design and methodology of choice feeding experiments with ruminant livestock

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Applied Animal Behaviour Science 140 (2012) 105– 120

Contents lists available at SciVerse ScienceDirect

Applied Animal Behaviour Science

jou rna l h om epa ge: www.elsev ier .com/ locate /applan im

esign and methodology of choice feeding experiments with ruminantivestock

anina Sarah Meier, Michael Kreuzer, Svenja Marquardt ∗

TH Zurich, Institute of Agricultural Sciences, Universitaetstrasse 2, 8092 Zurich, Switzerland

r t i c l e i n f o

rticle history:ccepted 5 April 2012vailable online 2 May 2012

eywords:hoicereferencealatabilityntakeeeduminant

a b s t r a c t

The purpose of this review is to describe design and methodology of choice feeding exper-iments applied in ruminant livestock by taking into account their potential for differentpurposes and animal-, feed-, and set-up specific aspects. The first choice feeding experi-ments took place around 1900 with rats, but they were rarely conducted with ruminantsuntil the 1950s. Choice feeding experiments can be used to assess the willingness of ani-mals to ingest certain experimental feeds in a choice situation, i.e., when offering differentfeeds separately at the same time, thus determining their preference for, or the palatabil-ity of, different feeds. Choice feeding experiments help to improve our understanding ofthe feeding behaviour of animals, and may allow adjustment of management strategiesto cover the nutritional requirements of the individual animal. Another application is forinstance to test the animal’s capability to cover their nutritional requirements, to avoiddetrimental effects or to perform self-medication (i.e., to specifically select and ingest sub-stances suitable to improve health and well-being). Choice feeding experiments can beconducted indoors or on pasture. The main target items to be determined in choice feedingexperiments are proportionate intake, intake rate or feeding behaviour. Assessing the lat-ter requires additional methods such as direct observations, automated recording systemsor video recordings in order to assess for instance time spent feeding and switching fre-quency between the different experimental feeds. Sometimes secondary response variables(performance, milk or meat quality, metabolic profile, greenhouse gas emissions) are alsorecorded in choice feeding experiments with ruminant livestock. In this review importantaspects and factors of influence to be considered in designing choice feeding experimentsare discussed. Factors of influence are related to the animal itself, such as previous experi-ence with the experimental feeds, adaptation processes, social influences or physiological
stage. Feed-specific aspects include the form and number of experimental feeds offered inchoice situations. Appropriate time schedules as well as advantages and disadvantages ofshort- or long-term choice feeding experiments are discussed. An important limitation ofsuch experiments is that the results are only valid for the specific situation tested and any expectations for other situations have to be developed with care.
© 2012 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Definition, implementation and intentions of choice feeding expe3. Design of choice feeding experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +41 44 632 2242; fax: +41 44 632 1128.E-mail address: [email protected] (S. Marquardt).

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106riments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

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106 J.S. Meier et al. / Applied Animal Behaviour Science 140 (2012) 105– 120

3.1. Indoor experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1083.2. Experiments on pasture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093.3. Target ruminant species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4. Methodology and measurements applied in choice feeding experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1104.1. Measurement of intake of individual feeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1104.2. Measurement of feeding behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1104.3. Measurement of secondary response variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5. Consideration on aspects important for successfully carrying out choice feeding experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.1. Animal-specific aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5.1.1. Importance of animal species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.1.2. Physiological stage of the animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.1.3. Number of animal replicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.1.4. Adaptation of the animals and consideration of previous experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.1.5. Assessments made in individually kept animals vs. those kept in groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.2. Feed-specific aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155.2.1. Number of feeds simultaneously tested . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155.2.2. Characteristics of experimental feeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

5.3. Set-up specific aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.3.1. Duration and time of exposure to the experimental feeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.3.2. Length of choice feeding periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116. . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction

Because of various motivations, animals choose amongfeeds when they get the opportunity to do so. A basicprinciple of feed selection is described by the ‘optimal for-aging theory’ (Emlen, 1968; MacArthur and Pianka, 1966)stating that animals aim to maximise energy intake in adefined period of time (Emlen, 1968). However, this prin-ciple may not be limited to energy but might also concernspecific nutrients such as protein and minerals (Belovsky,1978; Ceacero et al., 2010; Pyke et al., 1977). Ruminants areknown to be able to develop recognition mechanisms forfavourable or adverse feed constituents (Provenza, 1995),or to associate them with a characteristic taste, odour(Arnold et al., 1980) or flavour (Provenza et al., 1996),thus seemingly express ‘nutritional wisdom’ (Duncan andGordon, 1999; Provenza and Balph, 1990). Accordingly,ruminants seem to be capable of selecting diets which meettheir requirements (Görgülü et al., 1996), help to maintaina favourable rumen environment (Cooper et al., 1995) or toalleviate illness (Villalba and Provenza, 2007; Villalba et al.,2010a).

Ruminants grazing and browsing in natural habitatsselect a mixed diet due to several possible reasons (Duncanet al., 2003). This was shown in studies either conductedunder controlled conditions in barns (e.g., Fisher et al.,1999; Ginane et al., 2002a) or outdoors on pasture (e.g.,Parsons et al., 1994; Rutter, 2006). The diets ingestedwere more diverse and even ‘suboptimal’ in comparisonto what would have been expected when exclusively con-sidering the implications of the optimal foraging theory(Parsons et al., 1994; Prache and Damasceno, 2006). There-fore, the current intensive feeding practices, characterisedby a trend to increasingly implement diets with few sepa-
rate components, often offered in an inseparable mixture(total mixed ration) or even as only one component (forinstance a ryegrass ley), are standing in contrast to theassumed animals’ capabilities listed above (Villalba et al.,
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

2010b). Such homogenous and monotonous diet typeshave been proved to be very successful under controlledconditions and in high yielding livestock, as they are labourand cost efficient. However, with regard to animal welfareissues total mixed rations are not ideal, especially when theration is of inferior feed quality. Monotonous diets (totalmixed rations or monoculturous pastures) carry the risk ofnutrient imbalances followed by a higher risk of diseasesand mortality (Lynch et al., 1992). Lynch et al. (1992) forinstance stated that “The failure to provide an adequateration and to distribute it sufficiently to all individualsremains the greatest welfare problem of penned sheep.”Recently, some authors draw special attention on the linkbetween feed choice and animal welfare (see Mantecaet al., 2008). How many nutrients a distinct animal needsdepends on nutrient interactions within feeds, on feedproperties and on the physiology of the animal. Malnu-trition, i.e. under- or oversupply of nutrients, negativelyaffects animal welfare. Thus the availability of alterna-tives offers the individual animal the possibility to choosewhat fits best and to reduce stress resulting from inap-propriate rations (Manteca et al., 2008). Recently someresearchers promoted the idea of considering the situationin natural environments and consequently claim that offer-ing more diverse forages while giving individual animalsthe opportunity to choose is the best way to meet theirneeds (Provenza et al., 2007; Villalba et al., 2010b). Still,more experimental research on diet selection behaviour ofherbivores and factors of influence is needed (for a gen-eral overview see e.g., Duncan et al., 2003; Provenza et al.,1998).

Choice tests (i.e. choice feeding experiments) providea possibility to study factors of influence on the animal’sdecision of what to eat – and how much – under conditions
being widely controlled. Choice tests have been frequentlyused in ecological research (e.g., Raffa et al., 2002), butfewer have been used to obtain new insights into aspectsof ruminant nutrition.

Behaviour Science 140 (2012) 105– 120 107

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Table 1Choice feeding experiments with ruminants published until 1980.

Authors Experimentalanimals

Objectives

Nevens (1927) Dairy cows Ability to choose feed allowingto maintain supply withnutrients in the sameproportions as had been thecase in the previous diet

Gordon andTribe (1951)

Ewes Ability to select a diet whichallows ewes to bear and rearstrong, healthy lambs

Dent et al.(1956)

Calves Potential to counterbalancemineral deficiencies byoffering different mineralpreparations in free choice

Perry et al.(1956)

Fattening steers Comparison of performancewhen given the opportunity toselect components individuallyvs. offering a complete mixture

Olson et al.(1962)

Dairy cows Comparison of restricted vs. adlibitum access to roughage

Goatcher andChurch(1969)

Lambs Preference and nutrient intakewhen self-selection of dietarycomponents is allowed

Glimp (1971) Lambs Effect of energy level of thefeed on diet preference

Coppock et al.(1972)

Dairy cows Free-choice consumption ofcalcium-phosphorussupplements affected by theration and relationshipbetween requirements forcalcium and phosphorus andthe consumption of thesupplements

Pamp et al.(1975)

Lambs Ability to select minerals notprovided in adequate amountsin the ration

Rusterholz andTurner(1978)

Roe-deer Ability to distinguish betweendifferent concentrations ofnutritive substances in a feed

Suzuki et al. Steers Influence of feeds different in

J.S. Meier et al. / Applied Animal

The intention of the present review is to describe andiscuss various designs and techniques applied in choiceeeding experiments with ruminant livestock in barn orutdoors. Factors potentially influencing the outcome ofhoice feeding experiments are discussed in detail.

. Definition, implementation and intentions ofhoice feeding experiments

In ‘choice experiments’ or ‘choice feeding experiments’ith ruminant livestock two or more feeds are offered sep-

rately at the same time, and the proportions consumednd preferences for the feeds on offer are measured. Asreference and palatability are usually the target variables,his type of experiment is named either ‘preference test’Aderibigbe et al., 1982; Alonso-Díaz et al., 2009; Dumontnd Petit, 1995; Duncan and Young, 2002; Ginane et al.,005; Kyriazakis et al., 1998; Parsons et al., 1994; Simitzist al., 2008b; Villalba and Provenza, 1999), or ‘preferencetudy’ (Rogosic et al., 2006), ‘preference trial’ (Cataneset al., 2010; Clauss et al., 2003; Fisher et al., 1999), ‘palata-ility experiment’ (Scharenberg et al., 2007), ‘cafeteriaxperiment’ (Alonso-Díaz et al., 2008), ‘cafeteria trial’ (Deosa et al., 2002; Degen et al., 2010), ‘cafeteria method’Kalio et al., 2006), ‘cafeteria technique’ (Larbi et al., 1993),cafeteria feeding’ (Dikmen et al., 2009; Keskin et al., 2004)nd ‘cafeteria study’ (Sandoval-Castro et al., 2005). Severaltudies cited in the present review do not use one of theseerms for their choice feeding experiments. Because thexperimental designs fit to the definition given above, thesetudies were included in this review as well.

The term ‘cafeteria’ describes a situation where thehoice is given between several different alternatives sim-lar to a real cafeteria (or canteen) where there is choiceetween different dishes. Ben Salem et al. (1994) defined aafeteria-type experiment as an experiment with a “briefxposure of animals to several plant species”. Palatabil-ty plays an important role in preference and selectionreviewed by Baumont, 1996). Marten (1978) offers aefinition for ‘relative forage palatability’ as “plant char-cteristic(s) eliciting a proportional choice among two orore forages conditioned by plant, animal and environ-ental factors which stimulate a selective intake response

y the animal (. . .)”. Palatability includes acceptabilitytaste, flavour, structure, etc.), but not obligatorily desir-bility (Molyneux and Ralphs, 1992). Preference is definedy Hodgson (1979) as the animal’s discrimination betweenifferent feeds on offer and describes what the animalelects when having full option to do so without beingonstrained by external factors (Parsons et al., 1994).ccordingly, selection is a function of preference, but is

imited or changed by the possibilities and constraints pro-ided by the environmental context (Hodgson, 1979), suchs availability and accessiblity of the feeds. Thus diet selec-ion studies carried out in natural environments, where thenimals have the opportunity to select their diets from aeterogeneous mixture of feeds, were not considered as
hey do not register preference (see Rutter, 2006).
For the present review the term ‘choice feeding exper-ment’ is exclusively used. Only studies with ruminantivestock where different feeds were offered separately in a

(1980) physical form on feedingbehaviour

choice situation under controlled conditions were consid-ered. The term ‘experimental feed’ is here used for the testfeeds or forages used in indoor choice feeding experiments(in barns) as well as for different cultivars or sward typesoffered under controlled outdoor conditions on pasture.

Historically, choice feeding approaches have been per-formed since around 1900. These were performed inlaboratories with rats and there are no scientific publica-tions with ruminants known to the authors by that time(Table 1). Initially, there was a very low appreciation of theanimal’s ability to choose an appropriate diet. For instanceJordan (1901) stated in ‘The Rural Science Series’ in relationto the feeding of animals that it is “much safer to trust anagricultural chemist to select a ration than any cow evergrown”. This perception was disproved in the followingyears by the first choice feeding experiments. As one of thefirst, Evvard (1915) showed that it is more efficient to givepigs free choice among feeds than to offer them a mixed
ration. Osborne and Mendel (1918) found that rats alwayschoose the superior diet out of two diets when given thechoice. The term ‘cafeteria’ seems to go back to Richteret al. (1938) as this is the publication referred to when

Behavio
108 J.S. Meier et al. / Applied Animal
describing similar experimental designs (e.g., Chafetz et al.,1989). In fact, Richter et al. (1938) offered a choice of dif-ferent purified diets to rats and gave them the possibility todesign their diet on their own, although they never used theterm ‘cafeteria experiment’. One of the first choice feedingexperiments described with ruminants (here: dairy cows)is the Bachelor Thesis of Holler (1922; quoted by Nevens,1927). Still such experiments remained the exception untilthe 1950′s. From then on and until the 1980s an increas-ing number of such experiments have been performed andpublished with ruminants (Table 1).

The intentions behind assessing choice between, orpreference and palatability for, different experimentalfeeds in current choice feeding experiments are mani-fold and can either be more focused on the experimentalfeed or on the animal’s reaction to the feeds on offer.Some of these experiments were conducted to assess pref-erence or palatability and thus the general suitability ofcertain experimental feeds for feeding (e.g., Alonso-Díazet al., 2009; Ben Salem et al., 1994; Chapman et al., 2009;Kaitho et al., 1996; Kalio et al., 2006; Rogosic et al., 2006;van Dorland et al., 2007). Similar to testing different feedsindoors, choice feeding experiments on pasture allow theassessment of the preference between two different, sep-arately growing monocultures (e.g., Hirata et al., 2008;Penning et al., 1997; Rutter et al., 2004) or between dif-ferent sward attributes (Distel et al., 1995; Ginane et al.,2003). The effects of a trade-off situation between feedquality and quantity (e.g. Dumont et al., 1995a,b; Ginaneet al., 2003) or influences of different feed characteristics(e.g., Cooper et al., 1996; Distel et al., 2007; Ginane et al.,2002a; Villalba and Provenza, 1999) were also studied withchoice feeding experiments. Other researchers used choicefeeding experiments in order to test the animal’s ability fordifferent purposes. These abilities include maintaining anoptimal rumen environment by giving the animals a choicebetween feeds differing in energy density (Cooper et al.,1995) or in energy density and form (i.e., chopped, groundand pelleted, Cooper et al., 1996), choosing feeds accordingto requirements (Cooper et al., 1994; Fedele et al., 2002),foraging optimally (Prache and Damasceno, 2006), practic-ing self-medication (Villalba et al., 2010a), and expressing‘nutritional wisdom’ (Rusterholz and Turner, 1978).

The ability of the animals to associate previous experi-ence with preference for a feed was examined by exposinglambs in their sensitive period of conditioning to test feedsand testing the persistence of the preference pattern aftercertain time periods (Catanese et al., 2010; Simitzis et al.,2008a). Even influence of prenatal experience was tested(Simitzis et al., 2008b), and experienced animals were com-pared with inexperienced animals (Villalba et al., 2004).Some researchers added different toxins to feeds offered aschoices or as single feed opportunity (Burritt and Provenza,2000). Others conditioned animals with different doses of atoxin and performed repeatedly (0, 7, 21 and 49 d after con-ditioning) preference tests in order to test the magnitudeand persistence of conditioned flavour aversion (Kyriazakis
et al., 1998). Some researchers tested the reaction of ani-mals and the underlying mechanisms when offering feedswith increased levels of condensed tannins in choice situ-ations (Provenza et al., 1990) to investigate how animals
ur Science 140 (2012) 105– 120

deal with toxins. Villalba et al. (2010a) tested the abilityof sheep for self-medication by offering a binary choice offeed containing either 10% or no quebracho tannin which isassumed to have anthelmintic effects in nematode infectedruminants (see Athanasiadou and Kyriazakis, 2004). Theeffects of the physiological stage of the animal on feedchoices (Cooper et al., 1994; Dumont et al., 1995b; Newmanet al., 1994; Parsons et al., 1994) can also be studied withchoice feeding experiments.

Further studies were conducted to address methodolog-ical questions, such as the search for the appropriate lengthof assessing palatability (Ben Salem et al., 1994; Kaithoet al., 1996) or whether short-term experiments can beused to make predictions about feeding behaviour on along-term scale (Dumont et al., 1995a).

3. Design of choice feeding experiments

Several factors are relevant for setting up choice feed-ing experiments (Fig. 1). In the following section, differentdesigns of choice feeding experiments undertaken indoorsand on pasture with ruminants are described.

3.1. Indoor experiments

In most indoor choice feeding experiments the ani-mals are penned individually (e.g., Alonso-Díaz et al., 2008,2009; Atwood et al., 2001; Ben Salem et al., 1994; Burrittand Provenza, 2000; Catanese et al., 2009; Cooper et al.,1995; Dikmen et al., 2009; Distel et al., 2007; Duncan andYoung, 2002; Ginane et al., 2002a, 2005; Hadjigeorgiouet al., 2003; Keskin et al., 2004; Kyriazakis et al., 1998;Kyriazakis and Oldham, 1993; Lyman et al., 2008; Provenzaet al., 1990, 1996; Rogosic et al., 2007; Sandoval-Castroet al., 2005; Scharenberg et al., 2007; Scott and Provenza,1999). Sometimes individually penned animals are keptoutside of barns in open-air enclosures (e.g., Ben Salemet al., 1994). Keeping animals in groups and at the sametime measuring individual intake requires more sophisti-cated equipment (van Dorland et al., 2007, 2008) and istherefore done less frequently. Alternatively, in the case ofgroup housing, group intake is often measured (e.g., Askaret al., 2006; Atwood et al., 2006; Görgülü et al., 1996; Kalioet al., 2006; Larbi et al., 1993).

In order to provide the experimental feeds separatelyfrom each other, troughs (Scharenberg et al., 2007) or buck-ets (Distel et al., 2007; Ginane et al., 2002b; Kalio et al.,2006; Keskin et al., 2004) are used depending on the bulk-iness and the amount of the experimental feed offered.These feed containers have to be designed in a way thatminimises feed losses. To prevent a habit reflex, the posi-tions where the different experimental feeds are offered areoften randomly allocated for each day (e.g., Kaitho et al.,1996; van Dorland et al., 2007; Scharenberg et al., 2007)as it may happen that one position might be perceived asmore convenient by the animal than the other/s (Forbes,2007). However, especially when there is no difference in
sensory characteristics between the experimental feeds,changing of position might be confusing for the animal;in this case, the spatial memory might be the only hint forthe animal to differentiate between the experimental feeds

J.S. Meier et al. / Applied Animal Behaviour Science 140 (2012) 105– 120 109

Experimental Feed

Number Form Sequence

ArrangementTime of

Exposure

Duration of

Exposure

Housing Conditions

Single vs. GroupIndoors Outdoors

Adaptation Experience

Physiology

Ruminant Species

Number of

Animals

Duration of Experiment

Duration of Periods

Adaptation Period

Choice Feeding Experiment

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Fig. 1. Factors relevant for se

n offer (Forbes, 2007). For instance, learning from post-ngestive effects (see Section 5.1.4) is only possible if thenimal can relate the experimental feed to the respectiveonsequences. A good possibility to avoid position bias iso use replicates by offering the experimental feeds to partf the animals in one location of the test pen or plot and tohe others in the opposite location. The design of the placehere the feeds are offered is also important. It has to be

uaranteed that the animal has the same spatial accessibil-ty to each of the experimental feeds offered (see Section.1.5).

.2. Experiments on pasture

Outdoor choice feeding experiments on pasture areypically relying on different cultivated monocultures, fornstance to test the preference for different plant speciesultivated separately such as grass and clover (Newmant al., 1994; Parsons et al., 1994; Penning et al., 1997;utter et al., 2004). Others compared either different herba-eous forage plants such as birdsfoot trefoil, chicory andlfalfa (Chapman et al., 2009) or different grass speciesHirata et al., 2010; Prache and Damasceno, 2006). In otherxperiments the animals were given a choice between pas-ures with different sward characteristics. This includedifferent sward heights, sward densities or both (Distelt al., 1995) or different sward heights (i.e. accessibil-ty) and sward phenology (i.e. quality; Dumont et al.,995a; Ginane et al., 2003). Additionally, hay of differ-nt quality and accessibility can be tested in a specialesign aimed at assessing outdoor grazing preferences
nder indoor conditions (Dumont and Petit, 1995). How-ver, when including several factors the real preference forn experimental feed is no longer assessed, but only theariation of feed preference under distinct environmental
choice feeding experiments.

conditions (see Rutter, 2006). Choice feeding experimentsemploying alternative monoculture pastures need a longpreparation time because the pastures often have to besown 1 year (Chapman et al., 2009; Parsons et al., 1994)to 3 years (Harvey et al., 2000; Phillips and James, 1998) inadvance of the experiment in order to achieve steady pas-ture conditions. Existing pastures can be used when thechoice feeding experiments are designed to test the effectsof sward characteristics rather than that of different plantspecies. However, this still requires management efforts toobtain the desired differences in sward heights or densities.

The experimental feeds tested in choice feeding exper-iments on pasture can be offered in different designs (seeFig. 2). They can be provided on two adjacent plots (e.g.,Harvey et al., 2000; Hirata et al., 2006; Parsons et al., 1994;Penning et al., 1997) or arranged in alternating stripes(Dumont et al., 1995a; Gesshe and Walton, 1981; Ginaneet al., 2003). The plots with different feeds can be dividedby alleys (Hirata et al., 2008, 2010) or by an animal hold-ing area with concrete floor (Phillips and James, 1998).Another possibility is to distribute the different cultivars onseveral separated patches within one paddock (Chapmanet al., 2009; Distel et al., 1995; Ganskopp et al., 1997). Usinga different approach, Newman et al. (1992) offered twoexperimental feeds (grass and clover) not as pasture swardsbut as turves in small test pens in order to test individualsheep preferences. Others tested the choice between sup-plementary feeds such as grains (Scott et al., 1995), freshleaves of multipurpose trees and shrubs (Kalio et al., 2006)or feeds supplemented with different compounds such astannins and sucrose (Alm et al., 2002) offered in bowls (Alm
et al., 2002), troughs (Kalio et al., 2006), or on separate feed-ing areas (Scott et al., 1995) while the animals were grazingon pastures. The methodology of this type of experimentresembles that used for indoor experiments.

110 J.S. Meier et al. / Applied Animal Behaviour Science 140 (2012) 105– 120

(a) (b) (c) (d)

) adjacen
Fig. 2. Different types of choice feeding experiment setups on pasture: (aby an alley, (d) several separated patches within one paddock.
3.3. Target ruminant species

So far most indoor choice feeding experiments withruminants were conducted with sheep (e.g., Alonso-Díazet al., 2009; Catanese et al., 2010; Cooper et al., 1995;Görgülü et al., 1996; Kyriazakis et al., 1998; Scharenberget al., 2007; Villalba et al., 2010a) and goats (e.g., Alonso-Díaz et al., 2008; Duncan et al., 2006; Fedele et al., 2002;Ginane et al., 2005; Illius et al., 1999; Morand-Fehr, 2003;Provenza et al., 1990). Fewer experiments were carriedout with cattle (heifers: Atwood et al., 2001; Ginaneet al., 2002a; Sandoval-Castro et al., 2005; dairy cows:van Dorland et al., 2007, 2008). The use of cattle is morecommon under outdoor conditions (Distel et al., 1995;Ganskopp et al., 1997; Ginane et al., 2003; Hirata et al.,2006, 2010; Phillips and James, 1998; Rutter et al., 2004).Several outdoor studies have used sheep (Harvey et al.,2000; Parsons et al., 1994), goats (Penning et al., 1997) anddifferent deer species (fallow deer, Alm et al., 2002; roedeer, Clauss et al., 2003; white-tailed deer, Chapman et al.,2009).

4. Methodology and measurements applied inchoice feeding experiments

4.1. Measurement of intake of individual feeds

The main or primary variable to be recorded for assess-ing preference or palatability in choice feeding experimentsis the intake of the respective feed item(s) tested. Indoors,feed intake can be quantified manually or by a com-puter supported recording equipment with feeding troughson weight scales (Forbes, 2007). When the system isadditionally equipped with electronic identification units(responders) and control of access to the feed by electron-ically operated feeding doors (van Dorland et al., 2007),feed preference measurements can be made even underthe condition of group housing and, additionally, a possiblediurnal pattern in preference can be easily detected.

Methods how to assess preference under outdoor condi-tions were compiled by Rutter (2006) for designs based onadjacent monocultures. A simplified approach often usedin outdoor choice feeding experiments is the calculation ofintake from estimated intake rate (intake per unit of time)and time of grazing on the experimental feed (Harvey et al.,2000; Parsons et al., 1994). Intake rate can be estimated by
relating concurrent recordings of jaw movements (Harveyet al., 2000; Newman et al., 1994) for instance with the IGER(Institute of Grassland and Environmental Research, NorthWyke, Devon, UK) behaviour recorder as for instance used
t monocultures, (b) monocultures in stripes, (c) monocultures separated

by Rutter et al. (2004), or grazing time records (Prache andDamasceno, 2006) to get the time spent eating comple-mented by records of body weight changes during shortgrazing periods (Penning and Hooper, 1985) and assess-ments of the dry matter content of the experimental feeds.Another approach consists in using oesophageal fistulae,where samples of the ingested material are collected beforearriving in the rumen, in order to calculate bite weightfrom extrusa samples; the concomitant bite rate data ismeasured with a special sound recording system which dis-tinguishes between individual bites (see Distel et al., 1995).A simple method is to register the amounts of experimen-tal forages on offer and those remaining after grazing. Forthat the biomass growth in areas protected from grazingfor instance with the help of exclosure cages is concur-rently assessed (Chapman et al., 2009). However, individualanimal data are difficult to obtain with this method.

There are several other direct and indirect methods toestimate the intake of single plant species or the generalintake, such as direct observation, for calculating propor-tionate intake of the experimental feeds selected. Thiscan be achieved via bite counting and bite size or bitemass estimates (e.g., Agreil and Meuret, 2004; Genin et al.,1994; Reppert, 1960) or by another application of theoesophageal fistulae where samples of the ingested mate-rial are botanically separated for the respective proportionsof the experimental feeds (e.g., Hess et al., 2002), by faecalmicro-histological procedures (e.g., Mellado et al., 2006)and by indicator techniques with indigestible n-alkaneswhich differ in profile among plant species (Dove andMayes, 1996). Gordon (1995) describes further methodsgiving more detailed information. However, these methodsare mostly applied in diet selection studies.

4.2. Measurement of feeding behaviour

As intake data is difficult to generate under outdoorconditions, preference can also be assessed as the timethe animal spent eating from a certain experimental feedas a proportion of the total time spent eating (Corteset al., 2006; Dumont et al., 1995a; Newman et al., 1994;Penning et al., 1997). Intake rates might differ with differ-ent feed types (e.g., Parsons et al., 1994), as for instanceclover is easier to harvest by the animal than grass (Lynchet al., 1992) and thus intake measurements provide morereliable data on feed preference than considering just the
time spent grazing (see Rutter, 2006). However, in othersettings where intake rates might be differing becauseinfluenced by the physiological state of the animals suchas hunger proportional grazing time of the experimental

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eed might be a better parameter to measure prefer-nce (Newman et al., 1994). Feeding behaviour traits maynclude (i) eating time, eating rate and meal size per exper-mental feed in indoor measurements, (ii) grazing time,ntake rate, meal size and position in a pasture for out-oor measurements and (iii) the frequency of switchingetween the feeds on offer for indoor (e.g., Ginane et al.,002a) or outdoor measurements.

Feeding behaviour can be assessed by direct observationChapman et al., 2009; Cortes et al., 2006; Dumont et al.,995b; Hirata et al., 2008; Newman et al., 1992; Ruttert al., 2004), video recording (Distel et al., 1995; Hiratat al., 2006, 2008; Newman et al., 1994; Parsons et al., 1994;enning et al., 1997) and photographic recording (Prachend Damasceno, 2006). There are also different automatedecording systems as for instance with the ‘Ethosys’ col-ar (Ginane et al., 2003; described by Scheibe et al., 1998),ecording devices linked to an elastic noseband (Newmant al., 1994; Parsons et al., 1994), and the IGER behaviourecorder (Rutter et al., 2004, see Chapter 4.1). They are alsoseful for monitoring feeding behaviour at night (Gordon,995).

Direct observations can also be applied indoors, but thiss less frequently done than on pasture due to the availabil-ty of other appropriate techniques. Its application needsome precautions (reviewed by Mann, 1999). It is mostlyerformed by one or more persons observing the respectiveosition of the animal on the sward or the behavioural pat-ern of interest over a defined period of time. This gives wayo human errors especially because of exhaustion of thebserver, because direct observation requires permanentttention (Altmann, 1974). The animals have to be familiarith the presence of the observer and related soundscape,

s disturbances can affect behavioural patterns (Morand-ehr, 2003). The observer should be able to make accurateecords without biasing the animals’ behaviour by his pres-nce. He can be located outside the experimental plotsPrache and Damasceno, 2006) or hide in an elevated posi-ion inside the plots (Rutter et al., 2004). An advantagef direct observation is the possibility to register unusualvents and other observations which may not be obvious,r cannot be distinguished, from automated records. Theethod allows not only to continuously record the animal’s

ehaviour, but also to explore variations in feed prefer-nces with daytime or between days and to characterise theemporal organisation of feeding choices. Scan sampling,.e. the recording of individual behaviour of each subject

ithin a whole group at regular intervals (Martin andateson, 1993), is often applied when using direct observa-ions (Cortes et al., 2006; Ginane et al., 2003; Harvey et al.,000; Hirata et al., 2008; Prache and Damasceno, 2006;utter et al., 2004). Direct observation can also be used toecord further behavioural traits such as ruminating, stand-ng, moving and lying.

Video recording is a suitable alternative to direct obser-ation with respect to most traits to be recorded ands especially useful for the quantification of short-lived
ehaviour patterns or long-term progressive changes inehaviour (Weigensberg and Fairbairn, 1994). Its advan-ages are the preservation of an exact visual record of theehaviour and that these records can be played in slow
ur Science 140 (2012) 105– 120 111

motion or speeded up for analysis (Martin and Bateson,1993). Special software assists in accomplishing data eval-uation from video records. Recordings of grazing time mayallow estimating proportionate intakes. However, whenbite sizes and intake rate differ between experimentalfeeds, getting a complete picture requires supplementaryinformation on individual feed amounts ingested (see Sec-tion 4.1). In comparison to direct observation, the videoequipment is limited by the area covered and items pre-venting or obscuring full view both indoors and on pasture.Therefore, combinations of several methods are often usedin choice feeding experiments where video observation isapplied (Parsons et al., 1994).

Indoors, electronically operated devices offer an alter-native to direct observation or video observation. Feedingtroughs equipped with weight scales and electronic accesscontrol enable meal patterns of individual animals tobe continuously registered, and this in association withmeasuring actual intakes of the experimental feeds (vanDorland et al., 2007).

4.3. Measurement of secondary response variables

The aim of feeding strategies might not only be themaximisation of intake but also to achieve certain lev-els or changes in secondary response variables such asperformance, digestion, metabolism, quality of animal-source foods, and emission of noxious gases from animalhusbandry. Several experiments have assessed the possi-ble influences of choice feeding situations on secondaryresponse variables such as live weight changes (beef calves:Atwood et al., 2001; lambs: Dikmen et al., 2009), andmilk yield and milk composition (Görgülü et al., 2008; vanDorland et al., 2007). van Dorland et al. (2007) also inves-tigated digestibility and metabolic profiles of the cows asmeasured in the blood. Yurtseven and Öztürk (2009) andYurtseven et al. (2009) compared emissions of methaneand carbon dioxide under choice feeding conditions (Yurt-seven and Öztürk 2009) or in comparisons with total mixedrations (Yurtseven et al., 2009) using respiration chambers.

5. Consideration on aspects important forsuccessfully carrying out choice feedingexperiments

5.1. Animal-specific aspects

5.1.1. Importance of animal speciesRuminant livestock species might have an influence

on the results of choice feeding experiments even thoughdifferent ruminant species (cattle, sheep, goats) have sev-eral similar behavioural patterns and skills (Ekesbo, 2011).Sheep for instance are highly gregarious animals, and tokeep them separated from each other (see Section 5.1.5)might cause serious distress (Lynch et al., 1992). Thepreference for feed items might be also related to thefeeding type. Cattle and sheep are classified as ‘grass and
roughage eaters’ and goats belong to the ‘intermediatetype’ (Hofmann, 1989). Rogosic et al. (2006) using goatsand sheep found that these two species differed in intakeas goats had a higher level of shrub intake than sheep in

Behavio
a choice feeding situation, although the preference orderswere similar. Others found in a binary choice with ryegrassand clover over a period of 72 h that goats spent totally lesstime grazing than sheep and that that the longer grazingtime of sheep was mainly due to feeding longer on clover(Penning et al., 1997). However, when the animals got adlibitum access to different gramineous roughage feeds inbinary choices, or when woody feeds were offered dur-ing a short period, similar preferences between these twoanimal species in their preference pattern (Hadjigeorgiouet al., 2003), or preference pattern and intake behaviour(Degen et al., 2010) were found. Dumont et al. (1995b)observed a higher preference for the (scarcer) experimen-tal feed of higher quality in sheep than in cattle. A smallergut capacity in relation to body weight leads to higherenergy requirements of smaller ruminants such as sheepcompared to cattle (Rook et al., 2004). This and differ-ent muzzle anatomies (sheep for instance have narrowermouths and thus can select feeds more accurate than cattle;Lynch et al., 1992) might explain these differences in pref-erence patterns (Rook et al., 2004). A special indoor settingmimicking different pasture conditions such as differentquality and availability of the feeds (Dumont and Petit,1995) might help to overcome different selective grazingpatterns. When offering the pasture cultivars as hay, differ-ent grazing and especially harvesting strategies betweenruminant species apparent on pasture can be minimised(Dumont and Petit, 1995).

5.1.2. Physiological stage of the animalsIndividual nutritional requirements (Dikmen et al.,

2009; Kyriazakis and Oldham, 1993; Provenza et al., 1996;Scott and Provenza, 1999) and differences in the physiolog-ical stage such as growth, pregnancy, lactation or dry periodare factors that might influence individual preferences.Hunger seems to be one of the largest confounding fac-tors in choice feeding experiments. Hungry animals tend tostart consuming the very first feed accessible irrespectiveof its real preference (Scharenberg et al., 2007). Newmanet al. (1994) observed that hungry sheep, apart from havinggenerally higher intake rates and spending more time graz-ing, preferred clover less intensively than well-fed sheep.Hunger for a particular macronutrient was also found toinfluence feed choice. Lambs which were fed an unbalanceddiet chose the feed which compensates best the imbal-ance in a subsequent binary choice situation regardless ofthe feed’s structure (pellets, rolled or ground, Villalba andProvenza, 1999).

Fedele et al. (2002) investigated if goats are able toselect their diet according to their nutritional require-ments. The free-choice group had the possibility to chooseamong six feeds on offer during 13 periods including thenon-pregnant-non-lactating period, three stages of preg-nancy, and nine stages of lactation. This was compared togoats that were fed with flaked barley according to 50%of energy requirements and alfalfa hay ad libitum. Bothtreatments showed an increased feed intake in the preg-
nancy period and a decreased intake towards the end oflactation. The free-choice group also changed the exper-imental feed contribution to their diet in four out ofsix experimental feeds offered when switching from one
ur Science 140 (2012) 105– 120

physiological stage to another, suggesting that the goatscomposed their diet according to their physiological stagewithin the reproduction cycle. Cooper et al. (1994) offeredtwo sets of binary choices to pregnant and non-pregnantsheep consisting of experimental feeds contrasting in con-tents of energy or protein or both. At higher dietary energycontent, pregnant ewes showed a higher preference forthe feed with the higher crude protein content than non-pregnant ewes while this was not the case when both feedson offer had the respectively lower energy content. Parsonset al. (1994) found only a small and inconsistent effect oflactation compared to the dry period on preference in sheepoffered a choice between monoculture plots of ryegrass andclover. Morand-Fehr (2003) observed no apparent influ-ence of physiological stage on feed preferences in goats.

Animals suffering from gastrointestinal parasite bur-dens might show different feed preferences in choicefeeding experiments than healthy animals. This could bethe case when the experimental feeds offered contain alle-viating substances such as plant secondary compoundsand when the animals had the possibility to gain previ-ous knowledge about their alleviating effects. Villalba et al.(2010a) found for instance a tendency for an increasedpreference for a mixture of tannins and alfalfa in infectedcompared to non-infected lambs after the animals had theopportunity to experience the positive post-ingestive con-sequences.

It remains unclear whether sex might influence feedchoice (Aderibigbe et al., 1982; Rodríguez et al., 2008).

5.1.3. Number of animal replicatesIndividual animals differ in preference and in percep-

tion of palatability (Atwood et al., 2001; Dikmen et al.,2009; Provenza et al., 1996; Scott and Provenza, 1999). Aschoice feeding experiments need considerable resources,low animal numbers are often used as replicates (Cataneseet al., 2009; Provenza et al., 1996). In most indoor choicefeeding experiments the single animals are treated as repli-cates or experimental unit as they are penned or tetheredindividually. The number of animal replicates most fre-quently used are six (Atwood et al., 2006; Duncan andYoung, 2002; Rodríguez et al., 2008; Scharenberg et al.,2007; van Dorland et al., 2008) and eight (Dikmen et al.,2009; Distel et al., 2007; Provenza et al., 1996; Yurtsevenet al., 2009) animals per treatment. Indoor studies wherethe experimental animals were kept in group housing facil-ities with individual feed intake measurements also usedfive to six individual animals as repetitions per treatment(Aderibigbe et al., 1982; van Dorland et al., 2007). In choicefeeding experiments conducted on pasture the animalsare often kept in groups with group size varying fromtwo (Rutter et al., 2004) to three (Ganskopp et al., 1997;Hirata et al., 2006), four (Phillips and James, 1998) andsix (Ginane et al., 2003) animals per group. Sometimesentire herds of approximately 30 animals per group wereused (Hirata et al., 2008). In some studies the animals areused as replicates (e.g., Ginane et al., 2003). Sometimes one
‘focal’ animal per group was used for preference measure-ments (Newman et al., 1994; Parsons et al., 1994; Penninget al., 1997), sometimes the group was treated as repli-cate (Harvey et al., 2000; Rutter et al., 2004), based on the

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ften quoted consideration of Rook and Penning (1991)hat statistical independency of behaviour data of singlenimals is not assured when the animals are kept in groups.irata et al. (2008) reported that the switching between the

wo choices on offer occurred mostly within the group, inhis study consisting of about 30 animals. However, oth-rs observed independent grazing behaviour which wouldllow treating the single animals as a replicate (Ginanet al., 2003, see Section 5.1.5).

Applying the Latin square design is a promising wayo keep replicates and, consequently, time and resourceequirements minimal in choice feeding experiments, ast is flexible for various experimental designs (Bormant al., 1991). For instance Borman et al. (1991) used

× 4 and 3 × 3 Latin squares (rows (here: duration inays) × columns (here: feed positions)) with one squareer animal. Others used the number of animals asows and treatments as columns with one square pereriod (6 × 6: Cooper et al., 1996; 9 × 9: Ginane et al.,002b) or treatments as rows and number of periods asolumns with one square per animal (4 × 4: Favreau et al.,010; 3 × 3: Kyriazakis and Oldham, 1997). An impor-ant limitation of this design is that there may be aias from carry-over effects of the previous periods andreatments.

.1.4. Adaptation of the animals and consideration ofrevious experience

In indoor choice feeding experiments adaptation peri-ds for familiarising the animals with the barns are usuallyetween 1 week (Görgülü et al., 1996) and 2 weeksHadjigeorgiou et al., 2003; Kyriazakis and Oldham, 1997).

orand-Fehr (2003) chose 15–18 days to adapt goats tohe specific experimental conditions and the cages usedn the experiments. In most choice feeding experimentsesearchers practice adaptation by familiarising the ani-als with the feeds to be tested. These periods mostly vary

etween 1 week (e.g., Dikmen et al., 2009; Kyriazakis andldham, 1993) and 2 weeks (e.g., Alonso-Díaz et al., 2009),nd are needed unless the reaction at first confrontationith the experimental feeds is the aim of the experiment.

or the adaptation to novel feeds, in terms of intake andigestion at least 6 days are required (Demarquilly et al.,995). Reluctance of ingestion might occur due to neo-hobia, and intake might increase over the time the feedsre offered in a process of familiarisation (Launchbaught al., 1997), given that there are no negative post-ingestiveffects. Young animals might be less reluctant towards aovel feed than adults (Lynch et al., 1992). In choice feed-

ng studies aimed at assessing palatability, according toaitho et al. (1996) the animals should be given more than

day of adaptation to the experimental feeds in order toet more stable expressions of preference. Kyriazakis et al.1998) recommended even 10 days of training to ensurehat animals can associate each feed with its properties.

In outdoor studies, the animals mostly have previousrazing experience (Harvey et al., 2000; Hirata et al., 2006;
arsons et al., 1994; Penning et al., 1997). It is, there-ore, less common to conduct an adaptation period foramiliarisation to the feed and the experimental condi-ions. Rutter et al. (2004), however, familiarised animals
ur Science 140 (2012) 105– 120 113

during a 1-week adaptation period to the setting of a choicefeeding experiment. Penning et al. (1997) allowed 12 daysof adaptation to plots containing equal proportions of thetwo experimental feeds for levelling possible effects ofthe previous diet. Newman et al. (1994) even adapted formore than 14 days. Prache and Damasceno (2006) andCortes et al. (2006) conducted pre-experiments of 5 daysfor familiarisation with the feeds and measuring initialpreference.

For certain research questions it might be important togive the animals the possibility to previously gain expe-rience with the experimental feeds to be tested. Thus,learning processes are important especially when post-ingestive effects are taken into account (Duncan and Young,2002). Goats, for example, were found to need at least 1 h torecognise negative consequences induced by higher phe-nolic contents of a feed (Papachristou et al., 2003) andparasitised lambs required several days in order to learnabout the beneficial effects of tannins present in the exper-imental feed (Villalba et al., 2010a). The learning processabout a number of feeds and their possible (positive or neg-ative) consequences might be facilitated by offering theexperimental feeds first separately instead of simultane-ously because then animals are able to link the ingestedfeed to the respective post-ingestive effects (Duncan andYoung, 2002).

Previous experience with the experimental feeds orwith similar feeds might affect preference or intake pat-terns in choice situations (Parsons et al., 1994), especiallywhen the experience was gained early in life in sensitivephases (Distel et al., 1994). There is ample evidencethat mutual learning can influence preference and feedchoice (Lynch et al., 1992). The offspring can even developpreferences through prenatal experience (Simitzis et al.,2008b) or through learning from their dams or peers. Inthis respect, dams have a stronger influence than peers(Lynch et al., 1992; Provenza and Launchbaugh, 1999).Experience with feeds gained early in life could persistand influence future feeding behaviour (Simitzis et al.,2008a). Villalba et al. (2004) allowed weaned lambs to gainexperience with experimental feeds containing differentplant secondary compounds (PSC), here tannins, terpenesor oxalates. In subsequent choice feeding situations lambsfamiliar with such feeds consumed more of them thanlambs that were never exposed to PSC-containing feedsbefore, and these differences were still prevalent 8 monthslater. A similar consumption pattern was found with lambsthat either did or did not gain experience with low-qualityfeeds during their first months of age when exposed toa low-quality diet 9 months later (Distel et al., 1996).Sheep which had learned from negative post-ingestiveconsequences and had developed specific feed aversions,showed this aversion for at least about 50 days at the high-est dose rate of the negative stimulus (Kyriazakis et al.,1998). The intensity and persistence of this behaviour isprobably related to the magnitude of the aversion (Forbes,2007). Feeding behaviour found in adults thus may be at
least partly due to previous experiments or experienceobtained in the rearing period (Distel et al., 1996).
Carry-over effects of previous experience or diet couldalso be temporary (Illius et al., 1992). Ginane et al. (2002a)

Behavio
observed in choice situations a temporarily higher prefer-ence for an experimental feed which had not been offeredpreviously. They explained this by the motivation of theanimals for diversifying their diet. Pre-ingestive charac-teristics of the feeds and attempts of diversifying thediet might even overrule possible post-ingestive conse-quences on a short-term scale (Favreau et al., 2010). Thusexperimental feeds familiar to the animals or offered infamiliar forms (for instance as hay) might act as ‘rarity’and (temporarily) lead to an initial increase in intake pat-tern when replacing the previous diet. This was foundindoors (Favreau et al., 2010; Ginane et al., 2002a; Wangand Provenza, 1996) and outdoors (Newman et al., 1992;Parsons et al., 1994), and has to be excluded when thegoal is to determine the long-term preference. The wishof the animals to diversify their diet might be based onthe complementarity of feeds for instance in flavour ornutrient contents, and is independent whether the ani-mals are familiar or unfamiliar with the experimentalfeeds (Wang and Provenza, 1996). However, preferencesfor feeds already ingested may decrease over time evenwhen nutritionally appropriate due to satiety processes(Provenza et al., 1996).

Adaptation to the experimental feeds might occur whenoffering them over a longer time period. Adaptation effectsmay consist of an increase in intake of the experimentalfeeds (Ngwa et al., 2003; Papachristou et al., 2007). This isespecially expected for initially unpalatable feeds to whichthe animals may adapt in long-term studies (Ben Salemet al., 1994).

Preference may change with time due to changes in indi-vidual perception and increasing experience (Scharenberget al., 2007), especially when learning effects includepositive or negative post-ingestive feedbacks (Provenza,1995). For instance, the consumption of certain amountsof alkaloids when eating Delphinium barbeyi led to neg-ative post-ingestive feedbacks associated with a declinein intake resulting in a conditioned taste aversion (Pfisteret al., 1990). Feed aversion against a feed can be formedthrough the ability of ruminants to associate feeds withillness (Zahorik et al., 1990). Fluctuations in preferenceor intake pattern over the course of the time when feedscontaining PSC are offered or fed (tannins, Clauss et al.,2003; alkaloids, Pfister et al., 1990, 1997) might be relatedto post-ingestive effects as well. An increase in intake orpreference pattern might indicate adaptation processes ofthe animals (Ben Salem et al., 1994; Kalio et al., 2006) ormight just be part of a cyclical intake pattern with fluctua-tions in increase and reduction of intake of PSC-containingfeeds until they were detoxified (Pfister et al., 1997). Thesame cyclical intake pattern was found in other studies(Ngwa et al., 2003). A more stable pattern of preferencemight appear over the course of time with well acceptedfeeds, because changes in preference over time are typicallyhappening slower with feeds of high initial palatabilitythan with those with low initial palatability (Distel et al.,2007).

These aspects show the importance to take into accountthe individual feeding history of each individual in selectingthe experimental animals and to consider the appropriatelength for the choice period (see Section 5.3.2).

ur Science 140 (2012) 105– 120

5.1.5. Assessments made in individually kept animals vs.those kept in groups

Another factor of influence might be the stage offamiliarisation between the animals. Social interactionsbetween individual animals kept in a flock may causechanges in grazing behaviour (Harvey et al., 2000). Thismeans that the preferences expressed in the choice feedingexperiment might be influenced by the group members andmight suppress individual preferences (Hirata et al., 2008).It seems that the design of the experiments is decisivefor the extent of group behaviour and experimental set-tings might be favoured that allow the animals to expressindividual choices while being in visibility range and prox-imity to the other herd members (Dumont et al., 1995b;Ginane et al., 2003). The intention to stay in proximityof the herd, a behaviour which is also influenced by thenumber of accompanying animals, can be stronger thanthe intention to realise the choice for the preferred feedon offer. This was demonstrated with sheep kept in dif-ferent group sizes that had the possibility to graze a tallersward (preferred feed) located either 15 m or 50 m awayfrom a small herd of sheep serving as social attractants(Dumont and Boissy, 2000). In a special setup, Scott et al.(1995) found that both familiarisation, and thus the socialcontext of being reared together, and individual prefer-ences had an effect on the choice of feeding sites whensheep were familiar with each other. By contrast, in a non-familiar context the individual preferences seemed to bestronger than the social aspect. Therefore, in choice feed-ing experiments on pasture the animals should have thepossibility to express personal preferences without inter-ference by social constraints. Under indoor choice feedingconditions where the animals have their individual accessto troughs despite being kept in groups there is a lower riskof bias by social interaction and influence. However, if theaccess to the feeding place is restricted, dominant animalsmight deter subdominant individuals from feeding result-ing in lower feed intake and/or incapacity of expressing theindividual feed preferences (Fraser and Broom, 1997). Gen-erally, enough space should be provided in order to preventa decrease in intake, ideally giving all animals the possi-bility to feed at the same time. Although, it would seembetter to avoid social influence by keeping the animalsindividually, cattle might have a decreased intake whenfed individually compared to group feeding (Albright andArave, 1997).

Choosing an appropriate group size (at least >3 sheep:Penning et al., 1993; >6 sheep: Sevi et al., 1999) mightbe an important aspect to be taken into account whenassessing the grazing behaviour of highly gregarious ani-mals like sheep. Animals kept in small groups (or evenalone) might show grazing behaviour that differs from thatobserved in larger groups (Penning et al., 1993; Sevi et al.,1999). Repetitive use of the experimental plot/s for dif-ferent experimental groups of animals (e.g., Parsons et al.,1994; Rutter et al., 2004) after a certain period of restingmight be an option to overcome possible spatial limita-
tions. However, when the resting period is prolonged orwhen the repetitive use of the experimental plots cov-ers different seasons of the year, the seasonal changes inthe quantity and quality of the choice feeds as well as

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arry-over effects of previous grazing periods have to beaken into account.

.2. Feed-specific aspects

.2.1. Number of feeds simultaneously testedThe number of different experimental feeds offered

imultaneously in outdoor choice feeding experimentsith ruminants varies from binary choice situations (e.g.,irata et al., 2008) up to 10 different forages (Gesshe andalton, 1981). The same span ranging from two (Askar

t al., 2006; Catanese et al., 2009; Provenza et al., 1990;cott and Provenza, 1999) to 10 (Larbi et al., 1993) is foundor indoor experiments. Still, especially under outdoor con-itions, most choice feeding experiments were operatedith a low number of experimental feeds or sward types

nd were often testing dual choices (Distel et al., 1995;irata et al., 2008; Parsons et al., 1994; Rutter et al., 2004).n increasing number of experimental feeds makes it moreifficult for the animal to make an appropriate choice.ffering an increasing number of feeds may increase total

ntake (Rogosic et al., 2007). Independent from the numberf experimental feeds offered, it has to be ensured that thenimals have the same access to all experimental feeds inrder to prevent that different positions or arrangements ofhe feeds might influence the choice. Arranging the troughsn a circle may be a good technical solution to this problemAlm et al., 2002).

Outdoors, the number of choices is often limited bydditional factors. It might be difficult to distinguish onhich sward the animal is feeding when the swards areot clearly separated (Penning et al., 1997). In order tovoid that some observations cannot be explicitly allocatedo either of the experimental feeds (Harvey et al., 2000;ewman et al., 1994; Parsons et al., 1994), the boundariesetween the experimental feeds have to be clearly distin-uishable and visible by video camera or observer. Smalllleys may be helpful (see also Rutter, 2006) but, at theame time, may prevent that animals switch feeds as oftens they would when feeds would be closer together. Set-upsith monocultures are to be preferred over different mixed

wards as in the latter the exact composition of mixturesnd of what is consumed is more difficult to control (Cortest al., 2006) and replicates of the same mixed culture areardly composed the same (Hirata et al., 2006).

.2.2. Characteristics of experimental feedsThe physical characteristics of the experimental feeds

an have an influence on the choice between the individ-al feeds on offer (see also Morand-Fehr, 2003, concerningoats). In order to allow unbiased choices, the physicalharacteristics (e.g., form, length, density) of the feeds onffer should be as similar as possible unless these traitsre among the aspects to be tested. In the case of forages,ritical properties include dry matter content and physi-al structure such as particle length (Kenney et al., 1984).he physical form is especially important for forages with
ow inherent palatability. Grinding of shrub twigs witheaves could lead to a higher intake than when offeringlant parts (Ben Salem et al., 1994). Likewise, the removalf stems when feeding shrubs and trees might increase
ur Science 140 (2012) 105– 120 115

their palatability and preference. Additionally the presenceof mechanical plant defences such as thorns may influ-ence preference pattern (Papachristou et al., 2003). Evensward height on pasture may have an influence on choice(Penning et al., 1997). This was not the case in the study ofPhillips and James (1998) where monocultures of perennialryegrass and white clover were tested. Hirata et al. (2006)noted that cows preferred swards with higher bulk den-sity. van Dorland et al. (2008) found a stronger effect of thechoice situation in cows offered fresh forages than whengiven ensilaged forages resulting in differences in the pat-tern of intake across the day when comparing the groupreceiving a mixed diet vs. the choice group.

Villalba and Provenza (1999) showed that lambsingested more barley grains and alfalfa pellets whenthese were kept intact than when ground. Likewise, smallruminants may prefer concentrate where ingredients arecoarsely ground (Baumont et al., 2000). The palatabil-ity of dusty and finely ground experimental feeds canbe enhanced by moistening (reviewed by Baumont et al.,2000) and pelleting (Arave et al., 1983). Different prop-erties of the experimental feeds such as odour and tastemight influence the preference pattern as well. The impactof flavour on preference pattern or intake or both was forinstance demonstrated in choice feeding experiments withgoats (De Rosa et al., 2002) and sheep (Distel et al., 2007).There are several techniques available to exclude differ-ences in these sensory characteristics including sweetenersand aromatic substances.

Choice feeding experiments may also be hampered sim-ply by differences in the quality of the feeds to be compared.Ginane et al. (2003) pointed out that using experimentalfeeds of different quality or digestibility might result indifferences in rumen fill or passage rate thus possibly influ-ence feeding behaviour. Fibre properties influence intakerate (Favreau et al., 2010) and differences in the poten-tial intake rate affect feed choice (Cooper et al., 1996).In addition, presence and level of toxins and plant sec-ondary metabolites in the feed might have an influenceon intake behaviour (Burritt and Provenza, 2000). Whenthe basal diet fed is nutritionally unbalanced, preference(or avoidance) might be governed by the search for thenutrient missing or against that available in excess in thebasal ration (Villalba and Provenza, 1999). This bias canbe excluded by using control feeds with similar nutritionaland physical quality.

In studies where a special experimental feed, e.g., a feedcontaining toxins, is offered, a basal diet could ascertainat least a certain level of feed intake. This basal diet couldbe offered both before (Chriyaa et al., 1997) or after (BenSalem et al., 1994; Ginane et al., 2005; Papachristou et al.,2003; Sandoval-Castro et al., 2005) exposing the animalsto the experimental feeds given as a choice. For instancegrass (Sandoval-Castro et al., 2005), grass pellets (Ginaneet al., 2005), barley hay (Ben Salem et al., 1994), and bar-ley grain (Rogosic et al., 2007) have been used as basalfeeds. For the calculation of the palatability index (the ratio
of the feed biomass consumed to that offered; Ben Salemet al., 1994) e.g., for ranking shrubs, it seems advisable toinclude a basal diet (Ben Salem et al., 1994). However, whenthe basal diet is of a better quality than the experimental

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feeds, its amount offered has to be restricted, because oth-erwise the animals might show no or limited interest in theexperimental feeds to be tested in the choice situation.

5.3. Set-up specific aspects

5.3.1. Duration and time of exposure to the experimentalfeeds

Access to the experimental feeds can be provided eitherpermanently during the entire day and night (e.g., Atwoodet al., 2006; Distel et al., 2007; Harvey et al., 2000), orrestricted to a certain period. Restricted access can beallowed for short periods where feeds are offered for≤30 min (e.g., Ginane et al., 2002b; Kyriazakis et al., 1998;Morand-Fehr, 2003) and longer periods of >30 min to 8 h(e.g., Alonso-Díaz et al., 2009; Ben Salem et al., 1994;Papachristou et al., 2003). Not only time but also resourceuse might be considered as a decision criterion for lim-iting the duration of exposure to the experimental feeds.Newman et al. (1992) who offered choices between grassand clover as turves stopped the choice feeding periodwhen approximately 100 bites had been realised. This wasdone in order to prevent a bias in preference pattern as aconsequence of the differences occurring in the remainingavailability of the two experimental feeds. A prolonga-tion of the exposure time is especially important whenincreasing the number of experimental feeds as animalsfirst need to gather information about novel feeds to choosean optimal diet (Provenza and Balph, 1987, see also Section5.1.4). Therefore it has to be considered that short-termperiods for assessing preferences are often poor predictorsof long-term preference pattern or feeding behaviour andpreferences observed in short-term choice feeding experi-ments often cannot be extrapolated to long-term responses(Dumont et al., 1995a; Parsons et al., 1994). For certainexperimental questions, however, the immediate prefer-ence might be in the centre of interest.

Also the periods of the day of exposure to the experi-mental feeds may influence the outcome of choice feedingexperiments. Diurnal patterns of preference were repeat-edly determined in experiments where exposure to theexperimental feeds lasted for several hours or whole days(Catanese et al., 2009; Parsons et al., 1994; Rutter, 2006;Rutter et al., 2004; van Dorland et al., 2008). In choice feed-ing experiments using clover and ryegrass as experimentalfeeds, grazing sheep (Parsons et al., 1994) and cattle (Rutteret al., 2004) were found to prefer clover in the morning andryegrass in the afternoon and thus expressed a clear diur-nal pattern. Sheep (Lynch et al., 1992) and cattle (Albrightand Arave, 1997) are known to graze for up to 9 h per daywith peak grazing during the first hours after sunrise andthe last hours before sunset in temperate environments.Görgülü et al. (2008) observed generally higher intakesnear sunrise and sunset which might have been relatedto lower solar radiation intensity and temperature duringthese periods. Fisher et al. (1999) found higher preferencesfor the hay cut late in the afternoon, which was of higher
nutritional quality, as compared to the hay cut in the fol-lowing morning. In the study of Papachristou et al. (2007),intake measured over 8 days was significantly higher forboth PSC-containing and good-quality feeds individually
ur Science 140 (2012) 105– 120

and together (+10% total intake) when the PSC-containingfeeds where given for 4 h in the morning and the good-quality feeds for 4 h in the afternoon than when the orderwas reversed in a subsequent trial. Hunger and satiety areoften associated with the period of the day and thereforecould be as well important drivers of differences in the out-come of choice feeding experiments at different periods ofthe day (see Section 5.1.2).

5.3.2. Length of choice feeding periodsThe length of the entire choice period can be categorised

into short-term (total duration of the choice period foreach animal of less than 1 day; e.g., Distel et al., 1995),medium-term (1 day to 1 week; e.g., Harvey et al., 2000;Hirata et al., 2006; Parsons et al., 1994; Penning et al.,1997) and long-term (several weeks; e.g., Chapman et al.,2009; Ginane et al., 2003; Hirata et al., 2010; Phillips andJames, 1998; Rutter et al., 2004) (own categorisation). Addi-tionally there may be preceding periods of familiarisation,adaptation or learning (see Section 5.1.4). The exposureto the experimental feeds within the entire choice periodcan have different durations (see Section 5.3.1) and fre-quencies. Some choice feeding periods include a singularexposure to the experimental feeds over several days (e.g.,Atwood et al., 2006; Cooper et al., 1995), others use a designwith repeated exposures within 1 day (Distel et al., 1995)or over several days (e.g., Ben Salem et al., 1994; Kyriazakiset al., 1998). Harvey et al. (2000) recommended experi-mental lengths of more than 24 h as they found day-to-daydifferences in preference pattern when sub-dividing 48 hof outdoor choice feeding into two periods of 24 h. Ngwaet al. (2003) proposed experimental durations of over 12days to be appropriate for measuring reliable palatabilityindices. Because of the high variability of the intake withinthe first days, Kaitho et al. (1996) states that measurementsbetween 5 and 12 days yield the most accurate predic-tions for palatability trials, as at least 5 days are necessaryfor the animal to balance their diet in choice situations(Ginane et al., 2002a). In general, the length of the periodin such experiments has to be long enough to at least allowexpressing a stable pattern of preference (Clauss et al.,2003, see Section 5.1.4).

However, when post-ingestive effects are of no rel-evance, or shall be deliberately excluded, short-termdurations may be preferred (De Rosa et al., 2002; Villalbaand Provenza, 2000). Especially when a large number ofexperimental feeds is to be tested, this is a cost-and timeeffective option for testing of palatability or preferences forexperimental feeds (Morand-Fehr, 2003).

6. Conclusions

Choice feeding experiments with ruminants offer anexciting opportunity to obtain information about feedsgoing beyond the results of classical feed quality assess-ments as listed in the feed tables. They offer the possibilityto investigate various feed- and animal-related factors
influencing the feed choice decision of an animal in a con-trolled environment. Thus choice feeding experiments helpto better understand feeding behaviour and allow adjust-ment of management strategies to meet the nutritional

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equirements of the individual animal. Results of choiceeeding experiments could help to improve animal wel-are and health in commercially kept livestock by reducingtress related to unbalanced rations and by better meetingndividual needs. Even though being resource intensive, theumber of animals should be as high as possible, as thereight be considerable variability between the individuals.

onsidering animal welfare aspects, keeping the animals inroups would be preferable as this would in general lowerhe stress-induced effects as compared to animals keptn individual pens, especially in highly gregarious rumi-ants like sheep. However, mutual learning and previousrazing experience have to be taken into account. Out-oor choice feeding experiments should be designed in aay that animals can express their real preference without

ny social interventions. For instance offering the differenteeds in close proximity to each other such as in stripesr on small patches offers the opportunity for animals toraze their preferred feed without forcing the animal toeed far away from other group members. The number ofeeds tested should be limited to a number still allowingomparisons among the feeds. Given that a large numberf feeds shall be screened, short-term choice experimentsoth with respect to duration of exposure and entire lengthf the choice period might be preferred. However, long-erm choice feeding studies on few target feeds are usefuln demonstrating the possible occurrence, type and mag-itude of post-ingestive feedbacks and the influence ofdaptation on preference and feeding behaviour. Time andesource requirements are minimised when using a (repli-ated) Latin square design. It is advisable to include aontrol group to reduce incorrect findings due to environ-ental impacts. It is important to state that the outcomes

f choice feeding experiments are fully valid only for thepecific situation investigated. Thus, any extrapolation ofhe results has to be done with care.

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