Acquired aversions as the basis for varied diets of ruminants foraging on rangelands

Ruminants eat an array of plant species that vary in nutrients and toxins. This selection makes intuitive sense, but no theories adequately explain this diversity. Some maintain it reduces the likelihood of overingesting toxins, whereas others contend it meets nutritional needs. Nevertheless, herbivores seek variety even when toxins are not a concern and nutritional needs are met. I offer another explanation for this behavior, one which encompasses the avoidance of toxins and the acquisition of nutrients. A key concept in this theory is aversion, the decrease in preference for food just eaten as a result of sensory input (a food's taste, odor, texture, i.e., its flavor) and postingestive effects (effects of nutrients and toxins on chemo-, osmo-, and mechano-receptors) unique to each food. Aversions are pronounced when foods contain toxins or high levels of rapidly digestible nutrients; they also occur when foods are deficient in specific nutrients. Aversions occur even when animals eat nutritionally adequate foods because satiety (satisfied to the full) and surfeit (filled to nauseating excess) represent points along a continuum, and there is a fine line between satiety and aversion. Thus, eating any food is likely to cause a mild aversion, and eating a food too frequently or in excess is likely to cause a strong aversion. Aversions are involuntary and are not the result of conscious decisions by an animal. Aversions yield benefits (e.g., obtain a balanced diet, reduce ingestion of toxic foods, optimize foraging and rumination times, sample foods, maintain a diverse microflora in the rumen) that are often mistaken as the cause of varied diets. In this article, I discuss the subtle ways in which aversions diminish preference and cause animals to eat a variety of foods.

Experience early in life affects voluntary intake of blackbrush by goats

Low nutritional quality and high levels of condensed tannins adversely affect voluntary intake of blackbrush (Coleogyne ramosissima Torr.) by goats. We studied: (1) how consumption of blackbrush or alfalfa pellets by young goats affected their consumption of those foods later in life, and (2) whether previous ingestion of blackbrush or alfalfa pellets affected the excretion of condensed tannins and total phenols from blackbrush in urine and feces, production of proline-rich proteins in saliva, excretion of nitrogen in feces, and mass of the liver, kidneys, parotid glands, and reticuloramen in goats. From 6 to 26 weeks of age, experienced goats were exposed to blackbrush, while inexperienced goats ate alfalfa pellets. Following exposure, both groups were offered older-growth blackbrush twigs (OG) or a choice between OG and current season's blackbrush twigs (CSG). A similar feeding trial was repeated nine months after exposure, and, in addition, both groups were offered a choice between OG ad libitum and alfalfa pellets at six levels of availability. Immediately after exposure, experienced goats ingested 95% more (P < 0.01) OG per unit of body weight than inexperienced goats, but both groups rejected CSG. Nine months after exposure, experienced goats ingested 27% more (P < 0.01) OG than inexperienced goats. Experienced goats ingested 30% more OG than inexperienced goats at every level of alfalfa pellet availability. The fate of condensed tannins and total phenols was similar for both groups, but experienced goats excreted 63% more (P < 0.05) uronic acids per unit of body weight. Neither experienced nor inexperienced goats produced proline-rich proteins in saliva. Experienced goats excreted 32% more (P < 0.01) nitrogen in feces per unit of nitrogen ingested than did inexperienced goats. The mass of the reticulorumen was 30% greater (P < 0.05) for experienced than for inexperienced goats one month after exposure, but did not differ 10 months after exposure. The mass of the liver, kidneys, and parotid glands did not differ between treatments. The results show that experience early in life can have profound and persistent effects on consumption of diets high in chemical defenses and low nutrional quality. The results also suggest that several physiological and morphological factors are involved.

Multi-paddock grazing on rangelands: why the perceptual dichotomy between research results and rancher experience?

Maintaining or enhancing the productive capacity and resilience of rangeland ecosystems is critical for the continued support of people who depend on them for their livelihoods, especially in the face of climatic change. This is also necessary for the continued delivery of ecosystem services derived from rangelands for the broader benefit of societies around the world. Multi-paddock grazing management has been recommended since the mid-20th century as an important tool to adaptively manage rangelands ecosystems to sustain productivity and improve animal management. Moreover, there is much anecdotal evidence from producers that, if applied appropriately, multi-paddock grazing can improve forage and livestock production. By contrast, recent reviews of published rangeland-based grazing systems studies have concluded that, in general, field trials show no superiority of vegetation or animal production in multi-paddock grazing relative to continuous yearlong stocking of single-paddock livestock production systems. Our goal is to provide a framework for rangeland management decisions that support the productivity and resiliency of rangelands and then to identify why different perceptions exist among rangeland managers who have effectively used multi-paddock grazing systems and research scientists who have studied them. First, we discuss the ecology of grazed ecosystems under free-ranging herbivores and under single-paddock fenced conditions. Second, we identify five principles underpinning the adaptive management actions used by successful grazing managers and the ecological, physiological, and behavioral framework they use to achieve desired conservation, production, and financial goals. Third, we examine adaptive management principles needed to successfully manage rangelands subjected to varying environmental conditions. Fourth, we describe the differences between the interpretation of results of grazing systems research reported in the scientific literature and the results reported by successful grazing managers; we highlight the shortcomings of most of the previously conducted grazing systems research for providing information relevant for rangeland managers who aim to achieve desired environmental and economic goals. Finally, we outline knowledge gaps and present testable hypotheses to broaden our understanding of how planned multi-paddock grazing management can be used at the ranching enterprise scale to facilitate the adaptive management of rangelands under dynamic environmental conditions.

Preference for flavored wheat straw by lambs conditioned with intraruminal administrations of sodium propionate

We hypothesized that volatile fatty acids are feedback signals that condition food preferences in ruminants, and we tested two predictions based on this hypothesis: 1) low doses of propionate condition preferences for low-quality foods (Exp. 1 and 2) preferences are not caused by osmotic load (Exp. 2). In Exp. 1, lambs were offered chopped wheat straw flavored with either oregano or onion on odd days, whereas on even days flavors were switched and lambs received capsules containing sodium propionate. During four 8-d conditioning periods, the amounts of propionate delivered ranged from .7 to 1.4% of the daily DE intake (Period 1) or were fixed at .7% (Period 2) and 1% of the daily DE intake (Periods 3 and 4). After each 8-d conditioning period, lambs were offered oregano- and onion-flavored straw. Conditioning was then suspended and lambs were offered onion- and oregano-flavored straw at weekly intervals for 1 mo (extinction). Lambs preferred the flavor paired with propionate during conditioning (P < .001) and extinction (P < .07). During Exp. 2, a different group of lambs was conditioned as in Exp. 1, but sodium chloride was delivered at osmotic loads equivalent to those when propionate supplied .7% and 1% of the daily DE intake. Lambs strongly avoided the flavor paired with sodium chloride (P < .001). Thus, lambs acquired preferences for straw conditioned with doses of propionate typically considered ineffective in the regulation of food intake, and osmolalities generated by propionate did not cause, but probably attenuated, food preferences.

Preference for wheat straw by lambs conditioned with intraruminal infusions of starch

We hypothesized that feed preference depends on the interplay between flavour and postingestive effects, and we tested two predictions based on this hypothesis: (1) lambs acquire preferences for poorly nutritious feeds paired with starch; and (2) preferences persist when starch is no longer administered. Twenty lambs were randomly allocated to two groups and conditioned as follows: on odd-numbered days, lambs in group 1 received onion-flavoured wheat straw and lambs in group 2 received oregano-flavoured wheat straw. On even-numbered days, the flavours were switched and starch (2.5-9.4% of the digestible energy received/d) was infused into the rumen of all animals during straw consumption. Four periods of 8 d of conditioning were performed and on the 9th day of each period all animals were offered a choice between onion- and oregano-flavoured straw. After conditioning, starch administration was suspended and lambs were offered onion- and oregano-flavoured straw at weekly intervals for 8 weeks (extinction). Lambs strongly preferred the flavoured straw paired with starch, and this preference persisted during extinction. Thus, these results suggest that the postingestive effects of energy play an important role in the development of feed preferences of ruminants.

Preference for flavoured foods by lambs conditioned with intraruminal administration of nitrogen

We suggested that food preference depends on the interplay between flavour and post-ingestive effects, and we predicted that protein-restricted lambs would acquire preferences for foods paired with supplemental sources of N, including urea (Expts 1 and 2), casein (Expt 3), and gluten (Expt 4). In each experiment, twenty lambs, in two groups of ten, were conditioned as follows: on odd-numbered days, lambs in group 1 received wheat straw (Expts 1, 3, and 4) or ground barley (Expt 2) flavoured with a distinctive flavour, and lambs in group 2 received the same food but with a different flavour. On even-numbered days, flavours were switched and lambs received capsules containing different amounts of urea (ranging from 0.12 to 0.92 g N/d), casein (ranging from 0.23 to 0.69 g N/d), or gluten (ranging from 0.23 to 0.69 g N/d). After conditioning period of 8 d, lambs were given a two-choice test to determine preference for flavours paired with N. In Expts 1 and 2, lambs preferred the flavours conditioned with urea at lower doses (0.12 g N/d in Expt 1, 0.23 and 0.46 g N/d in Expt 2), but they avoided the flavour associated with urea at the highest dose (0.23 g N/d in Expt 1 and 0.92 g N/d in Expt 2). In Expts 3 and 4, lambs avoided the flavours associated with the lowest doses of casein or gluten (0.23 g N/d), but they preferred the flavours paired with casein or gluten at higher doses (0.46 and 0.69 g N/d). After conditioning, N administrations were suspended and lambs in Expts 3 and 4 were offered a choice of the two flavours at weekly intervals for 2 weeks (extinction); preferences persisted during extinction. Collectively, these results suggest that the post-ingestive effects of N in different forms and concentrations influenced the development of food preferences by lambs.

Nutrient-specific preferences by lambs conditioned with intraruminal infusions of starch, casein, and water

We hypothesized that lambs discriminate between postingestive effects of energy and protein and associate those effects with a food's flavor to modify food choices. Based on this hypothesis, we predicted that 1) lambs would acquire a preference for a poorly nutritious food (grape pomace) eaten during intraruminal infusions of energy (starch) or protein (casein) and that 2) shortly after an intraruminal infusion of energy or protein (preload), lambs would decrease their preferences for foods previously conditioned with starch or casein, respectively. Thirty lambs were allotted to three groups and conditioned as follows. On d 1, lambs in each group received grape pomace containing a different flavor and water was infused into their rumens as they ate the pomace. On d 2, the flavors were switched so each group received a new flavor and a suspension of starch (10% of the DE required per day) replaced the water infusion. On d 3, the flavors were switched again, and a suspension of casein (2.7 to 5.4% of the CP required per day) replaced the starch infusion. Conditioning was repeated during four consecutive trials. Lambs in Trial 1 had a basal diet of alfalfa pellets (e.g., free access from 1200 to 1700) and 400 g of rolled barley. Lambs in Trials 2, 3, and 4 received a restricted amount of alfalfa pellets (990 g/d) as their basal diet. After conditioning, all animals received an infusion of water, and, 30 min later, they were offered a choice of the three flavors previously paired with water, starch, or casein. On the ensuing days, the choice was repeated, but starch, casein, and barley replaced the water preload. The nutrient density of the infused preloads was increased during consecutive trials. Lambs preferred the flavors paired with starch > water > casein during Trial 1 (P < .05) and the flavors paired with starch > casein > water during Trials 2 (P < .05), 3 (P < .001), and 4 (P < .001). Preloads of casein decreased preferences for flavors previously paired with casein (P < .10 [Trial 2]; P < .001 [Trial 3], and increased preferences for flavors paired with starch (P < .05 [Trial 2]; P < .001 [Trial 3]). Preloads of energy (barley) had the opposite effect (P < .05 [Trial 3]). These results indicate that lambs discriminated between the postingestive effects of starch and casein and associated the effects with specific external cues (i.e., added flavors) to regulate macronutrient ingestion.

Influence of free-choice vs mixed-ration diets on food intake and performance of fattening calves

Research findings and management recommendations typically emphasize responses of the "average" individual, yet more than half of the animals in a group may differ significantly from the mean regarding food preference and intake. The productivity of a herd may be adversely affected if animals differing from the mean are fed a uniform diet formulated to meet the needs of the "average" individual. We compared the intake and performance of beef calves offered a choice or no choice among foods. Diets consisted of ad libitum access to either a chopped, mixed ration of rolled barley (31.3%), rolled corn (31.3%), corn silage (15.5%), and alfalfa hay (18.9%) (n = 16 calves) or a choice among those foods offered individually (n = 15 calves). Averaged across the 63-d trial, the two groups did not differ in ratios of protein to energy ingested (43 vs 43 g CP/Mcal ME; P = 0.50), but preference for foods high in energy or protein varied markedly for animals fed free-choice: on d 21 they had protein:energy ratios higher than those of animals fed the mixed ration, on d 2 the ratios were equal, and on d 40 they had protein:energy ratios lower than those of animals fed the mixed ration. Throughout the trial, no two animals consistently chose the same ingredients, and none selected a diet similar to the nutritionally balanced mixed ration, yet each animal ate a diet adequate to meet its needs. Animals offered the mixed ration tended to eat more than animals offered a choice (109 vs 102 g/kg MBW/d; P = 0.10), but they did not gain at a faster rate (0.89 vs 0.92 kg/d; P = 0.65). Gain/unit of food consumed also was similar for both groups (0.09 vs 0.10 kg/kg; P = 0.38). However, food cost/day was higher for animals fed the mixed ration than for those offered a choice ($1.58 vs $1.36; P = 0.03). Consequently, cost/kilograms of gain was higher for the mixed ration than for the choice group ($1.84 vs $1.49/kg; P = 0.045). These findings suggest that 1) animals can more efficiently meet their individual needs for macronutrients when offered a choice among dietary ingredients than when constrained to a single diet, even if it is nutritionally balanced; 2) transient food aversions compound the inefficiency of a single mixed diet by depressing intake even among animals suited to that nutritional profile; and 3) alternative feeding practices may allow producers to efficiently capitalize on the agency of animals, thus reducing illness and improving performance.

Preference of sheep for foods varying in flavors and nutrients

Our objective was to better understand the importance of flavor and nutrients in food preferences of lambs. Three foods differing in flavor and nutritional quality were created by grinding and mixing grape pomace, barley, alfalfa pellets, and soybean meal in different proportions. food 1 (2.21 Mcal/kg DE, 8.1% DP), food 2 (2.42 Mcal/kg DE, 11.0% DP), and food 3 (2.68 Mcal/kg DE, 13.8% DP). Intake of each food, offered singly and together, was assessed when foods 2 and 3 were flavored with 1% onion or 1% oregano. Lambs (n = 24) preferred food 3 > 2 > 1, regardless of flavor (P < .05), and they continued to prefer food 3 > 2 > 1, even when they received the toxin LiCl after eating one of the three foods (P < .05). When offered a choice, lambs always ate substantial amounts of all three foods, even though they might have been expected to eat food 3 exclusively. We hypothesize selection of a varied diet resulted from a decrease in preference for food just eaten as a result of sensory input (taste, odor, texture, i.e., a food's flavor) and postingestive feedback (effects of nutrients and toxins on chemo-, osmo-, and mechano-receptors) unique to each food. Thus, we submit that offering different foods of similar nutritional value, offering foods of different nutritional value, and offering the same food in different flavors are all means of enhancing food preference and intake.

Exploitation of secondary metabolites by animals: A response to homeostatic challenges

We propose that the exploitation of the bioactive properties of secondary metabolites (SMs) by animals can provide a "treatment" against various challenges that perturb homeostasis in animals. The unified theoretical framework for the exploitation of SMs by animals is based on a synthesis of research from a wide range of fields and although it is focused on providing generalized predictions for herbivores that exploit SMs of plants, predictions can be applied to understand the exploitation of SMs by many animals. In this review, we argue that the probability of SM exploitation is determined by the relative difference between the cost of a homeostatic challenge and the toxicity of the SM and we provide various predictions that can be made when considering behavior under a homeostatic perspective. The notion that animals experience and respond to costly challenges by exploiting therapeutic SMs provides a relatively novel perspective to explain foraging behavior in herbivores, specifically, and behavior of animals in general. We provide evidence that animals can exploit the biological activity of SMs to mitigate the costs of infection by parasites, enhance reproduction, moderate thermoregulation, avoid predation, and increase alertness. We stress that a better understanding of animal behavior requires that ecologists look beyond their biases that SMs elicit punishment and consider a broader view of avoidance or selection of SMs relative to the homeostatic state. Finally, we explain how understanding exploitation of SMs by animals could be applied to advance practices of animal management and lead to discovery of new drugs.

Influence of macronutrients and polyethylene glycol on intake of a quebracho tannin diet by sheep and goats

We determined if supplemental macronutrients or polyethylene glycol (PEG) influenced intake of a tannin diet. Sheep (lambs 5 mo age, 36 kg) and goats (kids 7 mo age, 32 kg) were fed supplements high in either energy or protein or offered a choice between the two supplements before and after receiving a meal containing 15% quebracho tannin. The effect of PEG, a compound that attenuates the negative effects of tannins, was assessed by offering PEG while animals consumed the tannin diet for 4 h/d. Intake of the tannin diet was influenced by both macronutrients and PEG. Animals that chose their own supplements or that received the high-protein supplement, consumed more of the tannin diet than animals fed the high-energy supplement: 34 and 36 vs 31 g/kg(0.75) (lambs) and 41 and 39 vs 34 g/kg(0.75) (kids), respectively (P < 0.05). Animals supplemented with PEG ate much more of the tannin diet than unsupplemented animals: 70 vs 39 g/kg(0.75) (lambs) and 63 vs 34 g/kg(0.75) (kids), respectively (P < 0.001). Sheep and goats consumed more tannin food when given PEG than when supplemented with macronutrients (51 and 38 g/kg(0.75), P < 0.001). Sheep and goats offered a choice between supplements consumed more CP than animals fed the high-energy supplement and more ME than animals fed the high-protein supplement (P < 0.05). In so doing, they selected a combination of foods that yielded a more balanced intake of macronutrients, while achieving high levels of intake of the tannin food. Sheep and goats can be used as an environmentally safe and economically sound means to reduce the abundance of tannin-rich vegetation. Macronutrients and PEG enhance use of tannin-containing plants, which may increase production of alternate forages and create a more diverse mix of species in a plant community.

Food aversion learning: ability of lambs to distinguish safe from harmful foods

We studied the ability of lambs to select safe foods in the presence of harmful foods. In a series of feeding experiments, 3- to 6-mo-old lambs were offered a choice between 1) a familiar, safe and a novel, harmful food, 2) a novel, safe and a novel, harmful food and 3) a familiar, harmful and a novel, safe food. All harmful foods were palatable feeds that had been treated with lithium chloride (LiCl), a non-lethal gastrointestinal poison. When lambs ingested a meal composed of a novel food containing LiCl and a familiar, safe food, lambs subsequently avoided the novel food. Lambs avoided the novel food even when the familiar food contained LiCl. The response of lambs varied when lambs were given a choice between a novel, safe food and a novel food containing LiCl. Lambs either avoided the novel food containing LiCl and ingested the safe food or they limited their intake of both novel foods. Their response was dependent on the novelty of the food containing LiCl. When a novel, palatable food contained 2% LiCl, lambs always ate some of the food, even after experiencing illness from ingesting it. Lambs experienced with foods containing LiCl displayed greater food neophobia than lambs naive to LiCl-treated foods. Thus, novelty was the major criterion that lambs used to associate foods with gastrointestinal illness.

What does it mean to be locally adapted and who cares anyway?

The availability of fossil fuels will likely decline dramatically during the first half of the twenty-first century, and the massive deficits probably will not be alleviated by alternative sources of energy. This seeming catastrophe will create opportunities for communities to benefit from foods produced locally in ways that nurture relationships among soil, water, plants, herbivores, and people to sustain their collective well being. Agriculture will be much more at the heart of communities than it is currently, but by necessity, it will no longer be so dependent on fossil fuels to power machinery or to produce fertilizers, herbicides, and insecticides to grow and protect plants in monocultures, antibiotics, and anthelmintics to maintain the health of herbivores, or nutritional supplements and pharmaceuticals to sustain humans. Rather, from soils and plants to herbivores and people, we will have to learn once again what it means to be locally adapted to the landscapes we inhabit. In the process of relearning these skills, plants will become more important as nutrition centers and pharmacies, their vast arrays of primary (nutrients) and secondary (pharmaceuticals) compounds useful in nutrition and health. There also will be a need, as in times past before our heavy reliance on fossil fuels, to produce livestock in easy-care systems that match seasonally-available forages with production needs and that match animals anatomically, physiologically, and behaviorally to local landscapes. This will mean reducing inputs of fossil fuels to increase profitability by 1) matching animal needs to forage resources; 2) selecting for animals that are adapted anatomically, physiologically, and behaviorally to local environments; 3) culling animals unable to reproduce with minimal help from humans; and 4) creating grazing systems that enhance the well-being of soils, plants, herbivores, and people.

Antiemetic drugs attenuate food aversions in sheep

Ruminants learn to avoid many foods that contain toxins by associating the flavor of the foods with aversive postingestive feedback. We hypothesized the emetic system is a cause of aversive feedback, and three experiments were conducted to determine whether antiemetic drugs (diphenhydramine, metoclopramide, dexamethasone) would attenuate food aversions caused by the toxicant lithium chloride (LiCl). Lambs were assigned to one of four treatments: antiemetics plus LiCl (A + L), antiemetics alone (A), LiCl alone (L), or neither antiemetics nor LiCl (C). The LiCl was administered immediately after sheep ate oats, wheat, and milo in Exp. 1, 2, and 3, respectively. The antiemetics were given 1 h before and at the time LiCl was administered to sheep. Lambs that received antiemetics (A and A + L) consistently ate more grain than lambs that did not receive the drugs (C and L) (Exp. 1, P < .08; Exp. 2, P < .05; Exp. 3, P < .08), and there was no interaction between antiemetics and LiCl. Thus, the results of all three experiments were consistent with the hypothesis that antiemetic drugs attenuate food aversions caused by the toxicant LiCl because sheep receiving antiemetic drugs (Group A + L) ate more grain than sheep not receiving the drugs (Group L). In addition, we suggest aversive postingestive feedback limited intake of grain because sheep receiving antiemetic drugs (Group A) ate somewhat more grain than sheep not receiving the drugs (Group C).

Sheep fed grain prefer foods and solutions that attenuate acidosis

We conducted experiments to determine whether lambs fed grain prefer foods and solutions containing sodium bicarbonate (NaHCO3) and lasalocid, compounds capable of attenuating acidosis. In Exp. 1, we determined whether lambs fed barley preferred flavored rabbit pellets (RP) containing NaHCO3 and lasalocid. Lambs in two groups (n = 10/group) were fed increasing amounts of barley on d 1 to 12 (300 to 1,100 g) and again on d 23 to 34 (300 to 1,350 g). After ingesting barley on d 1 to 12, lambs were fed ground RP containing lasalocid and NaHCO3 (i.e., medicated) and flavored with either 2% onion (group 1) or 2% oregano (group 2). During d 23 to 34, lambs were fed unmedicated RP containing NaCl and flavored with either 2% oregano (group 1) or 2% onion (group 2). During preference tests on d 35 to 40, lambs fed grain preferred RP with NaHCO3 to RP with NaCl (151 vs. 96 g; P < .01). In the Exp. 2, we determined whether wheat ingestion affected consumption of aqueous solutions containing NaHCO3. In trial 1, 28 lambs were assigned to four treatments: 1) low-wheat + 2% NaHCO3, 2) high-wheat + 2% NaHCO3, 3) low-wheat + water, and 4) high-wheat + water. For 12 d from 0800 to 0830, lambs in treatments 1 and 3 were fed 300 g of wheat and lambs in treatments 2 and 4 were fed up to 1,300 g of wheat; fluids (NaHCO3 and water) were then offered from 0930 to 1230 daily. Lambs drank more NaHCO3 on the high- than on the low-wheat diet (1,332 vs 890 g; P = .03); water consumption was similar for lambs on the high- and low-wheat diets (1,675 vs 1,700 g; P > .10). In trial 2, lambs in treatments 3 and 4 were offered a solution containing 1.4% NaCl. For 13 d from 0800 to 0830, lambs in treatments 1 and 3 were fed 500 g of wheat and lambs in treatments 2 and 4 were fed up to 1,700 g of wheat. Lambs had access to fluids from 0800 to 1200 daily. Lambs drank nearly twice as much NaHCO3 solution on the high- than on the low-wheat diet (1,066 vs 572 g), whereas they drank only 1.4 times more NaCl solution on the high- than on the low-wheat diet (888 vs. 634 g; P < .001). Fewer lambs showed signs of acidosis in treatment 2 than in treatment 4 in trials 1 (2 vs 9) and 2 (7 vs 17). Collectively, these results are consistent with the hypothesis that lambs fed grain prefer substances that attenuate acidosis.

Preference for flavored wheat straw by lambs conditioned with intraruminal infusions of acetate and propionate

We hypothesized that volatile fatty acids in rumen fluid are feedback signals that can condition food preferences or aversions in sheep. Three predictions were tested based on this hypothesis: 1) low doses of sodium propionate or sodium acetate condition preferences, but high doses condition aversions (Exp. 1 and 2); 2) preferences are not caused by osmotic load (Exp. 3 and 4); and 3) low doses of mixtures of acetate:propionate condition preferences (Exp. 4). In Exp. 1, 2, and 4, lambs were divided into four groups (10 lambs/group), and lambs in Exp. 3 were divided into two groups (five lambs/group). In all experiments, alfalfa pellets were the basal diet. On even days, half of the lambs were offered chopped wheat straw containing a distinctive flavor, whereas the other half received straw with a different flavor. During straw ingestion, different groups of lambs received intraruminal infusions of different concentrations (4, 8, or 12% of the daily DE received) of sodium propionate (Exp. 1), sodium acetate (Exp. 2), NaCl at osmotic loads equivalent to those when propionate supplied 4% of the daily DE received (Exp. 3), or different proportions of sodium acetate:sodium propionate (55:45 or 75:25% of the DE of the infusion [4% of the daily DE received]), or equimolar amounts of NaCl (Exp. 4). On odd days, the flavors were switched, and no infusions were administered. After 8 d of conditioning, lambs were offered a choice of wheat straw with the two distinctive flavors. Lambs preferred the flavor paired with the lowest doses of propionate (P = .07) and acetate (P = .08) but avoided the highest doses (P < .001). Excesses of VFA may condition aversions due to increases in rumen fluid osmolality and(or) excessive rates of supply of energy or sodium to the rumen. Lambs also preferred flavored straw associated with combinations of acetate and propionate (P < .001), especially at the highest concentration of propionate (P = .10). Lambs avoided NaCl in Exp. 3 (P < .001) and did not form preferences for NaCl in Exp. 4 (P > .05). Thus, osmolalities were not responsible for flavor preferences. In conclusion, our results support the hypothesis that food preferences and aversions reside along a continuum that depends on the amount of VFA infused.

Food flavor and nutritional characteristics alter dynamics of food preference in lambs

We addressed two questions involving food preference. First, we determined how a food's flavor and nutritional characteristics affected preference. In three trials, we offered lambs isonitrogenous foods differing in energy (trial 1, 90% TDN; trial 2, 100% TDN; trial 3, 110% TDN); each food was offered in apple and maple flavors. We hypothesized that preference for apple- or maple-flavored food would decrease with increasing duration of exposure (1, 2, or 4 d), and we speculated that the change in preference would intensify when food contained inadequate or excessive levels of energy. After eating food in one flavor, lambs preferred the alternative flavor, even after only a 1-d exposure, and preference for the alternative flavor was greater when the food had inadequate or excessive energy (P < .05). The second experiment determined whether eating a food with rapidly or slowly digestible sources of energy in the morning affected lambs' food preferences in the evening. We speculated that lambs fed rapidly digestible food in the morning may prefer a slowly digestible food in the afternoon because slowly digestible food better maintains nutrient status throughout the night or because preference for the rapidly digestible food decreases after exposure in the morning. We offered lambs isonitrogenous and isocaloric foods, that differed in rates of digestion, in apple and maple flavors. Lambs fed rapidly digestible food in the morning preferred slowly digestible food in the alternative flavor in the evening. However, lambs fed slowly digestible food in either flavor in the morning preferred slowly digestible food in both flavors in the evening (P < .05). These results show that lambs' preferences change as a result of food ingestion, and the degree of change in preference depends on the nutritional characteristics of the food. These findings further suggest food intake might be increased by providing a variety of foods to livestock on rangelands, pastures, or in confinement.

Influence of macronutrients and activated charcoal on intake of sagebrush by sheep and goats

We determined if supplemental macronutrients or activated charcoal influenced intake of sagebrush, a shrub that contains terpenes. Sheep (lambs 3 mo age, 33 kg) and goats (kids 5 mo age, 28 kg) were fed supplements high in energy, high in protein, or offered a choice between the two supplements before and after receiving sagebrush for 4 h/d. The effect of activated charcoal, a compound that reduces bioavailability of toxins, was assessed by offering charcoal mixed with a carrier or the carrier alone while animals consumed sagebrush for 4 h/d. The effects of supplemental macronutirents were tested before and after charcoal supplementation. Activated charcoal did not increase intake of sagebrush by sheep or goats (P > 0.05), but supplemental macronutrients influenced intake of sagebrush by both species (P < 0.05). Intake of sage-brush was markedly higher for animals fed the high-protein supplement than for animals fed the high-energy supplement (P < 0.05). Lambs (before and after charcoal supplementation) and kids (after charcoal supplementation) offered a choice between the two supplements used sagebrush at levels comparable to animals supplemented with protein. They consumed more CP than animals fed the high-energy supplement and more ME than lambs fed the high-protein supplement (P < 0.05). Thus, providing a choice between supplements enhanced intake of sagebrush and enabled animals to achieve a more balanced intake of macronutrients. Preference for supplement and sagebrush also depended on an animal's immediate past history with a supplement. Sheep and goats previously fed the high-energy or the high-protein supplement preferred, respectively, the high-protein or the high-energy supplement when given a choice (P < 0.05). The changes in preference for supplement caused a decreased intake of sagebrush by groups that preferred the energy concentrate, and an increased intake of sagebrush by groups that preferred the protein concentrate. Thus, supplements of different energy and protein densities consistently altered intake of sagebrush by sheep and goats.

Self-regulation of intake of polyethylene glycol by sheep fed diets varying in tannin concentrations

Tannins occur in many plant species, and they often suppress intake by reducing nutrient availability or by causing malaise. Polyethylene glycol (PEG) binds to tannins and may thereby increase the availability of macronutrients and decrease malaise. Supplemental PEG increases intake of tannin-containing plants by sheep, goats, and cattle. Given the strong response to supplemental PEG, we speculated that animals might self-regulate their intake of PEG when offered foods high in tannins. The objective of the first experiment was to determine if the amount of supplemental PEG (0, 25, 50, 75, or 100 g; molecular weight, 3,350) affected intake by lambs of a food (milo-tannin mix) containing 20% quebracho tannin. There was a linear relationship (Y = 272 + 1.2X; R2 = .86; P = .023) between the amount of supplemental PEG ingested and the subsequent intake of milo-tannin food by lambs. The objective of the second experiment was to determine whether lambs self-regulated intake of PEG when fed a ration that contained 0, 5, 10, 15, or 20% quebracho tannin and whether they adjusted their intake of PEG when tannin was removed from the diet. There was a positive relationship between the amount of PEG ingested and intake of food and tannin (P = .0001). Lambs fed high-tannin diets ate more PEG than controls (P = .03). Lambs fed the 20% tannin diet ate the most PEG, and controls ate the least PEG. Tannin limited intake of the diets, but PEG attenuated the response to a great degree (P = .065). Immediately after tannin was removed from the ration, lambs that formerly had been fed the 20% tannin ration ate more PEG than lambs fed the other rations (P = .0075). Ten of the lambs (5 from the 20% tannin group, 1 from the 15% tannin, and 2 each from the 10 and 5% groups) continued to eat PEG for 7 d after tannin was removed from their ration. When they were tested again 6 wk after the trial and offered tannin-free diets, their intake of PEG had decreased.

Tracking variable environments: There is more than one kind of memory

Three kinds of memory help herbivores track changes in the environment. The first is the collective memory of the species with genetic instructions that have been shaped by the environment through millennia. This includes skin and gut defense systems. Auditory and visual stimuli and sensations of pain impinge upon the skin defense system that evolved in response to predation. The taste of food and the sensations of nausea and satiety are an integral part of the gut defense system that evolved in response to toxins and nutrients in plants. The second kind of memory in social mammals is represented by the mother, a source of transgenerational knowledge, who increases efficiency and reduces risk of learning about foods and environments. The third kind of memory is acquired by individual experience. Postingestive feedback from nutrients and toxins enables individuals to experience the consequences of food ingestion and to adjust food preference and selection commensurate with a food's utility. The three memories interact, each linking the past to the present, and collectively shape the present and future of every individual. Thus, the dynamics of foraging involves appreciating the uniqueness of individuals and subgroups of animals, each with their own genetic and behavioral history, and recognizing that foraging behaviors may not be stable, optimal, or even predictable in the conventional sense.

Preference for polyethylene glycol by sheep fed a quebracho tannin diet

Tannins decrease food intake by reducing digestion and by causing illness, whereas polyethylene glycol (PEG) attenuates the aversive effects of tannins. Our objective was to determine whether sheep recognize the benefits of ingesting substances such as PEG when consuming tannins. If so, then ingestion of PEG should be 1) PEG-specific, 2) a function of previous experience with recovery from tannin-toxicosis, and 3) dependent on the presence/absence of tannins. During conditioning, lambs in Group 1 (n = 10) were offered a meal of high-tannin food, which presumably caused malaise, and then offered PEG (molecular weight, 3,350), which presumably led to recovery from malaise. Subsequently, lambs ingested a control food (wheat straw) that did not have the "medicinal" effects of PEG in the absence of the tannin diet. In contrast, lambs in Group 2 (n = 10) ingested PEG in the absence of the tannin diet, and they ingested the tannin diet only in association with wheat straw. Ingestion of PEG and straw by both groups of lambs increased as a function of the presence of tannins in the diet (P < 0.05). However, when offered a choice among the tannin diet, PEG and straw, or when given the tannin diet and then offered a choice between PEG and straw, lambs trained to associate PEG with tannins ate more PEG than lambs that ingested PEG without tannins (P < 0.05). The responses were apparently PEG-specific; straw intake did not differ between groups of lambs during testing (P > 0.05), and differences in PEG intake disappeared in the absence of tannins (P > 0.05). In summary, our results suggest that lambs fed high-tannin diets discriminated the effects of PEG from those provided by a "nonmedicinal" food (straw). Thus, it may be possible to formulate PEG supplements that allow herbivores to self-regulate intake of PEG under extensive management conditions.

Garlic in the ovine fetal environment

We investigated whether the odor of garlic can cross the placental barrier from the mother to the fetal environment. Samples of amniotic fluid, allantoic fluid, fetal blood, and maternal blood were collected 0, 50, 100, and 150 min after a pregnant ewe (approximately day 110 of gestation) was gavaged with 6 ml of Egyptian garlic oil. A panel of judges detected (p < 0.05) garlic odor in samples of allantoic fluid, fetal blood, and maternal blood collected 50, 100, and 150 min after the ewe was given garlic and in samples of amniotic fluid collected 100 min after treatment.

Conditioned food aversions: principles and practices, with special reference to social facilitation

Conditioned food aversion is a powerful experimental tool to modify animal diets. We have also investigated it as a potential management tool to prevent livestock from grazing poisonous plants such as tall larkspur (Delphinium barbeyi), white locoweed (Oxytropis sericea) and ponderosa pine (Pinus ponderosa) on western US rangelands. The following principles pertain to increasing the strength and longevity of aversions: mature animals retain aversions better than young animals; novelty of the plant is important, although aversions can be created to familiar plants; LiCl is the most effective emetic, and the optimum dose for cattle is 200 mg/kg body weight; averted animals should be grazed separately from non-averted animals to avoid the influence of social facilitation which can rapidly extinguish aversions. Social facilitation is the most important factor preventing widespread application of aversive conditioning. When averted animals see other animals eat the target food they will sample it, and if there is no adverse reaction they will continue eating and extinguish the aversion. However, if averted animals can be grazed separately, aversions will persist. Aversive conditioning may provide an effective management tool to prevent animals from eating palatable poisonous plants that cause major economic loss.

The effect of flavor concentration and toxin dose on the formation and generalization of flavor aversions in lambs

If an animal experiences gastrointestinal malaise after eating a novel feed, it develops a dislike for the feed called a conditioned flavor aversion (CFA). Understanding flavor aversions is important for diet-training procedures, understanding animal responses to poisonous plants, and preparing animals for new foraging environments. Our research objectives were to determine how variation in 1) flavor concentration (oregano) and 2) dose of gastrointestinal toxin (lithium chloride; LiCl) affected the establishment of CFA in lambs. In a series of experiments feeding lambs ground grains mixed with oregano, we examined how the formation of a CFA to one ground grain, with or without oregano, influenced the consumption of another oregano-flavored grain. We determined that 1) the higher the toxin dose, the stronger the CFA, the greater the generalization of the CFA to a similar feed, and the greater the avoidance of a novel feed; 2) lambs generalized aversions from familiar to novel feeds when both feeds had a flavor in common; and 3) the concentration or intensity of feed flavor apparently did not affect the acquisition or generalization of a CFA, but it did influence the acceptance of a novel feed. Our findings suggest that flavor aversions may be important in the acceptance of harvested or processed feeds.