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Amount of experience and prior illness affect the acquisition and persistence of conditioned food aversions in lambs
APPLIED ANIMAL BEHAVIOUR SCIENCE
ELSEVIER
Applied Animal Behaviour Science 48 (1996) 73-80
Amount of experience and prior illness affect the acquisition and persistence of conditioned food aversions in lambs Elizabeth A. Burr&t *, Frederick D. Provenza Department of Rangeland Resources, Utah State University, Lagan, UT 84322, USA Accepted 8 November 1995
Abstract
We studied the acquisition of food aversions in sheep. In the first experiment, we investigated how prior illness affected the formation of a conditioned food aversion. Lambs were offered small amounts ( < 200 g) of novel foods (wheat and rice) for 12 days, until lambs ate both foods readily. To establish an aversion to one of the foods, lambs were then given the toxin lithium chloride (LiCl) after eating one of the foods. On subsequent days, lambs were allowed to eat the food previously paired with LiCl until the aversion extinguished. When lambs later consumed a meal of wheat and rice and received LiCl, they avoided the food that had been paired with LiCI, but they did not reduce intake of the other food (P < 0.001). In the second experiment, we examined how amount of experience with a food affected the acquisition and persistence of a food aversion. Lambs were offered oats (novel food) at 1% body weight for either 28, 14, 7, or 1 days. An aversion was then induced by offering oats followed by LiCl. Lambs with at least 14 days exposure to novel foods formed the least persistent aversions (P = 0.002). Furthermore, the aversion to oats extinguished within 6 days in lambs offered oats for 7 days. Thus, lambs are not likely to abstain from a food they were trained to avoid (e.g. trees in orchards, poisonous plants) if they have more than 7 days experience with the food. Finally, lambs should be fed small amounts of highly nutritious novel foods (e.g. grains) during the first 7 days to avoid gastro-intestinal distress and more persistent aversions. Keywords: Aversion; Lithium chloride; Feeding; Sheep; Learning
??
Corresponding
author.
016%1591/96/$15.00 8 1996 Elsevier Science B.V. All rights reserved PII 0168-1591(95)01004-1
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E.A. Burritt,
F.D. Provenza /Applied
Animal
Behaviour
Science 48 (19%)
73-80
1. Introduction Animals probably acquire aversions to foods repeatedly during their lives (Provenza, 1995, Provenza, 1996). Aversions to nutritious foods are likely manifest as cyclical patterns of intake over days, for instance when ruminants eat nutritious foods like larkspur that contain toxic alkaloids (Olsen and Ralphs, 1986; Pfister et al., 1994) or when they eat high-energy foods like grain that are rapidly fermentable (Huber, 1976; Britton and Stock, 1987). Positive postingestive effects from nutrients cause intake to increase (Villalba and Provenza, 1996a; Villalba and Provenza, 1996b), but toxins in plants, and excess production of organic acids from starch digestion in grain, cause malaise which causes intake to decline when foods are eaten too frequently or in excess (Provenza et al., 1994a; Provenza, 1995, Provenza, 1996). Thus, aversions to nutritious foods are typically transient (Garcia, 1989) and often manifest by cyclic patterns of eating over days or weeks. Curiously, it is not known how ruminants identify which harmful familiar plants (e.g. larkspur) to stop eating after a meal of several foods. One explanation is that animals acquire aversions to foods that made them ill in the past. Our first objective was to discover which food(s) lambs avoid when they experience illness after a meal of two foods, one that made them ill previously and one that did not. Animals may also acquire less persistent aversions to nutritious foods like larkspur and grain as their familiarity with the foods increases. Thus, our second objective was to assess how the amount of experience with a novel food affects the formation and persistence of an acquired food aversion.
2. Materials and methods 2.1. Prior illness Twenty 4-month-old lambs were conditioned to eat wheat and rice, grains novel to them. Lambs were offered 200 g of wheat and rice on alternate days during Days 1 to 4 of the trial. On Day 5, lambs were offered a choice between 150 g of each of the grains for 15 min. Six lambs preferred wheat, one preferred rice, and the other 13 lambs ate both grains. On Days 6 and 7, lambs were offered 200 g of rice and wheat, respectively. On Days 8 to 11, lambs were offered 100 g of each grain, and on Day 12, they were offered and ate 175 g of each grain within 15 min. Lambs ate similar amounts of wheat and rice (1569 and 1533 g) during conditioning. On Day 13, ten lambs ate 250 g of wheat and ten lambs ate 250 g of rice in 15 min, and then received a mild dose of lithium chloride (LiCl; 150 mg kg-’ body weight (BW)), a non-lethal gastrointestinal poison, in a gelatin capsule. To test for an aversion on Day 14, lambs were offered a choice between 200 g of rice and 200 g of wheat for 15 min. To extinguish the aversion, we fed lambs 200 g of the food previously paired with LiCl; 1 to 6 days were required for lambs to eat 200 g of grain in 15 min, at which time we considered the aversion extinguished. Lambs were fed the same amount of both grams during extinction. When all lambs ate both foods readily, lambs were given 200 g of wheat and rice for 15 min. All lambs ate all of both foods and then received a mild dose of LiCl (150 mg
E.A. Burrin,
F.D. Provenza /Applied
Animal
Behaviour
Science 48 (1996)
73-80
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kg-’ SW> to insure they did not acquire an aversion to both foods. For the next 4 days, lambs could choose between 200 g each of wheat and rice for 15 min day-‘. Lambs were deleted from the trial if they did not acquire an aversion to the LiCl-paired food (three lambs from the rice-averse group) or if the aversion persisted longer than 6 days (one from each group). Lambs were fed alfalfa pellets CAP) ad libitum during conditioning and testing, except on days when LiCl was given, in which case they received AP 2 h after they received LiCl. All lambs had access to salt and water ad libitum. The repeated measures of analysis variance (Winer, 1971) had group (LiCl-paired with wheat or rice) as the main effect and lambs (n = 6 for LiCl-paired rice and n = 9 for LiCl-paired wheat) were nested within groups. Food (wheat and rice) was a subplot in the analysis and was crossed with lambs. The trial lasted 5 days. 2.2. Experience with a novel food Forty 4-month-old lambs were divided into five groups of eight lambs each and penned individually. Each group was fed oats (a novel food) at 1% BW for 42, 28, 14, 7, or 1 day(s). During conditioning, lambs received AP ad libitum for 2 h day-‘, 30 min after receiving oats. During the last 4 days of conditioning, lambs received AP ad libitum for 1 h day - ‘. Four of the lambs in each of the 42- and 28-day groups refused to eat oats at 1% BW and were deleted from the trial. Because of small sample size, these two groups were combined to form one group of lambs that ate oats for at least 28 days. One lamb in the 16day group did not eat oats and was removed from the trial. After conditioning, lambs received oats at 1% BW for 15 min and then received a strong dose of LiCl (200 mg kg-t) to insure that even lambs most familiar with oats would acquire an aversion. Lambs then received AP ad libitum for 1 h. The following 7 days at 08:OO h, lambs received oats at 1% BW for 15 min followed by AP ad libitum for 1 h. Intake was measured and expressed as %BW. The repeated measures analysis of variance had four treatments (1, 7, 14, 28 days) and lambs (n = 7 or 8) were nested within treatments. Foods (oats and AP) were analyzed separately. Intake was expressed as a percentage of BW in the analyses. The trial lasted 8 days. To further elucidate how exposure to foods affected the formation of food aversions, the data for treatments 1 and 14 days were expressed as a percent of food eaten prior to receiving LiCl, and analyzed as repeated measures with food (oats and AP) as the main effect. Lambs (n = 7 or 8) were crossed with food. Treatments (1 and 14 days only) were analyzed separately.
3. Results 3.1. Prior illness Lambs ate less of the food previously paired with LiCl (Fig. l(a) and (b); F = 6.8, df = 3,78, P < O.OOl), and they responded similarly to both rice and wheat (F = 0.28,
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Day Fig. 1. Intake (g) of wheat and rice by lambs. Rice or wheat ingestion by lambs was previously paired with lithium chloride (150 mg kg-’ BW). Bars represent LSD,,,.
df = 1,13, P = 0.61). Lambs increased consumption of the grain that had been paired with LiCl during the 4 days after receiving LiCl a second time. Seven lambs consumed 200 g each of wheat and rice the day after receiving the first dose of LiCl. Immediately after consuming both foods on Day 2, lambs received a second dose of LiCl. All seven lambs formed an aversion to the grain previously paired with LiCl after the second dose of LiCl.
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F.D. Prouenza/Applied
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Day Fig. 2. Intake (%BW) of oats by lambs after receiving lithium chloride (200 mg kg-’ BW). Lambs had either 1, 7, 14, or 28 days experience eating oats at 1% BW. LSD,,,,,,, = 0.156; LSD,.,,,,, = 0.227.
3.2. Experience with a novel food
Amount of experience with a novel food affected the formation and the persistence of a conditioned food aversion (F = 6.33, df = 3,27, P = 0.002). Lambs with 1 and 7 days experience ate similar amounts of oats the first 2 days of the trial, but lambs with 7 days experience consumed more oats than lambs with 1 day experience on the last 5 days of the trial (Fig. 2, treatment by day interaction = F = 2.45, df = 21,189, P < 0.001).
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Day Fig. 3. Intake of alfalfa pellets (%BW) by lambs after receiving lithium chloride (200 mg kg-’ BW). Bars represent LSD,,,,.
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Day Fig. 4. Consumption of oats and alfalfa pellets by lambs after receiving lithium chloride (200 mg kg-’ BW). Lambs had either 1 or 14 days experience eating oats at 1% BW. Intake data are expressed as a percent of food eaten prior to lambs receiving lithium chloride. Bars represent LsDo~s.
Lambs with 7 days experience ate less oats than lambs with 14 or 28 days experience early in the trial, but these three groups ate similar amounts of oats on the last 2 days of the trial. Lambs decreased intake of AP after receiving LiCl (Fig. 3, F = 49, df = 8,216, P < O.OOl), regardless of experience with oats. AP consumption returned to pre-treatment levels by the end of the trial. When the intake of oats and AP were expressed as a percentage of food eaten prior to receiving LiCl, lambs with 1 day exposure to oats consumed less oats than AP throughout the trial (Fig. 4(a), F = 2.48, df = 7,98, P = 0.022). The reverse was true for lambs with 14 days experience with oats (Fig. 4(b), F = 2.69, df = 7,84, P = 0.015).
E.A. Burritt, F.D. Provenza/Applied
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4. Discussion 4.1. Prior illness Lambs acquired an aversion to a food previously paired with illness after eating a meal of familiar foods. Burritt and Provenza (1991) found that lambs also acquire an aversion to a novel food when toxicosis follows a meal of one novel and four familiar foods; in that study, intake of one of the familiar foods (corn> also declined, evidently because lambs became ill when first introduced to corn 4 weeks prior to the onset of that study. The decrease in intake of larkspur, after cattle eat a meal of several familiar foods (Pfister et al., 1994), may also be due to prior gastro-intestinal distress from larkspur ingestion. Several lambs did not acquire an aversion after a single dose of LiCl (three in the group averse to rice and four in the group averse to wheat). Rather, on the next day, after they ate both wheat and rice and received a second dose of LiCl, they ate less of the grain paired with LiCl on the first day. This result is interesting because it suggests lambs experienced the effects of LiCl the first day, but showed no overt sign until after they received a second dose of LiCl, at which time they avoided the food paired with LiCl on Day 1. 4.2. Experience with a novel food Aversions were less persistent when lambs had at least 7 days experience with a novel food. Lambs with 1 or 7 days experience ate similar amounts of oats the day after receiving LiCl, but by the end of the trial lambs with 7, 14, and 28 days experience ate similar amounts of oats. Thus, lambs are not likely to acquire persistent aversions if they have at least 7 days experience with nutritious foods. When AP and oat consumption were expressed as a percentage of food eaten prior to receiving LiCl, lambs with 1 day experience ate less oats than AP (Fig. 4(a)), whereas lambs with 14 days experience ate more oats than AP daily except Day 8 (Fig. 4(b)). These data also suggest oats were a familiar food by Day 14. Lambs acquired an aversion not only to oats but also to AP (Fig. 3), a familiar food they ate with their mothers and after weaning. The decrease in AP consumption was similar for all groups, even lambs with only 1 day exposure to oats. Lambs ate less AP on the day they received LiCl than on the previous day, which suggests they became ill while eating AP. The aversion-inducing effects of LiCl are greatest within 1 h of administration, and the coincidence of eating AP and LiCl-induced toxicosis likely caused the aversion to AP (Provenza et al., 1993). Animals can acquire aversions to novel and familiar foods when illness and consumption coincide (Garcia and Koelling, 1967). Lambs also ate more AP (1.9% SW) than oats (1% SW) prior to experiencing illness, which undoubtedly further contributed to their aversion to AP (Provenza et al., 1994b). 5. Implications These results suggest animals in drylots and feedlots should be introduced to novel foods slowly. When ruminants are forced to eat high-grain rations, they may experience
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acidosis and acquire aversions to the novel foods. Small amounts of novel foods should be offered during the first few days (e.g. 7 days) to avoid more persistent aversions. Lambs in other settings may become familiar with new foods more or less quickly than those in our study, depending on conditions of exposure to new foods. Nevertheless, our results emphasize that if illness occurs soon after ruminants are placed on high-grain diets, they may be slow to ingest the diets and gain weight. Conversely, foods should be novel if aversions are to persist when animals are conditioned to avoid particular plants, for example poisonous plants or plants of economic value. Finally, the results show that experience can affect food aversions in ways that seem inexplicable without knowledge of past dietary history.
Acknowledgements This research was supported by grants from the Cooperative States Research Service and the Utah Agricultural Experiment Station. This paper is published with the approval of the Director, Utah Agricultural Experiment Station, Utah State University, Logan, as Journal Paper number 4736.
References B&ton, R.A. and Stock, R.A., 1987. Acidosis, rate of starch digestion and intake. In: Feed intake by beef cattle. Agric. Expt. Stn. MP 121, Oklahoma State University, pp. 125-137. Burr&, E.A. and Provenza, F.D., 1991. Ability of lambs to learn with a delay between food ingestion and consequences given meals containing novel and familiar foods. Appl. Anim. Behav. Sci., 32: 179-189. Garcia, J., 1989. Food for Tolman: cognition and cathexis in concert. In: T. Archer and L. Nilsson (Editors), Aversion, Avoidance and Anxiety. Erlbaum, Hillsdale, NJ, pp. 4.5-85. Garcia, J. and Koelling, R.A., 1967. A comparison of aversions induced by x rays, toxins and drugs in the rat. Radiation Res. Suppl., 7: 439-450. Huber, T.L., 1976. Physiological effects of acidosis on feedlot cattle. J. Anim. Sci., 43: 902-909. Olsen, J.D. and Ralphs, M.H., 1986. Feed aversion induced by intraruminal infusion with larkspur extract in cattle. Am. J. Vet. Res., 47: 1829-1833. Phster, J.A., Provenza, F.D. and Manners, G.D., 1994. Why do most cattle survive tall larkspur ingestion? In: SM. Colegate and P.R. Dorling (Editor), Plant Associated Toxins. CAB International, Wallingford, UK, pp. 484-489. Provenza, F.D., 1995. Postingestive feedback as an elemental determinant of food preference and intake in ruminants. J. Range Manage., 48: 2-17. Provenza, F.D., 1996. Acquired aversions as the basis for varied diets of ruminants foraging on rangelands. J. Anim. Sci., in press. Provenza, F.D., Lynch, J.J. and Nolan, J.V., 1993. Temporal contiguity between food ingestion and toxicosis affects the acquisition of food aversions in sheep. Appl. Anim. Behav. Sci., 38: 269-281. Provenza, F.D., Ortega-Reyes, L., Scott, C.B., Lynch, J.J. and Burr&, E.A., 1994a. Antiemetic drugs attenuate food aversions in sheep. J. Anim. Sci., 72: 1989-1994. Provenza, F.D., Lynch, J.J., Burr& E.A. and Scott, C.B., 1994b. How goats learn to distinguish between novel foods that differ in postingestive consequences. J. Chem. Ecol., 20: 609-624. Villalba, J.J. and Provenza, F.D., 1996a. Preference for wheat straw by lambs conditioned with intraruminal infusions of starch. Br. J. Nutr., accepted. Villalba, J.J. and Provenza, F.D., 1996b. Preference for wheat straw by lambs conditioned with intraruminal infusions of sodium propionate. J. Anim. Sci., submitted. Winer, B.V., 1971. Statistical Principles in Experimental Design. McGraw-Hill, New York.
ELSEVIER
Applied Animal Behaviour Science 48 (1996) 73-80
Amount of experience and prior illness affect the acquisition and persistence of conditioned food aversions in lambs Elizabeth A. Burr&t *, Frederick D. Provenza Department of Rangeland Resources, Utah State University, Lagan, UT 84322, USA Accepted 8 November 1995
Abstract
We studied the acquisition of food aversions in sheep. In the first experiment, we investigated how prior illness affected the formation of a conditioned food aversion. Lambs were offered small amounts ( < 200 g) of novel foods (wheat and rice) for 12 days, until lambs ate both foods readily. To establish an aversion to one of the foods, lambs were then given the toxin lithium chloride (LiCl) after eating one of the foods. On subsequent days, lambs were allowed to eat the food previously paired with LiCl until the aversion extinguished. When lambs later consumed a meal of wheat and rice and received LiCl, they avoided the food that had been paired with LiCI, but they did not reduce intake of the other food (P < 0.001). In the second experiment, we examined how amount of experience with a food affected the acquisition and persistence of a food aversion. Lambs were offered oats (novel food) at 1% body weight for either 28, 14, 7, or 1 days. An aversion was then induced by offering oats followed by LiCl. Lambs with at least 14 days exposure to novel foods formed the least persistent aversions (P = 0.002). Furthermore, the aversion to oats extinguished within 6 days in lambs offered oats for 7 days. Thus, lambs are not likely to abstain from a food they were trained to avoid (e.g. trees in orchards, poisonous plants) if they have more than 7 days experience with the food. Finally, lambs should be fed small amounts of highly nutritious novel foods (e.g. grains) during the first 7 days to avoid gastro-intestinal distress and more persistent aversions. Keywords: Aversion; Lithium chloride; Feeding; Sheep; Learning
??
Corresponding
author.
016%1591/96/$15.00 8 1996 Elsevier Science B.V. All rights reserved PII 0168-1591(95)01004-1
14
E.A. Burritt,
F.D. Provenza /Applied
Animal
Behaviour
Science 48 (19%)
73-80
1. Introduction Animals probably acquire aversions to foods repeatedly during their lives (Provenza, 1995, Provenza, 1996). Aversions to nutritious foods are likely manifest as cyclical patterns of intake over days, for instance when ruminants eat nutritious foods like larkspur that contain toxic alkaloids (Olsen and Ralphs, 1986; Pfister et al., 1994) or when they eat high-energy foods like grain that are rapidly fermentable (Huber, 1976; Britton and Stock, 1987). Positive postingestive effects from nutrients cause intake to increase (Villalba and Provenza, 1996a; Villalba and Provenza, 1996b), but toxins in plants, and excess production of organic acids from starch digestion in grain, cause malaise which causes intake to decline when foods are eaten too frequently or in excess (Provenza et al., 1994a; Provenza, 1995, Provenza, 1996). Thus, aversions to nutritious foods are typically transient (Garcia, 1989) and often manifest by cyclic patterns of eating over days or weeks. Curiously, it is not known how ruminants identify which harmful familiar plants (e.g. larkspur) to stop eating after a meal of several foods. One explanation is that animals acquire aversions to foods that made them ill in the past. Our first objective was to discover which food(s) lambs avoid when they experience illness after a meal of two foods, one that made them ill previously and one that did not. Animals may also acquire less persistent aversions to nutritious foods like larkspur and grain as their familiarity with the foods increases. Thus, our second objective was to assess how the amount of experience with a novel food affects the formation and persistence of an acquired food aversion.
2. Materials and methods 2.1. Prior illness Twenty 4-month-old lambs were conditioned to eat wheat and rice, grains novel to them. Lambs were offered 200 g of wheat and rice on alternate days during Days 1 to 4 of the trial. On Day 5, lambs were offered a choice between 150 g of each of the grains for 15 min. Six lambs preferred wheat, one preferred rice, and the other 13 lambs ate both grains. On Days 6 and 7, lambs were offered 200 g of rice and wheat, respectively. On Days 8 to 11, lambs were offered 100 g of each grain, and on Day 12, they were offered and ate 175 g of each grain within 15 min. Lambs ate similar amounts of wheat and rice (1569 and 1533 g) during conditioning. On Day 13, ten lambs ate 250 g of wheat and ten lambs ate 250 g of rice in 15 min, and then received a mild dose of lithium chloride (LiCl; 150 mg kg-’ body weight (BW)), a non-lethal gastrointestinal poison, in a gelatin capsule. To test for an aversion on Day 14, lambs were offered a choice between 200 g of rice and 200 g of wheat for 15 min. To extinguish the aversion, we fed lambs 200 g of the food previously paired with LiCl; 1 to 6 days were required for lambs to eat 200 g of grain in 15 min, at which time we considered the aversion extinguished. Lambs were fed the same amount of both grams during extinction. When all lambs ate both foods readily, lambs were given 200 g of wheat and rice for 15 min. All lambs ate all of both foods and then received a mild dose of LiCl (150 mg
E.A. Burrin,
F.D. Provenza /Applied
Animal
Behaviour
Science 48 (1996)
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kg-’ SW> to insure they did not acquire an aversion to both foods. For the next 4 days, lambs could choose between 200 g each of wheat and rice for 15 min day-‘. Lambs were deleted from the trial if they did not acquire an aversion to the LiCl-paired food (three lambs from the rice-averse group) or if the aversion persisted longer than 6 days (one from each group). Lambs were fed alfalfa pellets CAP) ad libitum during conditioning and testing, except on days when LiCl was given, in which case they received AP 2 h after they received LiCl. All lambs had access to salt and water ad libitum. The repeated measures of analysis variance (Winer, 1971) had group (LiCl-paired with wheat or rice) as the main effect and lambs (n = 6 for LiCl-paired rice and n = 9 for LiCl-paired wheat) were nested within groups. Food (wheat and rice) was a subplot in the analysis and was crossed with lambs. The trial lasted 5 days. 2.2. Experience with a novel food Forty 4-month-old lambs were divided into five groups of eight lambs each and penned individually. Each group was fed oats (a novel food) at 1% BW for 42, 28, 14, 7, or 1 day(s). During conditioning, lambs received AP ad libitum for 2 h day-‘, 30 min after receiving oats. During the last 4 days of conditioning, lambs received AP ad libitum for 1 h day - ‘. Four of the lambs in each of the 42- and 28-day groups refused to eat oats at 1% BW and were deleted from the trial. Because of small sample size, these two groups were combined to form one group of lambs that ate oats for at least 28 days. One lamb in the 16day group did not eat oats and was removed from the trial. After conditioning, lambs received oats at 1% BW for 15 min and then received a strong dose of LiCl (200 mg kg-t) to insure that even lambs most familiar with oats would acquire an aversion. Lambs then received AP ad libitum for 1 h. The following 7 days at 08:OO h, lambs received oats at 1% BW for 15 min followed by AP ad libitum for 1 h. Intake was measured and expressed as %BW. The repeated measures analysis of variance had four treatments (1, 7, 14, 28 days) and lambs (n = 7 or 8) were nested within treatments. Foods (oats and AP) were analyzed separately. Intake was expressed as a percentage of BW in the analyses. The trial lasted 8 days. To further elucidate how exposure to foods affected the formation of food aversions, the data for treatments 1 and 14 days were expressed as a percent of food eaten prior to receiving LiCl, and analyzed as repeated measures with food (oats and AP) as the main effect. Lambs (n = 7 or 8) were crossed with food. Treatments (1 and 14 days only) were analyzed separately.
3. Results 3.1. Prior illness Lambs ate less of the food previously paired with LiCl (Fig. l(a) and (b); F = 6.8, df = 3,78, P < O.OOl), and they responded similarly to both rice and wheat (F = 0.28,
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Day Fig. 1. Intake (g) of wheat and rice by lambs. Rice or wheat ingestion by lambs was previously paired with lithium chloride (150 mg kg-’ BW). Bars represent LSD,,,.
df = 1,13, P = 0.61). Lambs increased consumption of the grain that had been paired with LiCl during the 4 days after receiving LiCl a second time. Seven lambs consumed 200 g each of wheat and rice the day after receiving the first dose of LiCl. Immediately after consuming both foods on Day 2, lambs received a second dose of LiCl. All seven lambs formed an aversion to the grain previously paired with LiCl after the second dose of LiCl.
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Day Fig. 2. Intake (%BW) of oats by lambs after receiving lithium chloride (200 mg kg-’ BW). Lambs had either 1, 7, 14, or 28 days experience eating oats at 1% BW. LSD,,,,,,, = 0.156; LSD,.,,,,, = 0.227.
3.2. Experience with a novel food
Amount of experience with a novel food affected the formation and the persistence of a conditioned food aversion (F = 6.33, df = 3,27, P = 0.002). Lambs with 1 and 7 days experience ate similar amounts of oats the first 2 days of the trial, but lambs with 7 days experience consumed more oats than lambs with 1 day experience on the last 5 days of the trial (Fig. 2, treatment by day interaction = F = 2.45, df = 21,189, P < 0.001).
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Day Fig. 3. Intake of alfalfa pellets (%BW) by lambs after receiving lithium chloride (200 mg kg-’ BW). Bars represent LSD,,,,.
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Day Fig. 4. Consumption of oats and alfalfa pellets by lambs after receiving lithium chloride (200 mg kg-’ BW). Lambs had either 1 or 14 days experience eating oats at 1% BW. Intake data are expressed as a percent of food eaten prior to lambs receiving lithium chloride. Bars represent LsDo~s.
Lambs with 7 days experience ate less oats than lambs with 14 or 28 days experience early in the trial, but these three groups ate similar amounts of oats on the last 2 days of the trial. Lambs decreased intake of AP after receiving LiCl (Fig. 3, F = 49, df = 8,216, P < O.OOl), regardless of experience with oats. AP consumption returned to pre-treatment levels by the end of the trial. When the intake of oats and AP were expressed as a percentage of food eaten prior to receiving LiCl, lambs with 1 day exposure to oats consumed less oats than AP throughout the trial (Fig. 4(a), F = 2.48, df = 7,98, P = 0.022). The reverse was true for lambs with 14 days experience with oats (Fig. 4(b), F = 2.69, df = 7,84, P = 0.015).
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4. Discussion 4.1. Prior illness Lambs acquired an aversion to a food previously paired with illness after eating a meal of familiar foods. Burritt and Provenza (1991) found that lambs also acquire an aversion to a novel food when toxicosis follows a meal of one novel and four familiar foods; in that study, intake of one of the familiar foods (corn> also declined, evidently because lambs became ill when first introduced to corn 4 weeks prior to the onset of that study. The decrease in intake of larkspur, after cattle eat a meal of several familiar foods (Pfister et al., 1994), may also be due to prior gastro-intestinal distress from larkspur ingestion. Several lambs did not acquire an aversion after a single dose of LiCl (three in the group averse to rice and four in the group averse to wheat). Rather, on the next day, after they ate both wheat and rice and received a second dose of LiCl, they ate less of the grain paired with LiCl on the first day. This result is interesting because it suggests lambs experienced the effects of LiCl the first day, but showed no overt sign until after they received a second dose of LiCl, at which time they avoided the food paired with LiCl on Day 1. 4.2. Experience with a novel food Aversions were less persistent when lambs had at least 7 days experience with a novel food. Lambs with 1 or 7 days experience ate similar amounts of oats the day after receiving LiCl, but by the end of the trial lambs with 7, 14, and 28 days experience ate similar amounts of oats. Thus, lambs are not likely to acquire persistent aversions if they have at least 7 days experience with nutritious foods. When AP and oat consumption were expressed as a percentage of food eaten prior to receiving LiCl, lambs with 1 day experience ate less oats than AP (Fig. 4(a)), whereas lambs with 14 days experience ate more oats than AP daily except Day 8 (Fig. 4(b)). These data also suggest oats were a familiar food by Day 14. Lambs acquired an aversion not only to oats but also to AP (Fig. 3), a familiar food they ate with their mothers and after weaning. The decrease in AP consumption was similar for all groups, even lambs with only 1 day exposure to oats. Lambs ate less AP on the day they received LiCl than on the previous day, which suggests they became ill while eating AP. The aversion-inducing effects of LiCl are greatest within 1 h of administration, and the coincidence of eating AP and LiCl-induced toxicosis likely caused the aversion to AP (Provenza et al., 1993). Animals can acquire aversions to novel and familiar foods when illness and consumption coincide (Garcia and Koelling, 1967). Lambs also ate more AP (1.9% SW) than oats (1% SW) prior to experiencing illness, which undoubtedly further contributed to their aversion to AP (Provenza et al., 1994b). 5. Implications These results suggest animals in drylots and feedlots should be introduced to novel foods slowly. When ruminants are forced to eat high-grain rations, they may experience
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acidosis and acquire aversions to the novel foods. Small amounts of novel foods should be offered during the first few days (e.g. 7 days) to avoid more persistent aversions. Lambs in other settings may become familiar with new foods more or less quickly than those in our study, depending on conditions of exposure to new foods. Nevertheless, our results emphasize that if illness occurs soon after ruminants are placed on high-grain diets, they may be slow to ingest the diets and gain weight. Conversely, foods should be novel if aversions are to persist when animals are conditioned to avoid particular plants, for example poisonous plants or plants of economic value. Finally, the results show that experience can affect food aversions in ways that seem inexplicable without knowledge of past dietary history.
Acknowledgements This research was supported by grants from the Cooperative States Research Service and the Utah Agricultural Experiment Station. This paper is published with the approval of the Director, Utah Agricultural Experiment Station, Utah State University, Logan, as Journal Paper number 4736.
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