980). In cattle a frequentlyused device consists of a nose-plate press.The animals have to carry out a definedbehavioural sequence to be rewarded byfeed. The relation between the occurrencesof the responses (pushes) in the sequenceand the feed deliveries is called the sched-ule of reinforcement (Matthews, 1983). Com-monly, the schedules of reinforcement arebased on numbers of responses (ratioschedules) or elapsed times (interval sched-ules). For example, in the progressive ratioprocedure the number of responses requiredto be rewarded increases systematically. Ata sufficiently high ratio the animals ceaseto respond. This procedure was used toassess the relative preference values of arange of different feeds when they were pre-sented separately to individual sheep (Hut-son and Van Mourik, 1981 ). Operant con-ditioning procedures are also used toevaluate feed preference under choice con-ditions. One of the advantages of this tech-nique is that the measured behaviours ateach alternative are identical and indepen-dent of the act of eating (Matthews, 1983).Furthermore, by varying the parameters ofthe reinforcement schedule it is possible toavoid exclusive behaviours. Under choiceconditions operant procedures show howanimals maintain their choice for the pre-ferred feed as it becomes increasingly diffi- i-cult to obtain. Dumont and Petit (1995)developed this learning procedure in whichthe animals had a choice between a poorquality forage offered ad libitum and a goodquality one they could obtain in limitedamounts each time they walked back and

forth across the test area (fig 4). The pro-cedure was used to compare sheep andcattle preferences when accessibility andavailability of the preferred forage wereequivalent for the two species. The proce-dure is promising as a means of testing rel-ative palatability of different forages.

THE MAIN FEED COMPONENTSOF PALATABILITY

Physical characteristics

Since the work of Arnold (1966) it has been

recognized that the sense of touch plays arole in the response of the animal to thefeed. The sense of touch is used in selective

grazing to avoid thorny and sticky plants.Physical characteristics of the forage suchas dry matter content, particle size and resis-tance to fracture or height and density ofthe sward at pasture are known to affectease of prehension and thus intake rate(Jarrige et al, 1995). Many measurements ofvoluntary intake and feeding behaviour wereperformed on chopped and ground-dehy-drated forages by Jarrige et al (1973). When

the mean particle size of the foragedecreases voluntary intake increases, eat-ing time decreases and thus intake rateincreases markedly (see also Colebrook etal, 1985, fig 3). However, below a thresh-old of about 0.75 mm for legumes and about0.5 mm for grasses the animal’s response isreversed.

Physical characteristics of the feed actnot only mechanically on ease of prehen-sion, since it was shown in a choice situationthat sheep generally prefer the feeds theycan eat faster. With artificial sward boards,for a given density, the effect of sward heighton relative preference was closely relatedto its effect on intake rate (Black and Ken-ney, 1984). Likewise, for a given height, theeffect of density on relative preference isclosely related to its effect on intake rate.With dried forages, relative preferences formixtures with varying proportions of longand short particles were closely related tothe differences in intake rates (Kenney andBlack, 1984). Discrimination between thedifferent mixtures decreases as intake ratesof the feeds being compared increase.Accordingly, preference for short particlesis more pronounced for a slowly ingested

forage like straw than for rapidly ingestedhay (fig 5).

Preference for forages which can beeaten fast may partly explain the close neg-ative relationship between the energy nec-essary to grind the forage and voluntaryintake (Chenost, 1966). Resistance to grind-ing is related to the cell-wall content andthe percentage of stems in the forage(Chenost and Grenet, 1971). Thus, highresistance to grinding may explain the lowpalatability of straw and the high preferencefor straw in short versus long particles. Rye-grass selected for low leaf shear breakingload tends to be ingested with a higherintake rate (Inoue et al, 1994). Approxi-mately 4% of the energy available to asheep, from grass leaves, is estimated tobe used during chewing (Wright, 1992).

Sheep showed some preference forundried forage over the same forage driedwhen intake was expressed as wet matter(Kenney et al, 1984). However, whenexpressed as dry matter, the proportions oftotal intake coming from the undried andthe dried forage were similar. The influenceof dry matter content on feed preferencesthus seems to be of minor importance com-pared to that of particle size. In the same

study, the forage chopped to a 10 mm lengthwas preferred over the same forage cut to40 mm, irrespective of its dry matter con-tent. These findings are consistent withobservations made by studying the effectof mode of forage conservation. Making haygenerally reduces voluntary dry matterintake, but this reduction seems not to berelated to a drop in palatability since wellconserved hays are ingested at the samerate as the corresponding green forages(Jarrige et al, 1995). In dairy cows dryingthe grass increases voluntary intake whenthe dry matter content of the fresh grass islower than 15% (Vérité and Journet, 1970).In sheep, the lower intake of long choppedgrass silages than of short chopped silagesis related to a markedly lower intake rate.

Chemical characteristics

Factors other than physical characteristicsinfluence palatability. Without choice,legumes are always ingested at a higherrate than grasses, irrespective of particlesize (Jarrige et al, 1973). In a choice situa-tion, sheep develop a preference for lucerneover clover and for wheaten hay overwheaten straw when they are added to the

same basal diet in the ratio of 1 to 10 (Ken-ney and Black, 1984).

The effects of the primary tastes (sweet,salty, bitter, sour) were studied with watersolutions (Goatcher and Church, 1970b, c)and with sham intakes on oesophageal-fis-tulated sheep (Grovum and Chapman,1988). The results varied according to themethod used (table I) particularly for sweetand salty tastes. Grovum and Chapman(1988) argued that the absence of post-ingestive effects (eg, modifications of pHand osmotic pressure) in their study explainsthe difference between their results andthose of the trials performed with water solu-tions. However, Forbes (1995a) pointed outthat oesophageal-fistulated sheep can losesaliva through the fistula and becomesodium-deficient, thereby developing a pref-erence for salty feeds. The specific taste ofmonosodium glutamate is called umami andis reported to be a combination of sweetand salty tastes (Goatcher and Church,1970a). Its positive effect on palatability(table I) was also demonstrated by treatinghay with an aqueous solution ofmonosodium glutamate (Gherardi andBlack, 1991 ).

The effects of various odoriferous com-

pounds were analysed by Arnold (1970) andArnold et al (1980) by sprinkling the chem-

icals onto cotton wool pads placed in themanger. The difficulty was in controlling thelevel of odour being tested. Odoriferouscompounds were also studied with prefer-ence tests for different water solutions. The

difficulty then was to separate the responsethrough smell from the response throughtaste. For these reasons, anosmic and agus-tatory sheep were used in comparison withnormal ones (Arnold et al, 1980). As statedearlier, effects of odoriferous compoundscan vary in amplitude and sometimes in signaccording to whether the animals are inchoice situations or not. Amyl acetate has apositive effect on voluntary intake in a no-choice situation and a negative one in a freechoice situation (Arnold, 1970). Coumarindepresses voluntary intake in a no-choicesituation whereas sheep develop a likingfor coumarin in a free choice situation. Sev-eral results indicate that sheep can developa liking for butyric acid irrespective ofwhether they have a choice or not (Arnold etal, 1980; Gherardi and Black, 1991 How-ever, with water solutions butyric acid orbutyric sodium salt were shown to have neg-ative effects on feed preference as alsowere acetic acid and propionate in acid orsalt form (Goatcher and Church, 1970b, c).The negative effect of acetic acid was con-firmed in a preference test for water solu-tions, but not when tested as an odour con-

taminant (Arnold et al, 1980). The palata-bility of hay was decreased by treating it

with an aqueous solution of acetic acid

(Gherardi and Black, 1991) and the sham-intake of lucerne silage was linearlydecreased with increasing the addition ofacetic acid (Buchanan-Smith, 1990).

The low intake of silage is often attributedto a lack of palatability since digestibility isonly slightly different from that of the corre-sponding green forage (Demarquilly, 1978).Effects of smell and taste on silage intakewere studied with anosmic and agustatorysheep (Michalet-Doreau, 1975). The resultsmust be interpreted with caution since anos-mic sheep eat more than normal sheep(Arnold et al, 1980). Nevertheless, theincrease in silage intake by anosmic sheepcompared to normal animals was more pro-nounced with badly preserved silages(+33%) than with well preserved ones(+6.4%). However, silage intake by agus-tatory sheep was not modified. With dairycows, flavouring agents will increase grasssilage intake by about 8% over an 8 weekperiod (Weller and Phipps, 1989). The studyon sham-fed animals of the role of severalconstituents commonly found in silage juicegave unexpected results (Buchanan-Smith,1990). Only acetic acid added alone had aclear negative effect on sham intakeswhereas lactic and acetic acids togetherenhanced silage intake while acetic andbutyric acids together and also ammoniahad no effect on intake. A mixture of freeamino acids decreased intake only at thehighest concentration. A mixture of twoamines and gamma-amino butyric acidincreased intake at intermediate concen-trations. However, amines alone are sus-

pected to decrease palatability as in sheepinitial eating rate at the beginning of themeal was depressed by addition of aminesin the silage (Van Os et al, 1995). Theeffects of artificial flavours or of sweeten-ers seem to depend on the basal palatabil-ity of the treated forage. Sweeteners such as

molasses caused manure-affected plantsto be grazed (Marten and Donker, 1964).In contrast, the addition of molasses to nor-

mally palatable pelleted hay increases intakeonly slightly and for a short time (Kare, 1959,quoted by Demarquilly, 1978).

Most of the studies in which feeds aremodified are short-term studies with a timescale of minutes or hours. Arnold et al

(1980) added to pelleted hay small quantitiesof several compounds recognized to

decrease intake by their odour or todecrease preference for a water solution bytheir taste (table II). Over a 3 day period,significant depressions in intake wereobtained with coumarin, gramine, tannicacid, malonic acid and glycine. However,sheep that were both anosmic and agusta-tory were affected in the same way as nor-mal sheep. In vitro digestibility of the pel-leted hay was drastically depressed bytannic acid and gramine, and slightly bycoumarin and glycine, but was unaffectedby the other contaminants. In short-termpreference studies, palatability of wheatenhay could be increased by adding a com-bination of butyric acid plus monosodiumglutamate or decreased by magnesiumoxide (Gherardi and Black, 1991). In a long-term study over 25 days (Gherardi et al,1991 the voluntary intake of hay treatedwith butyric acid plus monosodium gluta-mate was 10% higher than that of untreatedhay. This increase in intake was associatedwith an increase in the apparent fractionalrate of digestion in the rumen. The high pref-erence for the treated hay was maintainedover 25 days. Voluntary intake of hay treatedwith magnesium oxide was not decreasedover 25 days, even though ruminal diges-tion was depressed and thus compensatedfor by an increase in rumen fill. In prefer-ence tests, however, aversion to hay treatedwith magnesium oxide was maintained.Long-term effects on feed preferences oron voluntary intake seem to be generally

associated with modification of digestiveparameters.

UTILIZATION OF THE SENSESIN FEEDING BEHAVIOUR

As judged from the long-term effects of mod-ifications of feed palatability, animals seemto be able to associate the sensory charac-teristics of a given feed with its nutritionalconsequences after feeding.

Learning the postingestive effectsof feeds

The senses that are stimulated in the pres-ence of feed enable the animal to anticipatethe postingestive effects of feed. This effecthas been extensively reviewed in rats byLe Magnen (1985). Provenza et al (1992)proposed a schematic representation of theprocesses involved in the learning of feedpreferences. The affective system integratesthe taste of a feed with postingestive feed-back and the cognitive system integrates

the odour and sight of the feed with its taste.Learned food aversions against toxic plantsor feeds experimentally laced with severalcompounds that cause malaise have beenclearly established in ruminants and in othermammals (Burrit and Provenza, 1989; duToit et al, 1991; Ralphs and Cheney, 1993).Learned preferences based on positive nutri-tional postingestive feedback are undoubt-edly also important for ruminants. Lambs,after 10 days adaptation, developed a strongpreference for non-nutritive flavours pairedwith glucose over the same flavours pairedwith saccharin (Burrit and Provenza, 1992;table 111). Thus, ruminants, like other mam-mals, develop preferences for feeds thatprovide more energy (Provenza, 1995). Insheep fed half orally and half intraruminallyoral consumption of straw was doubledwhen digestibility was increased by dosinggrass in the rumen instead of straw (fig 2). Incontrast, oral consumption of grassdecreased when straw was dosed in the

rumen instead of grass. These results,together with those stated earlier (Arnold etal, 1980; Gherardi et al, 1991), must be inter-preted as the consequences of learning thenutritive value of the diet. However in a free

choice situation, diet selection will not alwaysmaximize energy density of the diet. Sheepeat some straw to prevent rumen disorderseven though a more concentrated feed isalso on offer (Cooper et al, 1995). This isconsistent with the observation that sheepmaintain 10% of chopped forage in their dietwhen offered a free choice of the same for-

age chopped or pelleted (Greenhalgh andReid, 1974). Moreover, dietary experience inearly life was shown to have a significanteffect on consumption of low-qualityroughage and diet selection in free choicesituations (Distel et al, 1994). Processes oflearning from the mother, from conspecificsand through trial and error have beenreviewed (Provenza et al, 1992; Provenza,1995).

Animals use the senses of smell and

sight to detect subtle differences in feedsand select or avoid specific feed items. Theanticipation role of the senses may explainwhy in cows a barley supply significantlydecreased subsequent hay intake when fedorally, but not when added to the rumen(Baumont et al, 1994, table IV). The antici-patory role of the senses may also explainwhy anosmic sheep eat more than normalsheep (Arnold et al, 1980). These observa-tions are consistent with the finding that, forrats, olfactory cues are not only involved inpalatability but also in the control of energyintake (Larue and Le Magnen, 1972). Antic-ipation by the senses of postingestive effects

explains to a certain extent the diurnal pat-tern of preference (Parsons et al, 1994).

Hedonic behaviour

Intake is primarily a behaviour influencedby hunger, which is distressing, and by sati-ety, which is generally pleasurable (Read,1992). Recently, Forbes (1995b) postulatedthat &dquo;ruminants eat that amount of foodwhich leaves them with the most comfort-able feelings&dquo;. Qualitative, quantitative andaffective components can be discerned inthe complex sensations perceived in

humans during intake (Fantino, 1992). Thefirst two components afford recognition ofthe nature and amounts of feed ingested.The third component, the so-called hedo-nic perception, is related to pleasant orunpleasant feelings evoked by the feed.Hence, mechanisms of brain reward can tosome extent induce hedonic feedingbehaviour in competition with physiologicalfactors controlling intake. The palatability ofthe feed will stimulate hedonic behaviour toa greater or lesser extent. Total intake ofsheep was only 406 g/day when they atestraw and received grass in the rumen butrose to 901 g/day in the reverse situation,although digestibility of the total diet wassimilar (Greenhalgh and Reid, 1971; fig 2).Unpleasant feelings when eating straw mayexplain its very low hedonic value. With good

quality forages fed to wethers, observedvoluntary intakes greatly in excess ofrequirement can be explained by hedonicbehaviour (Baumont et al, 1989). The sen-sory response induced by a second distri-bution of fresh hay will override the satietysignals induced by the meal following thefirst distribution (Gatel, 1984). However, thesize of the second meal depends on the rel-ative palatability of the two hays distributed(Baumont et al, 1990). Sheep satiated withlow-quality meadow hay will eat 400 g oflucerne hay (fig 1 however, they are reluc-tant to eat meadow hay when satiated withlucerne.

Hedonic behaviour also competes withthe effort that has to be expended to earnthe reward. In a test situation, when ani-mals have to walk to obtain a good forage,the preference for the good forage dependson the amount offered the animals in reward

(Dumont and Petit, 1995).

CONCLUSION: PALATABILITYAND CONTROL OF INTAKE

The sensory response invoked by a feed isexpressed by the intake rate when no choiceis offered to the animal and by the feed pref-

erences in choice situation. It integrates thepostingestive effects that the animal haslearned to associate with its sensory prop-erties and it interacts with the nutrient

requirements of the animal. Meal size anddiet composition are mainly controlled byanticipation of postingestive effects to avoidnutritional excesses or deficiencies. Rumi-nants generally develop preferences forfeeds that will provide a high satiety levelrapidly. Feeds that can be ingested fast andthat are rapidly and highly digestible arevery palatable provided they do not containtoxic compounds. Nevertheless, for a givennutritive value, sensory properties of thefeed per se can stimulate or depress hedo-nic behaviour and thus intake. Indeed, resid-ual variation in predictive models of voluntaryintake based on nutritional characteristics

(ie, energy and nitrogen values, fill effect)remains generally high. Thus, palatabilitymeasured as the sensory response invoked

by a feed combines its nutritive and hedonicvalues (fig 6). Hedonic behaviour is proba-bly more important in low-producing ani-mals than in high-producing ones. More-over, the effect of hedonic behaviour onintake is probably more important whenchoice is offered to the animals. In a first

approach, the hedonic value of the feed canbe assessed by the difference between the

observed and the predicted intake asaffected by the nutritional characteristics.Several questions remain unanswered. Isit possible to increase intake in the long termby improving hedonic perception of the feedwithout also increasing the nutritive value?What exactly are the physical characteristicsand chemical compounds that are used byruminants to associate the sensory proper-ties of feed with its nutritive value? Modern

analytical methods to characterize textureand aromatic compounds in feeds may helpin gaining a better understanding of the com-ponents determining palatability.

REFERENCES

Arnold GW (1966) The special senses in grazing ani-mals. 2. Smell, taste and touch and dietary habitsin sheep. Aust J Agric Res 17, 531-542

Arnold GW (1970) Regulation of food intake in grazingruminants. In: Physiology of Digestion andMetabolism in Ruminants (AT Phillipson, ed), OrielPress, 264-276

Arnold GW (1985) Ingestive behaviour. In: World Ani-mal Science. 5. Ethology of Farm Animals (ANeimann-Sorensen, DE Tribe, eds), Elsevier Sci-ence, 183-198

Arnold GW, De Boer G, Boundy (1980) The influencesof odour and taste on food preferences and foodintake of sheep. Aust JAgric Res 31, 571-585

Baldwin BA (1978) Operant studies and preference teston the role of olfaction and taste in digestivebehaviour of pigs and ruminants. In: First Interna-tional Symposium on Palatability and Flavor Use inAnimal Feeds, Zurich, Switzerland, 10-11 October1978 (Institute of Animal Production, Swiss FederalInstitute of Technology), 43-49

Baumont R, Brun JP, Dulphy JP (1989) Influence of thenature of hay on its ingestibility and the kinetics ofintake during large meals in sheep and cow. In: XVlthInternational Grassland Congress, Nice, France (2)(R Jarrige, ed), French Grassland Society, 787-788

Baumont R, Seguier N, Dulphy JP (1990) Rumen fill,forage palatability and alimentary behaviour in sheep.J Agric Sci 115, 277-284

Baumont R, Daveau 0, Perpbre C (1994) A trial to quan-tify the roles of ruminal and oropharyngeal signals inthe control of food intake by cows. In: Proceedings ofthe Society of Nutrition Physiology. 8e International

Symposium on Ruminant Physiology, Willingen, Ger-many, 25-30 September 1994 (D Giesecke, ed),DLG-Verlag, 122

Black JL, Kenney PA (1984) Factors affecting diet selec-tion by sheep. 2. Height and density of pasture. AustJ Agric Res 35, 565-578

Buchanan-Smith JG (1990) An investigation into palata-bility as a factor responsible for reduced intake ofsilage by sheep. Anim Prod 50, 253-260

Burrit EA, Provenza FD (1989) Food aversion learning:ability of lambs to distinguish safe from harmful foods.J Anim Sci 67, 1732-1739

Burrit EA, Provenza FD (1992) Lambs form preferencesfor non-nutritive flavors paired with glucose. JAnimSci 70, 1133-1136

Chenost M (1966) L’indice de fibrosit6 des foins : mesureet relations avec la valeur alimentaire. Ann Zootech

15, 253-257

Chenost M, Grenet E (1971) L’indice de fibrosit6 desfourrages : sa signification et son utilisation pour la

prevision de la valeur alimentaire des fourrages. AnnZootech 20, 427-435

Church DC (1979) Taste, appetite and regulation ofenergy balance and control of food intake. I. Appetite,taste and palatability. In: Digestive Physiology andNutrition of Ruminants (DC Church, ed), OxfordPress, 281-290

Colebrook WF, Black JL, Kenney PA (1985) Effect ofsensory factors on diet selection by sheep. Proc NutrSoc (Austr) 10, 99-102

Cooper SDB, Kyriasakis I, Nolan JV (1995) Diet selec-tion in sheep: the role of the rumen environment inthe selection of a diet from two feeds that differ intheir energy density. Br J Nutr74, 39-54

Davis JD, Collins BJ, Levine MW (1978) The interac-tion between gustatory stimulation and gut feedbackin the control of the ingestion of liquid diets. In:

Hunger Models (DA Booth, ed), Academic Press,109-143

Demarquilly C (1978) Intake, palatability and flavor inruminant feeds. In: First International Symposiumon Palatability and Flavor Use in Animal Feeds,Zurich, Switzerland, 10-11 October 1978 (Institute ofAnimal Production, Swiss Federal Institute of Tech-nology), 91-98

Distel RA, Villalba JJ, Laborde HE (1994) Effects of earlyexperience on voluntary intake of low-qualityroughage by sheep. JAnim Sci72, 1191-1195

du Toit JT, Provenza FD, Nastis AS (1991) Conditionedfood aversions: How sick must a ruminant get beforeit detects toxicity in foods? Appl Anim Behav Sci 30,35-46

Dumont B (1997) Diet preference of herbivores on pas-ture. Ann Zootech (in press)

Dumont B, Petit M (1995) An indoor method for studyingthe preferences of sheep and cattle at pasture. ApplAnim Behav Sci 46, 67-80

Fantino M (1992) ftat nutritionnel et perception affectivede I’aliment. In : Plaisir et preferences alimentaires(I Giachetti, ed), Polytechnica, 31-45

Faverdin P (1985) Regulation de l’ingestion des vacheslaitibres en debut de lactation. These de doctorat

Ina-PG, France, 131 pFaverdin P, Baumont R, Ingvartsen KL (1995) Control

and prediction of feed intake in ruminants. In: RecentDevelopments in the Nutrition of Herbivores. Pro-ceedings of the IVth International Symposium on theNutrition of Herbivores. Clermont-Ferrand, France,1995 (M Journet, E Grenet, MH Farce, M Th6riez, CDemarquilly, eds), INRA Editions, 95-120

Forbes JM (1986) Dietary factors affecting intake. In:The Voluntary Intake of Farm Animals. Butterworths,London, UK, 86-113 3

Forbes JM (1995b) Voluntary Food Intake and DietSelection in Farm Animals. CAB International, 532 p

Forbes JM (1995a) Physical limitation of feed intake inruminants and its interactions with other factors affect-

ing intake. In: Ruminant Physiology: Digestion,Metabolism, Growth and Reproduction. Proceed-ings of the Eighth International Symposium on Rumi-nant Physiology (WV Engelhardt, S Leonhard-Marek,G Breves, D Giesecke, eds), Ferdinand Enke Verlag,217-232

Gallouin F, Le Magnen J (1987) Evolution historiquedes concepts de faim, satiété et app6tit. Reprod NutrDev27, 109-128

Gatel F (1984) Signification de la satiete a court termechez le mouton : influence de la qualit6 du fourrageet des stimuli associes a la prise de nourriture. AnnZootech 33, 111-118 8

Gherardi SG, Black JL (1991) Effect of palatability onvoluntary feed intake by sheep. 1. Identification ofchemicals that alter the palatability of a forage. AustJ Agric Res 42, 571-584

Gherardi SG, Black JL, Colebrook WF (1991) Effect ofpalatability on voluntary feed intake by sheep. 2.The effect of altering the palatability of a wheatenhay on long-term intake and preference. AustJAgricRes 42, 585-598

Goatcher WD, Church DC (1970a) Review of some nutri-tional aspects of the sense of taste. J Anim Sci 31,973-981

Goatcher WD, Church DC (1970b) Taste responses inruminants. 1. Reactions of sheep to sugars, sac-charin, ethanol and salts. J Anim Sci 30, 777-783

Goatcher WD, Church DC (1970c) Taste responses inruminants. 2. Reactions of sheep to acids, quinine,urea and sodium hydroxide. J Anim Sci 30, 784-790

Greenhalgh JFD, Reid GW (1971) Relative palatability tosheep of straw, hay and dried grass. Br J Nutr 26,107-116 6

Greenhalgh JFD, Reid GW (1974) Long- and short-termeffects on intake of pelleting a roughage for sheep.Anim Prod 19, 77-86

Grovum WL (1988) Appetite, palatability and control offeed intake. In: The Ruminant Animal, DigestivePhysiology and Nutrition (DC Church, ed), PrenticeHall, 202-216 6

Grovum WL, Chapman (1988) Factors affecting the vol-untary intake of food by sheep. 4. The effect of addi-tives representing the primary tastes on sham intakesby oesophageal-fistulated sheep. BrJ Nutr59, 63-72

Hodgson J (1979) Nomenclature and definitions in graz-ing studies. Grass Forage Sci 34, 11-18 8

Hutson GD, Van Mourik SC (1981) Food preferencesof sheep. Aust J Exp Agric Anim Husb 21, 575-582

Inoue T, Brookes IM, John A, Kolver ES, Barry TN (1994)Effects of leaf shear breaking load on the feedingvalue of perennial ryegrass (Lolium perenne) forsheep. 2. Effects on feed intake, particle breakdown,rumen digesta outflow and animal performance. JAgric Sci 123, 137-147

Jarrige R, Demarquilly C, Journet M, Beranger C (1973)The nutritive value of processed dehydrated forageswith special reference to the influence of physicalform and size particle. In: Proceedings of the FirstInternational Green Crop Drying Congress. E & E

Plumridge, 99-118 8

Jarrige R (1988) Alimentation des bovins, ovins etcaprins. INRA Publications, 471 p

Jarrige R, Dulphy JP, Faverdin P, Baumont R, Demar-quilly C (1995) Activit6s d’ingestion et de rumina-tion. In: Nutrition des Ruminants Domestiques (RJarrige, Y Ruckebusch, C Demarquilly, MH Farce, MJournet, eds), INRA, 123-181

Kenney PA, Black JL (1984) Factors affecting diet selec-tion by sheep. I. Potential intake rate and accept-ability of feed. Aust J Agric Res 35, 551-563

Kenney PA, Black JL, Colebrook WF (1984) Factorsaffecting diet selection by sheep. 3. Dry matter con-tent and particle length of forage. Aust J Agric Res35, 831-838

Kertz AF, Koepke MK, Davidson LE, Betz NL, NorrisJR, Skoch LV, Cords BR, Hopkins DT (1982) Factorsinfluencing intake of high urea-containing rations bylactating dairy cows. J Dairy Sci 65, 587-604

Larue C, Le Magnen J (1972) The olfactory control ofmeal pattern in rats. Physiol Behav9, 817-821 1

Le Magnen J (1985) Hunger. Problems in theBehavioural Sciences. Cambridge University Press,151 p

Marten GC, Donker JD (1964) Selective grazing inducedby animal excreta. I. Evidence of occurrence and

superficial remedy. J Dairy Sci 47, 773-776

Matthews LR (1983) General introduction. In: Mea-surement and scaling of food preferences in dairycows: concurrent schedule and free-access tech-

niques. PhD Thesis, University of Waikato, NewZealand, 236 p

Matthews LR, Kilgour R (1980) Learning and associ-ated factors in ruminant feeding behaviour. In: Diges-tive Physiology and Metabolism in Ruminants (YRuckebusch, P Thivend, eds), MTP, 123-144

Mertens DR (1994) Regulation of forage intake. In: For-age Quality, Evaluation and Utilization (GC Fahey,ed), 450-493

Michalet-Doreau B (1975) Recherches sur les causesdes variations des quantités d’ensilage d’herbe

ing6rbes par les ruminants. These Docteur-Ing6nieurUniversite de Nancy, Nancy, France, 99 p

Morand-Fehr P, Hervieu J, Legendre D, Gutter A, DelTedesco L (1987) Rapid tests to assess concentratefeed acceptability by goats. Ann Zootech 36, 324

Newman JA, Parsons AJ, Harvey A (1992) Not all sheepprefer clover: diet selection revisited. J Agric Sci 119,275-283

Parsons AJ, Newman JA, Penning PD, Harvey A, Orr RJ(1994) Diet preference of sheep: effect of recentdiet, physiological state and species abundance.J Anim Ecol 63, 465-478

Provenza FD, Pfister JA, Cheney CD (1992) Mecha-nisms of learning in diet selection with reference tophytotoxicosis in herbivores. J Range Manage 45,36-45

Provenza FD (1995) Role of learning in food prefer-ences of ruminants: Greenhalgh and Reid revisited.In: Ruminant Physiology: Digestion, Metabolism,Growth and Reproduction. Proceedings of the EighthInternational Symposium on Ruminant Physiology(WV Engelhardt, S Leonhard-Marek, G Breves, DGiesecke, eds), Ferdinand Enke Verlag, 233-247

Ralphs MH, Cheney CD (1993) Influence of cattle age,lithium chloride dose level, and food type in the reten-tion of food aversions. J Anim Sci 71, 373-379

Read NW (1992) Role of gastrointestinal factors inhunger and satiety in man. Proc Nutr Soc 51, 7-11 1

Van Niekerk Al, Greenhalgh JFD, Reid GW (1973)Importance of palatability in determining the feedintake of sheep offered chopped and pelleted hay. BrJ Nutr 30, 95-105

Van Os M, Dulphy JP, Baumont R (1995) The effect ofprotein degradation products in grass silages onfeed intake and intake behaviour in sheep. Br J Nutr73, 51-64

Vérité R, Journet M (1970) Influence de la teneur eneau et de la dbshydratation de I’herbe sur sa valeuralimentaire pour les vaches laiti!res. Ann Zootech 19,255-268

Weller RF, Phipps RH (1989) Preliminary studies on theeffect of flavouring agents on the dry-matter intake ofsilage by lactating dairy cows. J Agric Sci 112, 67-71

Wright W (1992) The Fracture Properties of Grassesand their Relevance to Feeding in Herbivores. Uni- i-

versity of Reading, 174 p