Effects of food structure and nutritional quality and animal nutritional state on intake behaviour and food preferences of sheep

Applied Animal Behaviour Science 63 Ž1999. 145–163

Effects of food structure and nutritional quality and animal nutritional state on intake behaviour and food preferences of sheep Juan J. Villalba ) , Frederick D. Provenza Department of Rangeland Resources, Utah State UniÕersity, Logan, UT 84322-5230, USA Accepted 19 November 1998

Abstract There is evidence of the independent effects of plant physical and chemical characteristics on foraging, but little has been done to determine how these traits interact to affect food selection. We determined if the nutritional state of lambs Ž OÕis aries . affected intake behavior and preference for foods with different ratios of proteinrenergy Žalfalfa: high proportion proteinrenergy; barley: high proportion energyrprotein., presented in different physical forms Žwhole or ground.. Changes in nutritional state were induced by altering the nutritional composition of the basal diet Žhigh wEpx or low wePx proportion of energyrprotein.. We determined preference for barley and alfalfa Žwhole or ground. when lambs were fed Ž1. a nutritionally balanced diet with a 15-h Žovernight. fast ŽPeriod 1., Ž2. an unbalanced diet ŽEp or eP. with a 15-h Žovernight. fast ŽPeriod 2., and Ž3. an unbalanced diet without a fast ŽPeriod 3.. Averaged across the three periods, all lambs preferred barley to alfalfa Ž P - 0.001., and they spent more time feeding on barley than alfalfa Ž P - 0.001.. Lambs also preferred whole to ground foods Ž P - 0.05.. Nevertheless, the basal diet strongly affected preference for barley and alfalfa. During Period 2, lambs fed basal diet Ep ate less barley Ž142 vs. 232 g. and more alfalfa Ž71 vs. 9 g. than lambs fed basal diet eP Ž P - 0.001., especially when alfalfa was pelleted and barley was ground or rolled. Lambs fed basal diet eP strongly preferred barley to alfalfa, even if barley was ground and alfalfa was pelleted. These differences were even more pronounced after a meal of basal diet Ep or basal diet eP in Period 3. Lambs fed basal diet Ep consumed less energy during preference tests than lambs fed basal diet eP Ž P - 0.001.. Lambs fed the two unbalanced basal diets consumed similar

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Corresponding author. Tel.: q1-801-797-2539; fax: q1-801-797-3796; e-mail: [email protected]

0168-1591r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 Ž 9 8 . 0 0 2 3 8 - X

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amounts of protein during preference tests Ž P ) 0.05.. These differences in intake of energy and protein by lambs fed the unbalanced diets led to a steady increase in the ratio of proteinrenergy selected by lambs fed the balanced basal diet and diet Ep during Periods 1, 2, and 3 Ž7.9, 11.7, 13.9 g CPrMJ., and a constant ratio of proteinrenergy selected by lambs fed the balanced basal diet and diet eP during Periods 1, 2, and 3 Ž7.6, 8.1, 8.1 g CPrMJ; P - 0.001.. Thus, lambs fed basal diet Ep selected barley and alfalfa in a way that achieved a higher ratio of proteinrenergy in their diet than lambs fed basal diet eP Ž12.7 vs. 8.2 g CPrMJ DE, P - 0.001.. Collectively, these results suggest that food structure and biochemical composition interacted with a lamb’s nutritional state to determine preference. When a lamb’s need for a particular macronutrient was high, a food’s biochemical composition was more important than its structure in determining preference. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Food preference; Lambs; Intake rate; Food structure; Food quality; Nutritional state

1. Introduction Herbivores select diets from an array of plant species that differ in physical and chemical characteristics ŽDove, 1996; Hodgson et al., 1999; Provenza et al., 1999.. The outcome is a diet higher in nutrients and lower in toxins than the average available in the environment ŽIllius and Gordon, 1993; Parsons et al., 1994a; Provenza, 1995a,b, 1996.. Several factors influence the process. Plants differ in physical structure, and plant attributes Žheight, width, density. that confer high rates of intake are preferred ŽBlack and Kenney, 1984., a finding consistent with the hypothesis that herbivores maximize rates of food intake ŽBelovsky, 1978; Stephens and Krebs, 1986.. Plants also contain an array of biochemicals ŽProvenza, 1995a,b, 1996., and foods that meet needs for energy ŽBurritt and Provenza, 1992; Ralphs et al., 1995; Villalba and Provenza, 1996, 1997a,b. and nitrogen ŽVillalba and Provenza, 1997c. are preferred. Fasting affects preference ŽJung and Koong, 1985; Edwards et al., 1994; Newman et al., 1994., presumably by altering needs for macronutrients ŽWang and Provenza, 1996; Villalba and Provenza, 1999.. These findings for macronutrients are consistent with the hypothesis that preference originates from the interrelationship between taste and postingestive feedback, determined by an animal’s physiological state and a food’s chemical characteristics ŽProvenza, 1995a,b, 1996; Provenza et al., 1999.. Although a food’s structure and chemical composition and an animal’s nutritional state affect preference, interactions among these variables have not been explored. Under most conditions, food structure and quality are confounded because foods with high nutrient content are typically easy to harvest and highly digestible Že.g., green forage, low tensile strength., whereas plants with low nutrient content are usually difficult to harvest and poorly digestible Že.g., mature forage, high tensile strength.. Thus, we sought to determine how preference is affected by the interaction between a food’s physical and chemical characteristics and an animal’s nutritional state. To do so, we determined how preferences for foods that differed in macronutrients and physical form were affected by changes in the nutritional state of lambs fed different basal diets and fasted for different periods of time.

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2. Animals, materials and methods 2.1. Experimental subjects Eighteen Rambouillet–Columbia–Finn–Targhee crossbred wethers Ž27 kg. were penned individually at the Green Canyon Ecology Center, Utah State University, Logan. All animals had free access to mineral blocks and fresh water. Lambs were approximately three months of age at the beginning of the experiment and they had been raised together. All lambs had previous experience with alfalfa pellets and rolled barley grain. After being penned individually, lambs received alfalfa pellets Žad libitum. and rolled barley Ž300–400 g. daily for 10 days. 2.2. Intake behaÕiour 2.2.1. Period 1— lambs fed a balanced diet r 15-h fast Our first objective was to determine intake of barley Žrolled and ground. and alfalfa Žpelleted and ground. when lambs were fed a balanced basal diet. For 10 days, all lambs received a basal diet formulated to meet their needs for energy and protein ŽNRC, 1985; Table 1.. At 0800 h daily, lambs were fed rolled barley Ž400 g. and alfalfa pellets Žad libitum—approximately 2.5 kgrday. in separate containers. After 10 days of exposure to the balanced diet, we measured rates of food intake Ž4 days. and food preference Ž4 days., as outlined below ŽIndividual test foods.. During this 8-day period, refusals of the basal diet were collected at 1700 h and lambs were without food from 1700 h until 0800 h. 2.2.1.1. IndiÕidual test foods. At 0800 h, lambs were fed alfalfa or barley Žcompared to barley, alfalfa supplies less DE and more CP, Table 1. in two forms ŽTable 2., whole Žpelleted alfalfa and rolled barley. or ground Ž1–2 mm particle size., according to the following schedule: Day 1: Alfalfa pellets Day 2: Ground alfalfa pellets Day 3: Rolled barley grain Day 4: Ground barley grain

Table 1 Nutritional characteristics of foods and diets used in the study Ždry matter basis. Food

Crude proteina Ž%.

Digestible energy b ŽMJrkg.

Alfalfa Barley Diet Ep c Diet eP c

17.4 12.0 11.0 32.5

9.29 15.86 13.60 8.79

a

Nitrogen was determined by the Kjeldahl method ŽAOAC, 1975.. Crude protein calculated as N=6.25. Calculated values for digestible energy are based from values obtained from NRC Ž1985.. c Diet Ep was composed of Corn:Pomace:NaBentonite:NaBicarbonate Ž80.0:16.5:2.5:1.0., whereas diet eP was composed of Soybean Meal:Pomace Ž40:60.. b

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Table 2 Basal diets and test foods offered during the three feeding periods Feeding periods Period

Basal diet

Offered to

Fasting period before test foods

1 2

Balanced Unbalanced Ep eP Unbalanced Ep eP

All lambs

15-h Žovernight.

Group 1 Group 2

15-h Žovernight.

3

Group 1 Group 2

No fast: 1-h Preload Ep 1-h Preload eP

Test Foods Ž5 min tests.

IndiÕidual test foods Alfalfa Alfalfa Barley grain Barley grain

Physical Form

Day

Pellets Ground Rolled Ground

1 2 3 4

Ground–Ground Pellets–Ground Pellets–Rolled Ground–Rolled

5 6 7 8

Preference tests Alfalfa–Barley grain

The order in which foods and physical form combinations were presented was preestablished arbitrarily and was kept constant throughout the study so that the potential influence of consuming particular test foods in different physical forms on ensuing days of intake determinations was consistent across feeding periods. We recorded the time each lamb spent eating each food during 5 min. Lambs were observed individually and bouts of inactivity or behaviours not directed toward food mastication and deglutition were excluded from calculations of feeding time. We determined the amount of food ingested by lambs as the difference between the weight of food offered and that remaining after 5 min. Intake rates were calculated as food consumed divided by feeding time. 2.2.1.2. Preference tests. After measuring intake rates for 4 days, each lamb received alfalfa and barley simultaneously for 5 min ŽTable 2.. The foods were offered in two forms, whole Žpelleted alfalfa and rolled barley. or ground Ž1–2 mm particle size., according to the following schedule: Day 5: Ground alfalfa pellets and ground barley grain. Day 6: Alfalfa pellets and ground barley grain. Day 7: Alfalfa pellets and rolled barley grain. Day 8: Ground alfalfa pellets and rolled barley grain.

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We recorded time spent feeding and amount of food consumed Ždifference between the weight of food offered and that remaining after 5 min.. Intake rates for each food were calculated as the amount of food consumed divided by the time spent eating each food. 2.2.2. Period 2— lambs fed unbalanced diets, 15-h fast After we determined intake of alfalfa and barley when lambs were fed the balanced basal diet, we randomly allocated lambs to two groups Žnine per group. and fed them basal diets that differed in macronutrients: Group 1 received a basal diet ŽEp. high in digestible energy ŽDE. and low in crude protein ŽCP. Žground and mixed grape pomace:corn grain w20:80x; 13.8 MJrkg DE, 10% CP., whereas Group 2 received a basal diet ŽeP. low in DE and high in CP Ž8.8 MJrkg DE, 32.5% CP. ŽTable 1.. Lambs in each group received 200 g of the new diet initially, and this amount was increased by 200 g each day, while the amounts of barley Ž200 g. and alfalfa Ž400 g. fed as part of the original basal diet were reduced daily. The new diets were introduced gradually to minimize acidosis Ždiet Ep. and ammonia toxicity Ždiet eP.. Nevertheless, the day after offering 1000 g of Ep, lambs showed signs of acidosis Že.g., anorexia, diarrhea., so we suspended feeding of both diets for 7 days during which lambs received alfalfa pellets ad libitum. After 7 days, we again fed diet Ep, but with buffers Žsodium bentonite, 2.5%; sodium bicarbonate, 1%. added to prevent acidosis ŽTable 1.. We increased the amounts of Ep and eP offered by 200 g daily until lambs received 200 g of alfalfa pellets plus 1400 g of Ep ŽGroup 1. or eP ŽGroup 2. at 0800 h daily. During the next 10 days, lambs ate an average of 1400 grday of Ep, and 650 " 39.4 grday ŽMean " SE. of eP. We next Žafter 10 days. determined the effects of nutritional state on ingestive behaviour following a 15-h Žovernight. fast ŽTable 2.. We determined rates of food intake Ž4 days. and preference Ž4 days.. During this 8-day period, refusals of the basal diet were collected at 1700 h and lambs were without food from 1700 h until 0800 h the next day Ži.e., for 15-h.. 2.2.2.1. IndiÕidual test foods. At 0800 h all lambs received alfalfa or barley, and rates of food intake were calculated, as described previously for Period 1 ŽTable 2.. 2.2.2.2. Preference tests. After measuring intake for 4 days, all lambs received alfalfa and barley simultaneously to determine preference, as described previously for Period 1 ŽTable 2.. 2.2.3. Period 3— preloads We also wanted to determine the effects of the basal diet ŽEp or eP. on intake and preference without an overnight fast. After another 8 days of eating a basal diet of Ep or eP, lambs received a meal of their respective basal diets ŽGroup 1: Ep; Group 2: eP. for 1 h at 0800 h. Immediately following the meal, intake rates and preference tests for alfalfa and barley were determined, as described previously for Period 1 ŽTable 2.. Lambs consumed on average 422 " 34 g of diet Ep and 261 " 22 g of diet eP ŽMean " SE. during the 1 h preload.

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2.2.4. Preference for the same food in different physical forms Following the preload-trials, we determined preference for ground and whole foods after lambs were fed the balanced basal diet previously described ŽTable 1. for 7 days. After 7 days, all lambs were offered alfalfa pellets and ground alfalfa for 5 min at 0800 h on one day, and rolled and ground barley for 5 min at 0800 h the next day. Intake was determined as the difference between food offered and food that remained. 2.3. Data analyses For single-food tests, responses Žintake, calculated intakes of DE and CP, feeding time, intake rates. were analyzed as a split-plot design with lambs nested within basal

Table 3 Effects of basal diet on intake of barley, rate of barley intake, and active feeding time when barley was offered in two different physical forms for 5 min Measure

Feeding period

Physical form Rolled

SEM Ground

Group ŽBalanced diet. 1

2

1

2

Basal diet ŽUnbalanced dietrpreload.

Barley Intake Žg.

Energy Intake) ŽMJ.

Protein Intakea Žg.

Rate of Barley Intake Žgrmin. Rate of Energy Intake) ŽMJrmin. Rate of Protein Intakea Žgrmin. Active Feeding Time Žmin. a,b,c,d

Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload

Ep

eP

Ep

eP

261a 300a 175a 3.72 a 4.27 a 2.51a 28.4 a 32.7 a 19.1a 52.4 a 63.5a 44.2 a 0.75a 0.92 a 0.63 a 5.71a 6.92 a 4.82 a 4.9 a 4.7 a 3.4 a

269a 308a 339b 3.81a 4.39 a 4.81b 29.3 a 33.6 a 36.9 b 54.7 a 64.1a 71.2 b 0.79 a 0.92 a 1.00 b 5.96 a 6.99 a 7.76 b 4.9 a 4.9 a 4.8 b

170b 121b 147c 2.43 b 1.72 b 2.09 c 18.5 b 13.2 b 16.0 c 34.0 b 28.1b 32.1c 0.50 b 0.42 b 0.46 c 3.70 b 3.06 b 3.50 c 5.0 a 3.5 b 3.7 a

199c 197c 234d 2.85 c 2.80 c 3.31d 21.7 c 21.4 c 25.5d 40.1c 40.0 c 47.7 a 0.59 c 0.59 c 0.67 a 4.37 c 4.36 c 5.2 a 5.0 a 4.9 a 4.9 b

14.23

0.21

1.55

2.78

0.04

0.30

0.22

Means within a row lacking a common superscript letter differ Ž P - 0.05.. )Values of energy intake calculated as: wDigestible energy ŽMJrg. )barley intake Žg. or )rate of barley intake Žgrmin.x. a Values of protein intake calculated as: wg proteinrg of barley )barley intake Žg. or )rate of barley intake Žgrmin.x.

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diet ŽPeriod 1: balanced basal diet wGroups 1 and 2x; Periods 2 and 3: Ep wor Group 1x and eP wor Group 2x.. Basal diet was the between-subject factor, and day Žwhere different physical forms of test foods we.g., ground or wholex were offered. was the within-subject factor in the split-plot. For preference tests, responses Žfood intake, calculated intakes of DE and CP, feeding times, intake rates. were analyzed as a split-plot design with lambs nested within basal diet ŽPeriod 1: balanced basal diet wGroup 1 and 2x; Periods 2 and 3: Ep wor Group 1x and eP wor Group 2x.. Basal diet was the between-subject factor, and day Žwhere two foods we.g., alfalfa and barleyx were offered in a choice in different physical forms we.g., whole or groundx. was the within-subject factor. To compare initial responses Ži.e., when all lambs received the same balanced basal diet wPeriod 1x. to those obtained after subjecting the animals to two different unbalTable 4 Effects of basal diet on intake of alfalfa, rate of alfalfa intake, and active feeding time when alfalfa was offered in two different physical forms for 5 min Measure

Feeding period

Physical form Pelleted

SEM Ground

Group ŽBalanced Diet. 1

2

1

2

Basal diet ŽUnbalanced dietrpreload.

Alfalfa Intake Žg.

Energy Intake) ŽMJ.

Protein Intakea Žg.

Rate of Alfalfa Intake Žgrmin. Rate of Energy Intake) ŽMJrmin. Rate of Protein Intakea Žgrmin. Active Feeding Time Žmin.

Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload Balanced Diet Unbalanced Diet Preload

Ep

eP

Ep

eP

130 258 285 1.09 2.18 2.38 20.5 40.8 45.0 26.2 57.8 60.6 0.21 0.50 0.50 4.14 9.13 9.57 4.9 4.5 4.8

126 238 266 1.05 2.00 2.22 19.9 37.6 42.1 25.9 50.8 56.8 0.05 0.42 0.46 4.10 8.03 8.98 4.9 4.7 4.7

55 93 83 0.46 0.79 0.71 8.6 14.7 13.1 11.7 19.3 18.0 0.08 0.17 0.17 1.86 3.05 2.84 4.6 4.8 4.7

58 73 79 0.50 0.63 0.67 9.1 11.5 12.5 11.8 15.1 16.3 0.08 0.13 0.13 1.86 2.38 2.58 4.9 4.9 4.8

10.64

0.08

1.68

2.14

0.02

0.34

0.14

)Values of energy intake calculated as: wDigestible energy ŽMJrg. )alfalfa intake Žg. or )rate of alfalfa intake Žgrmin.x. a Values of protein intake calculated as: wg proteinrg of alfalfa )alfalfa intake Žg. or )rate of alfalfa intake Žgrmin.x.

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anced basal diets Že.g., Ep and eP wPeriods 2 and 3x. we also divided the lambs into two groups for Period 1 Ži.e., Groups 1 and 2.. These groups corresponded to the animals that received diets Ep ŽGroup 1. or eP ŽGroup 2. during Periods 2 and 3. Thus, we compared intake behaviour across different basal diets Žbalanced, unbalanced, preloads. fed in successive periods Ž1, 2, 3.. Period was a repeated measure in the model. For all analyses, means were compared by LSD when F-ratios were significant Ž P - 0.05..

3. Results 3.1. IndiÕidual foods 3.1.1. Barley Physical form of the food, basal diet, and length of fast interacted to affect intake of barley by lambs Žintake: F2,46 s 6.69; rate of intake: F2,46 s 4.29; P - 0.05.. The responses for DE and CP followed the same patterns we describe for barley. Physical form of the food affected intake. Averaged across all three periods, lambs ate more rolled than ground barley Ž276 vs. 178 g; 3.93 vs. 2.55 MJ DE; 30.0 vs. 19.4 g CP; F1,16 s 78.62; P - 0.001.. They also ate rolled barley at a higher rate than ground

Fig. 1. Intake of alfalfa and barley by two groups of lambs Ž ns9. during 5-min preference tests. Values are averages across two periods where lambs were fed unbalanced basal diets Ždiet Ep whigh proportion energyrproteinx, and diet eP wlow proportion energyrproteinx. with a 15-h fast ŽPeriod 2., and with no fast Žpreload for 1 h; Period 3.. Lambs fed Ep ate less barley and more alfalfa than lambs fed eP Ž P - 0.001., regardless of food form.

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barley Ž59 vs. 37 grmin; 0.84 vs. 0.54 MJrmin; 6.4 vs. 4.0 g CPrmin; F1,16 s 103.5; P - 0.001.. Basal diet also affected intake. Averaged across Periods 2 and 3, lambs fed Ep ate less barley than lambs fed eP Ž186 vs. 270 g; 2.64 vs. 3.83 MJ DE; 20.2 vs. 29.4 g CP; F2,46 s 11.99; P - 0.001.. They also ate barley at a lower rate than lambs fed eP Ž42 vs. 56 grmin; 0.61 vs. 0.79 MJrmin DE; 4.6 vs. 6.1 g CPrmin; F s2,46 s 9.58; P - 0.001.. The response to the basal diet depended on the length of time lambs were fasted. With a 15-h fast, lambs fed Ep ate less ground barley than lambs fed eP, and they also ate ground barley at a lower rate than lambs fed eP ŽTable 3.. Immediately after a preload of Ep or eP, the amount and rate of barley intake were lower for lambs fed Ep than for lambs fed eP, regardless of whether barley was rolled or ground ŽTable 3.. Feeding time did not differ between physical forms Žrolled vs. ground barley: 4.6 vs. 4.5 min, F1,16 s 1.02; P ) 0.05.. Averaged across Periods 2 and 3, lambs fed Ep spent less time feeding than lambs fed eP Ž3.8 vs. 4.9 min, F2,46 s 5.80; P - 0.05.. When lambs were fed the balanced diet ŽPeriod 1., time spent eating rolled or ground barley did not differ between groups ŽTable 3.. Following exposure to the unbalanced diets ŽPeriod 2., feeding time was lower for lambs fed Ep than for lambs fed eP when barley was ground ŽTable 3.. After a preload of Ep or eP ŽPeriod 3., feeding time was lower for lambs in Ep than eP, regardless of food structure ŽTable 3..

Fig. 2. Intake of alfalfa and barley by two groups of lambs Ž ns9. during 5-min preference tests. Values are for three periods where lambs were fed a balanced basal diet ŽG1 and G2. with a 15-h fast ŽPeriod 1., unbalanced basal diets Ždiet Ep whigh proportion energyrproteinx, and diet eP wlow proportion energyrproteinx. with a 15-h fast ŽPeriod 2., and unbalanced diets Ždiets Ep and eP. with no fast Žpreload for 1 h; Period 3.. When the basal diets were unbalanced, and especially after Period 3, lambs fed Ep consumed less barley and more alfalfa than lambs fed eP Ž P - 0.001..

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3.1.2. Alfalfa Averaged across the three periods, lambs ate more alfalfa pellets than ground alfalfa Ž215 vs. 73 g; 1.80 vs. 0.63 MJ; 33.9 vs. 11.6 g CP; F1,16 s 194.71; P - 0.001., and they ate pellets at higher rates Ž46 vs. 15 grmin; 0.38 vs. 0.13 MJrmin; 7.2 vs. 2.4 g CPrmin; F1,16 s 194.70; P - 0.001.. Feeding time did not differ for pelleted or ground alfalfa Ž4.7 vs. 4.8 min; F1,16 s 0.85; P ) 0.05; Table 4.. Basal diet did not affect the amount of alfalfa ingested, the rate of alfalfa intake, or the time spent feeding, as revealed by non-significant basal diet effects Ž F1,16 s 0.22; F1,16 s 0.47; F1,16 s 1.90, respectively; P ) 0.05., and basal diet by physical form by period interactions Ž F2,46 s 0.09; F2,46 s 0.23; F2,46 s 0.98, respectively, P ) 0.05; Table 4.. 3.2. Preference tests Averaged across the three periods, lambs preferred barley to alfalfa Ž188 vs. 40 g, F1,64 s 206.50; P - 0.001., and they spent more time feeding on barley than alfalfa Ž3.8 vs. 0.98 min, F1,64 s 217.30; P - 0.001.. Lambs also preferred whole to ground foods Žalfalfa pellets vs. ground alfalfa: 62 vs. 18 g; F1,64 s 6.71; P - 0.05; rolled vs. ground barley: 227 vs. 149 g; F1,64 s 15.90; P - 0.001.. The basal diet strongly affected preference for barley and alfalfa. Alfalfa was consumed more and barley less by lambs receiving Ep than by lambs receiving eP,

Fig. 3. Intake of energy and protein by two groups of lambs Ž ns9. during 5-min preference tests conducted on three consecutive feeding periods. Energy and protein intake were calculated by adding the digestible energy and crude protein consumed with each food during preference tests. Values are for three periods where lambs were fed a balanced basal diet ŽG1 and G2. with a 15-h fast ŽPeriod 1., unbalanced basal diets Ždiet Ep whigh proportion energyrproteinx, and diet eP wlow proportion energyrproteinx. with a 15-h fast ŽPeriod 2., and unbalanced diets Ždiets Ep and eP. with no fast Žpreload for 1 h; Period 3.. During Periods 2 and 3, lambs fed Ep consumed less energy than lambs fed eP Ž P - 0.001..

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regardless of food form Žbasal diet by test food by day interaction, F3,64 s 1.70; P ) 0.05; Fig. 1.. Averaged across Periods 2 and 3, lambs fed Ep ate less barley Ž112 vs. 233 g. and more alfalfa Ž101 vs. 14 g. than lambs fed eP Ž F1,64 s 61.28; P - 0.001., especially when alfalfa was pelleted and barley was ground or rolled ŽFig. 1.. Lambs fed eP distinctly preferred barley to alfalfa, even if barley was ground and alfalfa was pelleted, and the differences were even more pronounced when barley was rolled and alfalfa was pelleted or ground ŽFig. 1.. Lambs differed in preference for barley and alfalfa across the three periods Žbasal diet by day by period interaction, F2,232 s 45.59; P - 0.001.. During exposure to the balanced basal diet ŽPeriod 1., lambs strongly preferred barley to alfalfa. However, when the basal diet was unbalanced ŽPeriod 2., lambs fed Ep had a higher preference for alfalfa and a lower preference for barley than lambs fed eP. These differences were even more pronounced after preloads of Ep and eP ŽPeriod 3; Fig. 2.. Basal diet affected consumption of DE and CP during daily trials ŽFig. 3.. During Periods 2 Žunbalanced diet. and 3 Žpreloads. lambs fed Ep consumed less energy during preference tests than lambs fed eP Ž2.44 vs. 3.42 MJ; F2,108 s 10.67; P - 0.001.. Intake of energy decreased across periods during preference tests for lambs fed diet Ep, whereas intake of energy increased across periods for lambs fed diet eP Žbasal diet by period interaction F2,108 s 10.67; P - 0.001; Fig. 3.. All lambs consumed similar amounts of protein, but protein intake tended to increase across periods for lambs fed Ep

Fig. 4. Time spent eating alfalfa and barley by two groups of lambs Ž ns9. during 5-min preference tests. Values are averages across two periods where lambs were fed unbalanced basal diets Ždiet Ep whigh proportion energyrproteinx, and diet eP wlow proportion energyrproteinx. with a 15-h fast ŽPeriod 2., and with no fast Žpreload for 1 h; Period 3.. Lambs fed Ep spent more time eating alfalfa and less time eating barley than lambs fed eP Ž P - 0.001., regardless of food form.

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Žbasal diet by period interaction F2,108 s 1.95; P s 0.14; Fig. 3.. These differences in intake of energy and protein by lambs in the two groups led to a steady increase in the ratio of proteinrenergy selected by lambs fed the balanced basal diet and subsequently diet Ep Ž7.9, 11.7, 13.9 g CPrMJ., and a constant ratio of proteinrenergy selected by lambs fed the balanced basal diet and subsequently diet eP Ž7.6, 8.1, 8.1 g CPrMJ. during Periods 1, 2, and 3, respectively Žbasal diet by period interaction F2,108 s 24.77, P - 0.001.. Thus, lambs fed Ep selected barley and alfalfa in amounts that led to a higher ratio of proteinrenergy in their diet than lambs fed eP Ž12.7 vs. 8.2 g CPrMJ DE; F1,16 s 18.31; P - 0.001.. Feeding time was not affected by the physical form of the food Žpelleted vs. ground alfalfa: 1.12 vs. 0.84 min; F1,64 s 2.43; P ) 0.05; rolled vs. ground barley: 3.73 vs. 3.76 min, F1,64 s 0.0003; P ) 0.05., but the basal diet affected time spent eating alfalfa and barley. Averaged across Periods 2 and 3, lambs fed Ep spent more time eating alfalfa Ž2.51 vs. 0.27 min. and less time eating barley Ž1.98 vs. 4.54 min. than lambs fed eP Ž F1,64 s 83.44; P - 0.001; Fig. 4.. Lambs offered a balanced basal diet ŽPeriod 1. and subsequently diet Ep ŽPeriods 2 and 3. fed more on alfalfa and less on barley, whereas lambs offered a balanced basal diet ŽPeriod 1. and subsequently diet eP ŽPeriods 2 and 3. did not change time spent eating alfalfa and barley across periods Žbasal diet by day by period interaction; F2,232 s 51.51; P - 0.001; Fig. 5..

Fig. 5. Time spent eating alfalfa and barley by two groups of lambs Ž ns9. during 5-min preference tests. Values are for three periods where lambs were fed a balanced basal diet ŽG1 and G2. with a 15-h fast ŽPeriod 1., unbalanced basal diets Ždiet Ep whigh proportion energyrproteinx, and diet eP wlow proportion energyrproteinx. with a 15-h fast ŽPeriod 2., and unbalanced diets Ždiets Ep and eP. with no fast Žpreload for 1 h; Period 3.. Lambs offered a balanced basal diet and subsequently diet Ep increased the time spent eating alfalfa and decreased the time spent eating barley, whereas lambs offered a balanced basal diet and subsequently diet eP did not change time spent eating alfalfa and barley across periods Ž P - 0.001..

Food

Physical form

Alfalfa:

Ground

Pelleted

Pelleted

Ground

Average group

Barley:

Ground

Ground

Rolled

Rolled

1

Group ŽBalanced basal diet. 1

2

1

2

Basal diet 2

1

2

1

2

Ep

eP

44.9 a 61.4 a 58.3 a

47.6 b 50.4 b 52.5 b

Basal diet ŽUnbalanced dietrpreload.

Barley Diet Balanced Unbalanced Preload SEM Alfalfa Diet Balanced SEM Unbalanced SEM Preload SEM a,b

Ep

eP

Ep

eP

Ep

eP

Ep

eP

35.1 44.3 43.0

34.7 37.1 40.7

35.0 48.9 47.9

37.7 46.0 39.9

53.6 69.5 82.0

57.7 61.1 65.2

55.8 79.2 66.2

60.1 57.2 64.4

2.57

– – 18.6 21.1 22.5 16.7

15.0 – 13.7 – 20.0 –

– – 59.2 19.2 69.4 16.7

24.0 33.3 63.4 27.2 48.1 33.3

28.9 – 66.1 19.2 64.4 16.7

Means within a row lacking a common superscript letter differ Ž P - 0.05..

1.30

– – 58.5 – 53.9 –

18.5 – 18.6 17.8 56.1 15.7

– – 19.4 – 24.3 33.3

23.7 33.3 40.6 9.6 53.2 8.2

21.0 27.2 38.8 19.3 36.4 19.3

J.J. Villalba, F.D. ProÕenzar Applied Animal BehaÕiour Science 63 (1999) 145–163

Table 5 Rate of intake of barley and alfalfa Žgrmin. during 5 min preference tests

157

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Rates of barley intake were higher for lambs in Group 2 than Group 1 when the basal diet was balanced ŽPeriod 1., but this reversed when the basal diets were unbalanced ŽPeriod 2., and after a preload ŽPeriod 3. Žbasal diet by period interaction, F2,91 s 6.50; P - 0.05; Table 5.. Rates of barley intake were lower when barley was ground than when it was rolled Ž41 vs. 64 grmin; F1,48 s 221.10; P - 0.001; Table 5.. Rates of alfalfa intake were not compared statistically due to the large number of 0 values recorded for alfalfa intake, as a consequence of its low preference relative to barley, especially when lambs were fed a balanced diet and diet eP ŽTable 5.. Only 15 of 108 possible values could be calculated for lambs receiving a balanced basal diet and subsequently diet eP during the four preference tests during the three consecutive periods. 3.3. Preference for the same food in different physical forms Lambs consumed more pelleted than ground alfalfa Ž91 vs. 41 g; F1,16 s 7.45; P - 0.05., and they ate more rolled than ground barley Ž205 vs. 82 g; F1,16 s 6.18; P - 0.05.. 4. Discussion 4.1. Nutritional state The rate of food intake, defined as the product of bite rate and bite mass ŽAllden and Whittaker, 1970., is generally regarded only as a function of the interaction between animal morphology Žmouth size and shape, mastication and swallowing. and sward structure Žtiller density and height. ŽLaca et al., 1994; Ungar, 1996.. As a result, mathematical formulations of the relationship between intake rate and food supply Že.g., functional responses. have not accounted for the interaction between an animal’s nutritional state and a food’s nutritional quality and availability ŽDemment and Laca, 1994; O’Reagain et al., 1996.. Nevertheless, in our study intake rate was affected by a lamb’s physiological state and a food’s nutritional characteristics. Lambs fed an energy-rich basal diet ŽEp. spent less time feeding on barley and consumed less barley at lower rates than lambs fed a basal diet with a higher proteinrenergy ratio ŽeP., even after an overnight fast; the differences were even more pronounced immediately after preloads of Ep or eP. Thus, even when rate of food intake has been mainly attributed to physical structure Že.g., Wilman et al., 1996., the chemical composition of the food can have a significant impact on intake as well. Bite rate decreases when the nutritional quality of the herbage Že.g., concentration of soluble carbohydrates. declines ŽBirrell, 1989., even when food availability is high ŽO’Reagain and Owen-Smith, 1996., which suggests that animals may forage at rates lower than their potential ŽOwen-Smith, 1993.. Cattle and sheep achieve substantially higher intake rates than expected from sward structure alone, apparently related to their nutritional state and the food’s nutritional quality ŽO’Reagain et al., 1996.. When grazing in monocultures Že.g., grass or clover. sheep also alter grazing time in relation to their physiological state and intake rates ŽPenning et al., 1995..

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Lambs fed a basal diet of Ep ate more alfalfa and less barley than lambs fed eP. Compared to barley, alfalfa is higher in protein and lower in energy ŽTable 1.. Thus, by modifying the proportion of alfalfa and barley selected during preference tests, lambs fed Ep selected higher ratios of proteinrenergy during preference tests than lambs fed eP ŽFig. 3.. When offered an appropriate choice, lambs maintain a constant ratio of protein to energy in their diet ŽEgan, 1980; Provenza et al., 1996., and they typically show stronger preferences for energy Žin grams of starch. than for protein Žin grams of crude protein. ŽWang and Provenza, 1996; Villalba and Provenza, 1999.. The high preference for energy over protein is presumably because ruminants require nearly five times more energy Žin grams of starch. than protein Žin grams of crude protein. on a daily basis ŽNRC, 1985., they can readily store excess energy, and because they are so efficient at recycling nitrogen ŽOwens, 1988.. As a result, intake of protein is limited to a greater extent than energy ŽWebster, 1993., a result observed in tests comparing the effects of protein and energy on preference ŽVillalba and Provenza, 1999.. In addition, soluble carbohydrates in barley are readily fermented and can decrease food intake ŽBaile and Forbes, 1974., which may have lowered barley intake by lambs fed the energy-dense basal diet ŽEp.. Collectively, our results suggest that lambs displayed macronutrient-specific food preferences, which is consistent with other findings. Sheep fed six isocaloric foods that varied in protein select a diet which maximized growth rate by avoiding foods with too high or too low proportions of proteinrenergy ŽKyriazakis and Oldham, 1993.. Lambs’ preference for energy Žprotein. decreases after a meal high in energy Žprotein., whereas their preference for energy Žprotein. increases after a meal high in protein Ženergy. ŽVillalba and Provenza, 1999, unpublished data.. 4.2. Effects of fasting Fasting increases subsequent rates of food intake ŽEdwards et al., 1994; Newman et al., 1994., and consistent with previous studies, fasted lambs fed an energy-rich diet ŽEp. had higher rates of intake of rolled barley than non-fasted lambs. The response of the non-fasted lambs can be attributed to the satiating effects of the energy-dense preload ŽEp.; preloads of energy-rich foods attenuate preferences for flavours of foods previously paired with starch ŽVillalba and Provenza, 1999.. Contrary to previous findings, we found that non-fasted lambs fed the nitrogen-rich diet ŽeP. had higher rates of intake of barley than fasted lambs. The increased rate of eating by non-fasted lambs is likely due to an increased need to balance the excess nitrogen supplied by the nitrogen-rich preload ŽeP. with soluble carbohydrates present in the energy-dense test food Žbarley; see energy intake values in Table 3.. Lambs balance the ingestion of rumen degradable protein with soluble carbohydrates ŽKyriazakis and Oldham, 1997., and they increase preferences for non-protein sources of nitrogen when the basal diet supplies a synchronous source of fermentable carbohydrates ŽVillalba and Provenza, 1997c.. We also found that diet selection varied between groups of equally fasted animals Ž15 . h fed basal diets of different nutritional characteristics Ži.e., energy- or nitrogen-rich.. Fasted lambs fed Ep consumed less barley and more alfalfa than equally fasted lambs

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fed eP, and the differences were even more pronounced when lambs were given preloads of eP or Ep. Thus, our results suggest that fasting not only affects the rate of food intake ŽGreenwood and Demment, 1988; Dougherty et al., 1989., but food selection as well ŽNewman et al., 1994.. Moreover, fasting did not cause a general increase in eating. Intake of alfalfa was lower in fasted than non-fasted lambs fed Ep, whereas intake of barley was higher in fasted than non-fasted lambs fed Ep, but similar for fasted and non-fasted lambs fed eP. Balancing the supply of soluble carbohydrates and nitrogen optimizes protein synthesis and maximizes the retention of nitrogen in ruminants ŽSinclair et al., 1993.. In contrast, lambs fed eP maintained an almost absolute preference for barley, and thus an almost constant ratio of proteinrenergy, which was maintained after preloads of eP. Collectively, our results show that besides the fastingrnon-fasting dichotomy examined in previous studies Že.g., Greenwood and Demment, 1988; Newman et al., 1994., the specific nutritional characteristics of the basal diet also affect rates of food intake and preferences by herbivores. 4.3. Effects of physical structure on food intake preference Lambs ingested barley and alfalfa at higher rates when foods were whole than when they were ground, which is consistent with predictions from optimality models ŽStephens and Krebs, 1986; Newman et al., 1995. and experimental analyses Že.g., Kenney and Black, 1984. that suggest herbivores forage to optimize rate of food intake. Lambs also preferred whole foods Žhigher rates of intake. to ground foods Žlower rates of intake., which is consistent with previous findings ŽKenney and Black, 1984; Black and Kenney, 1984.. Nevertheless, even though diet selection has been explained as a strategy to maximize intake rate ŽBelovsky, 1978; Ungar and Noy-Meir, 1988; Parsons et al., 1994b., little experimental evidence fully supports this assertion ŽIllius and Hodgson, 1996.. Herbage height is an important determinant of rate of food intake Že.g., Ungar et al., 1991; Laca et al., 1992., but tall ŽBazely, 1990. or short ŽIllius et al., 1987. sward patches may be preferred when their nutritional quality is higher. In our study, the nutritional state of the lambs and the chemical composition of the foods overroad the importance of intake rate on food preference. When the quality of the basal diet ŽEp, eP. and test foods Že.g., barley, alfalfa. differed Žhigh and low proportions proteinrenergy., physiological state and food quality were more important than food structure Žwhole vs. ground foods., and the rate at which foods were ingested Žwhole foods faster than ground foods.. Lambs in different states maintained the same pattern of preference for foods with high Žalfalfa. or low Žbarley. proportions of proteinrenergy, regardless of food structure. Even when the energy-rich test food Žbarley. was consumed at its highest rate Žrolled., lambs on an energy-rich basal diet consumed less barley and more ground alfalfa than lambs on a protein-rich basal diet. 5. Conclusion Food quality Že.g., Laca et al., 1992. and structure Že.g., Villalba and Provenza, 1999. have been controlled in previous studies to determine the effects of a food’s physical

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and chemical characteristics on preference, but the interaction between these two variables has not been explored ŽHodgson et al., 1999.. Our results suggest that nutritional state influences diet selection to a degree that may diminish the importance of a food’s structure on intake rate. Thus, we suggest that state-dependent food choices should be considered in conjunction with the effects of fasting to better understand food selection. Food structure affected the amount of food lambs consumed, but food choice was affected to a greater extent by physiological state than by food structure. The influence of food quality and structure operate along a continuum. When food quality is similar, structure becomes an important determinant of food choice. In contrast, when food quality is different and the need for particular nutrients increasesrdecreases, the biochemical composition of a food becomes crucial in food selection.

Acknowledgements This research was supported by grants from the Cooperative States Research, Education, and Extension Service; United States Department of Agriculture and the Utah Agric. Expt. Stn. This paper is published with the approval of the Director, Utah Agric. Expt. Stn., Utah State Univ. as Journal paper no. 7043. The authors gratefully acknowledge J. Witt and J. Rogosic for help in data collection.

References Allden, W.G., Whittaker, I.A.McD., 1970. The determinants of herbage intake by grazing sheep: the interrelationships of factors influencing herbage intake and availability. Aust. J. Agric. Res. 21, 755–766. AOAC ŽAssociation of Official Analytical Chemists., 1975. Official methods of analysis of the Association of Official Analytical Chemists, 12th edn. Washington, DC, USA. Baile, C.A., Forbes, J.M., 1974. Control of feed intake and regulation of energy balance in ruminants. Physiol. Rev. 54, 160–214. Bazely, D.R., 1990. Rules and cues used by sheep foraging in monocultures. In: Hughes, R.N., ŽEd.., Behavioural Mechanisms of Food Selection. NATO ASI Series, Vol. G20, Springer-Verlag, Berlin, pp. 343–367 Belovsky, G.E., 1978. Diet optimization in a generalist-herbivore: the moose. Theor. Popul. Biol. 14, 105–134. Birrell, H.A., 1989. The influence of pasture and animal factors on the consumption of pasture by grazing sheep. Aust. J. Agric. Res. 40, 1261–1275. Black, J.L., Kenney, P.A., 1984. Factors affecting diet selection by sheep: II. Height and density of pasture. Aust. J. Agric. Res. 35, 565–578. Burritt, E.A., Provenza, F.D., 1992. Lambs form preferences for nonnutritive flavors paired with glucose. J. Anim. Sci. 70, 1133–1136. Demment, M.W., Laca, E.A., 1994. Reductionism and synthesis in the grazing sciences: models and experiments. Proc. Aust. Soc. Anim. Prod. 20, 6–16. Dougherty, C.T., Bradley, N.W., Cornelius, P.L., Lauriault, L.M., 1989. Short-term fasts and the ingestive behaviour of grazing cattle. Grass and Forage Sci. 44, 295–302. Dove, H., 1996. Constraints to the modelling of diet selection and intake in the grazing ruminant. Aust. J. Agric. Res. 47, 257–275. Edwards, G.R., Newman, J.A., Parsons, A.J., Krebs, J.R., 1994. Effects of the scale and spatial distribution of the food resource and animal state on diet selection: an example with sheep. J. Anim. Ecol. 63, 816–826.

162

J.J. Villalba, F.D. ProÕenzar Applied Animal BehaÕiour Science 63 (1999) 145–163

Egan, A.R., 1980. Host animal–rumen relationships. Proc. Nutr. Soc. 39, 79–87. Greenwood, G.B., Demment, M.W., 1988. The effect of fasting on short-term cattle grazing behaviour. Grass and Forage Sci. 43, 377–386. Hodgson, J., Cosgrove, G.P., Woodward, S.J.R., 1999. Research on foraging strategy: progress and priorities. Proceedings of the XVIII International Grassland Congress, in press. Illius, A.W., Gordon, I.J., 1993. Diet selection in mammalian herbivores: constraints and tactics. In: Hughes, R.N., ŽEd.., Diet Selection: An Interdisciplinary Approach to Foraging Behaviour. Blackwell, Oxford, pp. 157–180. Illius, A.W., Hodgson, J., 1996. Progress in understanding the ecology and management of grazing systems. In: Hodgson, J., Illius, A.W., ŽEds.., The Ecology and Management of Grazing Systems. Commonwealth Agricultural Bureau International, Wallingford, UK, pp. 429–457. Illius, A.W., Wood-Gush, D.G.M., Eddison, J.C., 1987. A study of the foraging behaviour of cattle grazing patchy swards. Biol. Behav. 12, 33–44. Jung, H.G., Koong, L.J., 1985. Effects of hunger satiation on diet quality by grazing sheep. J. Range Manage. 38, 302–305. Kenney, P.A., Black, J.L., 1984. Factors affecting diet selection by sheep: I. Potential intake rate and acceptability of feed. Aust. J. Agric. Res. 35, 551–563. Kyriazakis, I., Oldham, J.D., 1993. Diet selection in sheep: the ability of growing lambs to select a diet that meets their crude protein Žnitrogen=6.25. requirements. Br. J. Nutr. 69, 617–629. Kyriazakis, I., Oldham, J.D., 1997. Food intake and diet selection of sheep: the effect of manipulating the rates of digestion of carbohydrates and protein of the foods offered as a choice. Br. J. Nutr. 77, 243–254. Laca, E.A., Ungar, E.D., Seligman, N., Demment, M.W., 1992. Effects of sward height and bulk density on bite dimensions of cattle grazing homogeneous swards. Grass and Forage Sci. 47, 91–102. Laca, E.A., Ungar, E.D., Demment, M.W., 1994. Mechanisms of handling time and intake rate of a large mammalian grazer. Appl. Anim. Behav. Sci. 39, 3–19. NRC ŽNational Research Council., 1985. Nutrient Requirements of Sheep. National Academy Press, Washington, DC, USA. Newman, J.A., Penning, P.D., Parsons, A.J., Harvey, A., Orr, R.J., 1994. Fasting affects intake behaviour and diet preference of grazing sheep. Anim. Behav. 47, 185–193. Newman, J.A., Parsons, A.J., Thornley, J.H.M., Penning, P.D., Krebs, J.R., 1995. Optimal diet selection by a generalist grazing herbivore. Functional Ecol. 9, 255–268. Owen-Smith, R.N., 1993. Evaluating optimal diet models for an African browsing ruminant, the kudu: how constraining are the assumed constraints?. Evol. Ecol. 7, 499–524. O’Reagain, P.J., Owen-Smith, R.N., 1996. Effect of species composition and sward structure on dietary quality in cattle and sheep grazing South African sourveld. J. Agric. Sci. ŽCamb.. 127, 261–270. O’Reagain, P.J., Goetsch, B.C., Owen-Smith, R.N., 1996. Effect of species composition and sward structure on the ingestive behaviour of cattle and sheep grazing South African sourveld. J. Agric. Sci. ŽCamb.. 127, 271–280. Owens, F.N., 1988. Protein metabolism of ruminant animals. In: Church, D.C., ŽEd.., The Ruminant Animal. Digestive Physiology and Nutrition. Prentice Hall, NJ, USA, pp. 227–249. Parsons, A.J., Newman, J.A., Penning, P.D., Harvey, A., Orr, R.J., 1994a. Diet preference of sheep: effects of recent diet, physiological state and species abundance. J. Anim. Ecol. 63, 465–478. Parsons, A.J., Thornley, J.H.M., Newman, J.A., Penning, P.D., 1994b. A mechanistic model of some physical determinants of intake rate and diet selection in a two-species temperate grassland sward. Functional Ecol. 8, 187–204. Penning, P.D., Parsons, A.J., Orr, R.J., Harvey, A., Champion, R.A., 1995. Intake and behaviour responses by sheep, in different physiological states, when grazing monocultures of grass or white clover. Appl. Anim. Behav. Sci. 45, 63–78. Provenza, F.D., 1995a. Postingestive feedback as an elementary determinant of food preference and intake in ruminants. J. Range Manage. 48, 2–17. Provenza, F.D., 1995b. Role of learning in food preferences of ruminants: Greenhalgh and Reid revisited. In: Engelhardt, W.V., Leonhard-Marek, S., Breves, G., Giesecke, D., ŽEds.., Ruminant Physiology: Digestion, Metabolism, Growth and Reproduction. Proceedings of the VIII International Symposium on Ruminant Physiology. Ferdinand Enke Verlag, Stuttgart, Germany, pp. 233–247.

J.J. Villalba, F.D. ProÕenzar Applied Animal BehaÕiour Science 63 (1999) 145–163

163

Provenza, F.D., 1996. Acquired aversions as the basis for varied diets of ruminants foraging on rangelands. J. Anim. Sci. 74, 2010–2020. Provenza, F.D., Scott, C.B., Phy, T.S., Lynch, J.J., 1996. Preference of sheep for foods varying in flavors and nutrients. J. Anim. Sci. 74, 2355–2361. Provenza, F.D., Villalba, J.J., Augner, M., 1999. The Physics of Foraging. Proceedings of the XVIII International Grassland Congress, in press. Ralphs, M.H., Provenza, F.D., Wiedmeyer, R., Bunderson, F.B., 1995. The effects of energy source and food flavor on conditioned preferences in sheep. J. Anim. Sci. 73, 1651–1657. Sinclair, L.A., Garnsworthy, P.C., Newbold, J.R., Buttery, P.J., 1993. Effect of synchronizing the rate of dietary energy and nitrogen release on rumen fermentation and microbial protein synthesis in sheep. J. Agric. Sci. ŽCamb... 120, 251–263. Stephens, D.W., Krebs, J.R., 1986. Foraging Theory. Princeton Univ. Press, NJ, USA. Ungar, E.D., 1996. Ingestive behaviour. In: Hodgson, J., Illius, A.W., ŽEds.., The Ecology and Management of Grazing Systems, Commonwealth Agricultural Bureau International, Wallingford, UK, pp. 185–218. Ungar, E.D., Noy-Meir, I., 1988. Herbage intake in relation to availability and sward structure: grazing processes and optimal foraging. J. Appl. Ecol. 25, 1045–1062. Ungar, E.D., Genizi, A., Demment, M.W., 1991. Effects of sward height, bulk density and height heterogeneity on bite dimensions and the short-term intake rate of cattle. Agron. J. 83, 973–978. Villalba, J.J., Provenza, F.D., 1996. Preference for flavored wheat straw by lambs conditioned with intraruminal administrations of sodium propionate. J. Anim. Sci. 74, 2362–2368. Villalba, J.J., Provenza, F.D., 1997a. Preference for wheat straw by lambs conditioned with intraruminal infusions of starch. Br. J. Nutr. 77, 287–297. Villalba, J.J., Provenza, F.D., 1997b. Preference for flavored wheat straw by lambs conditioned with intraruminal administrations of propionate and acetate. J. Anim. Sci. 75, 2905–2914. Villalba, J.J., Provenza, F.D., 1997c. Preference for flavored foods by lambs conditioned with intraruminal administrations of nitrogen. Br. J. Nutr. 78, 545–561. Villalba, J.J., Provenza, F.D., 1999. Nutrient-specific preferences by lambs conditioned with intraruminal infusions of energy, nitrogen and water. J. Anim. Sci., in press. Wang, J., Provenza, F.D., 1996. Food preference and acceptance of novel foods by lambs depend on the composition of the basal diet. J. Anim. Sci. 74, 2349–2354. Webster, A.J.F., 1993. Energy partitioning, tissue growth and appetite control. Proc. Nutr. Soc. 52, 69–76. Wilman, D., Mtengeti, E.J., Moseley, G., 1996. Physical structure of twelve forage species in relation to rate of intake by sheep. J. Agric. Sci. 126, 277–285.