Review: Closing nutrient cycles for animal production - Current and future agroecological and socio-economic issues

We face an urgent and complex challenge to produce large amounts of healthful animal and plant foods for an estimated 10 billion people by 2050 while maintaining essential ecosystem services. To compound this challenge, we must do so while not further degrading our environment and conserving essential nutrients such as copper, magnesium, phosphorus, selenium, and zinc that are in short supply for fertilization. Much good research has been done, but to meet this challenge, we need to greatly increase on-farm and watershed-scale research including on-farm evaluations and demonstrations of the putative best combinations of stewardship techniques over multiple years in real-world settings, which are backed by data on nutrient inputs, soil, air, and water chemistry (fluxes) and water discharge. We also need to work with farmers, specialists, and generalists in highly creative interdisciplinary teams that resist forming silos and that use combinations of techniques linked to agroecology and industrial ecology in combination with state-of-the-art engineering. Some of these research and demonstration farms need to be in catchments prone to pollution of aquatic and terrestrial ecosystems with nitrogen, phosphorus, and other nutrients. Some promising approaches include mixed crop-livestock systems, although these alone may not be productive enough without updating to meet the dietary needs of an estimated 10 billion people by 2050. Other approaches could be state-of-the-art multi-trophic production systems, which include several species of plants integrated into production with vertebrates (e.g., ruminants, pigs, poultry), invertebrates (e.g., insects, earthworms) and fish, shrimp, or crayfish to utilize wasted feed and excreta, and recycle nutrients back to the animals (via plants or invertebrates) in the systems. To cut costs and increase desirable outputs, we must recycle nutrients much better within our food production systems and produce both animal and plant foods more efficiently as nutrients cycle through systems.

Palates link soil and plants with herbivores and humans

Palates link animals with landscapes. An attuned palate, which enables animals to meet needs for nutrients and self-medicate, evolves from flavour–feedback associations, availability of biochemically rich foods, and learning in utero and early in life to eat nourishing combinations of foods. Unlike our ancestors who ate biochemically rich diets, the palates of many individuals are no longer linked in healthy ways with landscapes. Selection for yield, appearance and transportability diminish phytochemical richness of vegetables and fruits, which adversely affects the flavour and nutritive value of produce for humans. Likewise, phytochemically impoverished pastures and feedlot diets can unfavourably affect the health of livestock and the flavour and nutritive value of meat and dairy for humans. Not coincidentally, as the flavours of meat, dairy and produce became blander, processed foods became more desirable as people in industry learned to link feedback from energy-rich compounds with artificial flavours that obscure nutritional sameness and diminish health. Thus, the roles plants and animals once played in nutrition and health have been usurped by processed foods fortified and enriched in ways that adversely affect preferences by stimulating appetite for processed over wholesome foods. The health of soil, plants, herbivores and humans could be improved by creating cultures that know how to produce and combine biochemically rich foods into meals that nourish and satiate.

Modelling preference and diet selection patterns by grazing ruminants: a development in a mechanistic model of a grazing dairy cow, MINDY

The work presented here represents additions to the mechanistic and dynamic model of a grazing dairy cow (MINDY). The additions include a module representing preference and selection, based on two theories, namely, post-ingestive feedback and discomfort. The model was evaluated by assessing its ability to simulate patterns of preference and selection in response to a variety of feeding management. The improvements detailed here enable a realistic simulation of patterns of food selection by grazing ruminants, based on a range of feeding situations from different studies with cattle and sheep. These simulations indicate that the concepts encoded in MINDY capture several of the underlying biological mechanisms that drive preferences and selective behaviour. Thus, simulations using MINDY allow prediction of daily and diurnal patterns of selection based on preference, derived from some post-ingestive feedbacks and total discomfort. Estimates of herbage intake and parallel measurements of ingestive behaviour, rumen function and metabolism in grazing ruminants pose experimental and technical difficulties, and matching these processes to animal preference and selective behaviour is a greater challenge. As a consequence, advances in knowledge of foraging behaviour and dietary choice are slow and costly. On completion of more thorough testing, MINDY can be used as a tool for exploratory mechanistic research, to design and organise experimental programs to address a range of factors that control intake and its ecology, helping advance knowledge faster and at a low cost.

Understanding and manipulating diet choice in grazing animals

Conventional models of foraging, such as optimal foraging theory, generally take the univariate approach to explain the decisions of consumers on the basis of the intrinsic properties of foods, including nutrient concentration and abundance. However, the food environment is inherently diverse and, as a consequence, foraging decisions are influenced by the interactions among multiple food components and the forager. Foraging behaviour is affected by the consumer’s past experiences with the biochemical context in which a food is ingested, including the kinds and amounts of nutrients and plant secondary compounds in a plant and its neighbours. In addition, past experiences with food have the potential to influence food preference and intake through a mechanism, namely, food hedonics, which is not entirely dependent on the classical homeostatic model of appetite control. Research on the impacts of experience with food context and its behavioural expression in natural settings should pioneer innovative management strategies aimed at modifying food intake and preference of herbivores to enhance their nutrition, health and welfare, as well as the health and integrity of the landscapes they inhabit.

Synthesis: foraging decisions link plants, herbivores and human beings

Herbivores make decisions about where to forage and what combinations and sequences of foods to eat, integrating influences that span generations, with choices manifest daily within a lifetime. These influences begin in utero and early in life; they emerge daily from interactions among internal needs and contexts unique to biophysical and social environments; and they link the cells of plants with the palates of herbivores and humans. This synthesis summarises papers in the special issue of Animal Production Science that explore emerging understanding of these dynamics, and suggests implications for future research that can help people manage livestock for the benefit of landscapes and people by addressing (1) how primary and secondary compounds in plants interact physiologically with cells and organs in animals to influence food selection, (2) temporal and spatial patterns of foraging behaviours that emerge from these interactions in the form of meal dynamics across landscapes, (3) ways humans can manage foraging behaviours and the dynamics of meals for ecological, economic and social benefits, and (4) models of foraging behaviour that integrate the aforementioned influences.

Importance of grass-legume choices on cattle grazing behavior, performance, and meat characteristics

We determined if tall fescue in a mixture with either tannin-containing sainfoin or saponin-containing alfalfa affected cattle foraging behavior, performance, meat quality, and consumer acceptance of meat. Foraging behavior, BW, and pasture biomass before and after grazing were monitored when cattle strip-grazed 3 replications of 2 treatments from May through September 2010 (12 calves/replication) and from June through September 2012 (8 calves/replication). Animals were allowed a choice between tall fescue and sainfoin (SAN) or tall fescue and alfalfa (ALF) growing in strips (fescue, legume, and fescue-legume mixture). Incidence of use (scan samples) of legumes increased from the beginning to the end of the trials (P < 0.05), suggesting that cattle learned to mix legumes with tall fescue. Scan samples and assessments of pasture biomass removal revealed greater use of sainfoin than alfalfa when both legumes were abundant in the spring (P < 0.05); that pattern reversed toward the end of the growing season (P < 0.05) as the abundance of sainfoin declined more than that of alfalfa. Intake of forage per kilogram of gain was greater for SAN than for ALF in 2010 (P = 0.0003) and showed a tendency to be greater for SAN than ALF in 2012 (P = 0.19). There were no differences in ADG between SAN and ALF in either year (1 kg/d; P > 0.05). No incidences of bloat were detected. When cattle (3 calves/treatment in 2010 and 8 calves/treatment in 2012) were slaughtered in September to assess meat quality and consumer acceptance, carcasses were lean (4% to 6% fat content of the longissimus muscle), but compared with ALF, SAN had higher marbling scores, quality grades (Select versus Standard), and back fat thicknesses (P < 0.05) in 2012. Steaks from cattle finished on SAN were redder in color than steaks from cattle finished on ALF (P < 0.05; 2012). Meat samples from SAN and ALF showed some differences in unsaturated fatty acid profiles and volatiles concentrations (P < 0.05), but consumer acceptance did not differ between SAN and ALF (P > 0.05). Thus, cattle incorporated a lower-quality grass into their diets even when legumes were available ad libitum during most grazing trials. Choices between tall fescue and legumes differing in the type of secondary compound (tannins vs. saponins) and bloating potential influenced patterns of forage use by cattle and led to differences in meat quality.

The relative importance of mother and toxicosis in the selection of foods by lambs

A lamb's mother and postingestive feedback both influence learning about foods, but their relative importance is unknown. We conducted a study to compare the ingestion of elm (1) by lambs whose mothers avoided elm because ingestion of elm by mother was previously paired with the toxin lithium chloride (LiCl) (M), (2) by lambs who received a mild dose of LiCl after they ingested elm, and whose mothers also avoided elm (M + L), and (3) by lambs who received LiCl after they ingested elm, but whose mothers ate elm avidly (M vs. L); in treatment (4) neither mothers nor lambs were given LiCl (C). In all four treatments, mothers and lambs ate poplar. Each lamb was exposed with its mother for 5 min/day to poplar on days 1,2, 5, 7, 9, and 11 and to elm on days 3, 4, 6, 8, 10, and 12. Following exposures to elm on days 8, 10, and 12, lambs in treatments M + L and M vs. L were given a low dose of LiCl (100 mg/kg body weight orally in a capsule) when they ingested elm to produce an aversion to elm. During the first test after weaning, lambs could choose between elm and poplar. Lambs in C took more bites of elm than did lambs in M, M + L, and M vs. L (13, 2, 2, < 1;P >F=0.003), and they also took a higher percentage of bites from elm (42, 11, 6, 1;P >F < 0.001). The number of bites of poplar did not differ among treatments (M=31, M + L=26, M vs. L=42, C=27;P >F =0.458). During the second test, when lambs were offered only elm, lambs in C took significantly more bites of elm than those in the other treatments, and lambs in M took more bites of elm than lambs in M + L, but not M vs. L (C=38, M=16, M + L=3, M vs. L=6;P >F < 0.001). During the third test, when lambs again could choose between elm and poplar, lambs in C again took more bites of elm (C=14, M=3, M + L=< 1, M vs. L=< 1;P >F=0.034), and they also took a higher percentage of bites from elm (C=26, M=5, M + L=2, M vs. L=2;P >F < 0.001), than did lambs in the other three treatments. The number of bites of poplar did not differ among treatments (M=47, M + L=43, M vs. L=62, C =41;P >F=0.223). We conclude that the response by lambs to the toxin LiCl was more important than was the mother as a social model because lambs that received LiCl avoided elm whether or not the mother ate it.

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.

Cattle preferences differ when endophyte-infected tall fescue, birdsfoot trefoil, and alfalfa are grazed in different sequences

We determined if sequence of ingestion affected use of endophyte-infected tall fescue (TF) when cattle also grazed birdsfoot trefoil (BFT) or alfalfa (ALF). Based on chemical characteristics of TF (alkaloids), BFT (tannins), and ALF (saponins), we hypothesized that cattle first allowed to graze ALF or BFT would subsequently spend more time grazing TF than cattle that first grazed TF followed by ALF or BFT. Sixteen bred heifers (478 ± 39 kg initial BW) were randomly assigned to 4 replicated pasture units. Each replicated unit consisted of 4 treatment sequences (TF → BFT, TF→ALF, BFT → TF, or ALF→TF), with 2 cows per sequence. Pastures were in the vegetative stage of growth at a height of 20 to 30 cm and provided ad libitum forage to cattle. We recorded foraging on TF, BFT, and ALF using scan sampling of individuals at 2-min intervals. The study was conducted in 4 phases run sequentially, for a total of 30 d. In phases 1 and 3, cattle in group 1 grazed TF pastures for 45 min and were then moved to BFT pastures for the next 45 min (TF→BFT); cattle in group 2 grazed in the reverse sequence (BFT → TF). In phases 2 and 4, cattle in group 1 grazed TF pastures for 45 min and then subsequently grazed ALF pastures for the remaining 45 min (TF→ALF); cattle in group 2 grazed in the reverse sequence (ALF→TF). Sequence of plant ingestion affected food selection. In phase 1, scans revealed grazing of TF by heifers was cyclic, and heifers tended to have more scans (P = 0.52) grazing TF when they grazed BFT → TF; scans for heifers grazing TF were consistently greater (P < 0.05) throughout phase 3 of the trial. In phase 2, heifers that grazed in the sequence ALF→TF spent considerably more scans (P = 0.03) foraging on TF from d 4 to 10 than heifers that grazed in the sequence TF→ALF, and they remained greater throughout phase 4 of the trial. Although the sequence ALF→TF appeared to be more effective than BFT → TF, consistent with the hypothesis of a complementary relationship between the steroidal alkaloids in TF and saponins in ALF, tannin concentrations in BFT were minimal (1.8%), which likely reduced the presumed inactivation of alkaloids by tannins. We also speculate that heifers needed to learn about the positive postingestive influence of sequence, a notion consistent with more similar scans spent foraging BFT and TF early in phases 1 (BFT → TF) and 2 (ALF→TF), and with the consistent and marked increase in scans spent foraging on TF for animals foraging in phases 3 (BFT → TF) and 4 (ALF→TF).

Rumen distension and contraction influence feed preference by sheep

Distension of the rumen limits feed intake by livestock. Ruminal dysfunctions due to bloat, which causes distension by accumulation of excessive gas within the rumen, also reduce feeding. We hypothesized that excessive levels of rumen distension cause feed aversions and that preference increases for feeds eaten in association with recovery from bloat. To test these hypotheses, we determined whether 12 commercial crossbred lambs (average initial BW of 43 +/- 2 kg) could associate ingestion of specific feeds with the consequences of increased intraruminal pressure and its subsidence. Six of the lambs were fitted with rumen cannulas and offered ground alfalfa for 30 min after a rubber balloon was inserted into the rumen of each animal and distended with air to volumes of 1.8, 2.5, or 4.5 L. Subsequently, balloons were deflated and alfalfa was offered again for a second period of 30 min. Feed intake was not affected when the balloon was not distended (P = 0.45 to 0.93), but distension reduced feed intake (P < 0.001) in direct proportion to the magnitude of distension at all 3 volumes (R(2) = 0.70). Relief from distension promoted a compensatory increase in feed intake (P = 0.006). During conditioning to determine if lambs acquired a preference for a feed associated with recovery from distension, fistulated lambs were offered novel feeds: wheat bran (group 1; n = 3) and beet pulp (group 2; n = 3), and the balloon was distended for 30 min. Feeds were then switched and the balloons were deflated (recovery). Control lambs (n = 6) received the same feeding protocol without the balloons. Lambs formed strong aversions to feeds associated with distension and preferred feeds associated with recovery (P = 0.001 to P = 0.10). No preferences or avoidances were observed in control lambs conditioned without rumen distension (P = 0.17 to P = 0.87). Thus, rumen distension and recovery from distension induced feed aversions and preferences, respectively, which may be critical in learning avoidance of bloat-inducing plants and preferences for plants and supplements that relieve the incidence of bloat.

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.

Terpenes and carbohydrate source influence rumen fermentation, digestibility, intake, and preference in sheep

We hypothesized that toxins and nutrients in foods interact to influence foraging behavior by herbivores. Based on this hypothesis we predicted that 1) terpenes in big sagebrush (Artemisia tridentata) influence intake and preference in sheep for diets varying in sources of nonstructural (barley grain) and structural (sugar beet pulp) carbohydrates, and 2) these effects are due to the differential effects of terpenes on fermentation products and apparent digestibility of each class of carbohydrates. Lambs were fed 2 isoenergetic and isonitrogenous diets with varying proportions of the same ingredients (beet pulp- and barley grain-based diet) or offered a choice between the 2 diets; all feeds were fed without and with terpenes, in consecutive periods. We also compared intake and preference of the beet pulp-and barley-based diets before and after the lambs ate a meal of sagebrush. Finally, we assessed the effect of terpenes on ruminal variables and in vivo digestibility. Lambs ate less when fed beet pulp or when they were offered a choice of diets with terpenes (P < 0.001), and intake of the beet pulp-based diet was the most affected (P < 0.05). Lambs preferred the beet pulp-to the grain-based diet with terpenes, but their preference reversed when terpenes were removed from the diets (P < 0.05). When lambs were offered both diets, intake and preference did not differ (P > 0.20) before eating sagebrush, but they preferred the beet pulp-based diet after eating sagebrush (P < 0.05). Intake of sagebrush did not differ among groups consuming the test diets (P = 0.21). Addition of terpenes to both diets increased the digestibility of DM, NDF, and ADF and decreased concentrations of total VFA and acetate (P < 0.05). Terpenes also depressed butyrate concentration in the barley-based diet (P < 0.05). Propionate concentrations were not affected by terpenes in either feed (P = 0.63). In summary, the predominant type of feed ingredient (beet pulp, grain) ingested with terpenes influenced fermentation products, intake, and preference in lambs. The source of energy from supplements, or other plants in the diet, is likely to influence intake and preference for sagebrush in sheep foraging on rangelands. Moreover, ingesting terpenes from sagebrush may also influence intake and preference for other plant species or supplements.

Phosphorus appetite in sheep: Dissociating taste from postingestive effects1,2

We hypothesized that lambs discriminate the postingestive effects of P and associate those effects with feed flavor to modify feed choices. Three predictions were tested based on this hypothesis: 1) lambs will modify preference for arbitrary flavors eaten during intraruminal infusions of NaH2PO4, 2) changes in preferences are more specific than changes in osmotic load induced by salts of Na; and 3) preference for P is inversely related to the concentration of inorganic P in blood. Thirty lambs were depleted of P by the offer of a P-deficient diet, allocated to 3 groups (10 lambs/group), and conditioned during 3 periods as follows: During conditioning period 1, lambs in each of 3 groups ate a poorly nutritious feed (grape pomace), flavored differently for each group, while water was infused into the rumen. During conditioning periods 2 and 3, lambs again ate grape pomace, with 2 new flavors now paired with infusion of an aqueous solution (126 mmol) of NaCl (conditioning period 2) or NaH2PO4 (conditioning period 3), rather than with water. After conditioning, all lambs were offered a choice of the 3 flavors during preference tests immediately after conditioning (period 1) and every 3 wk thereafter (periods 2, 3, and 4). During period 1, when serum inorganic P levels were greatest, lambs preferred flavors paired with water > NaCl > NaH2PO4 (P < 0.05). During periods 2 and 3, as inorganic P concentrations decreased in serum, lambs preferred flavors paired with NaH2PO4 > NaCl (period 2, P = 0.10; period 3, P = 0.05). Lambs preferred flavors paired with water > NaH2PO4 in period 2 (P < 0.001), but those differences disappeared in periods 3 and 4 (P > 0.05). During period 4, lambs preferred flavors paired with NaCl > NaH2PO4 (P < 0.10). The estimate of the slope for the linear relationship between intake of flavors paired with NaH2PO4 and serum inorganic P was negative (P < 0.0001), whereas estimates of the slopes for the relationships between intake of flavors paired with NaCl or water and serum inorganic P were not different from 0. Thus, preference for P was inversely related to the concentration of serum inorganic P. Our results suggest lambs discriminated among the postingestive effects of NaH2PO4, NaCl, and water and associated those effects with specific flavors. Lambs avoided flavors paired with NaH2PO4 during periods of P replenishment, and they increased preference for those flavors during periods of P need.

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.

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.

Polyethylene glycol influences selection of foraging location by sheep consuming quebracho tannin

Tannins are a heterogeneous group of phenolic polymers that can induce detrimental effects when consumed by herbivores. Polyethylene glycol (PEG) binds to tannins and thus attenuates their negative effects. Our objective was to determine whether sheep actively seek PEG when fed tannins and thus modify their foraging location as a function of the spatial distribution of PEG. Lambs were first trained to recognize the beneficial effects of PEG by offering a meal high in quebracho tannin (QT), which presumably caused malaise, and then PEG (MW, 3,350), which presumably led to recovery from malaise. Animals were then tested in an experimental area where they could forage at two different locations that contained in Trial 1 1) PEG and QT in adjacent food boxes (PEG+QT) or 2) QT and in Trial 2 1) PEG or 2) QT. Preference for foraging locations was tested under three conditions: 1) no preload meal, 2) a meal high in quebracho tannin 1 h before testing, and 3) a basal diet high in quebracho tannin. Lambs spent more time (P = 0.012) and ate more tannin-containing food (P = 0.022) at locations where PEG was present (PEG+QT) than where it was absent (QT; Trial 1). Lambs responded to increased tannins in their bodies (Conditions 2 and 3) by reducing intake (P = 0.0001; Trial 2) at sites containing only QT and by increasing intake (P = 0.0001, Trial 1; P = 0.001, Trial 2) and foraging time (P = 0.072, Trial 1; P = 0.0001, Trial 2) at locations where PEG was available. Thus, PEG influenced foraging location by sheep consuming quebracho tannin. Accordingly, it may be possible to formulate range blocks that enable herbivores to ingest PEG in tannin-rich habitats. Strategic distribution of PEG in those habitats may attract animals to underutilized feeding sites.

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.

Changes in preferences of gestating heifers fed untreated or ammoniated straw in different flavors

We determined how a food's flavor and digestibility, along with an animal's recent experiences, influenced food preference and intake. In three experiments, pregnant heifers were fed a basal ration (7.75 kg/animal) of alfalfa, barley, corn silage, and a vitamin/mineral supplement from 1500 to 2200. Exp. 1 determined the influence of recent exposure to flavored straw. Animals were divided into two groups (n = 16/group) and fed either untreated or ammoniated straw with digestibilities of 43 and 58%, respectively. Within each group, half of the heifers were fed maple-flavored straw and the other half were fed coconut-flavored straw from 1100 one day to 0900 the next day, with no base ration. We then offered straw in both flavors from 1000 to 1200 for the next 5 d. Animals fed maple-flavored straw for 1 d generally preferred coconut- over maple-flavored straw for the next 5 d, whereas animals previously fed coconut-flavored straw preferred maple-flavored straw (P < 0.001). The change in preference was stronger when animals were fed untreated compared with ammoniated straw. Experiments 2 and 3 determined the influence of offering straw in different flavors, either in sequence (Exp. 2) or simultaneously (Exp. 3). In Exp. 2, we offered heifers (n = 16) straw in three flavors (maple from 0900 to 1100, coconut from 1100 to 1300, and unflavored from 1300 to 1500) and compared their intake with that of heifers (n = 16) offered unflavored straw throughout the day. In Exp. 3, we compared intake of heifers (n = 16) simultaneously offered straw in three flavors (coconut, maple, and unflavored) with that of heifers (n = 16) offered only unflavored straw from 1000 to 1500. In both experiments, straw intake and preference differed between heifers offered straw in a variety of flavors as opposed to only unflavored straw (P < 0.05), but animals fed a variety of flavors did not consistently eat more than those fed only one flavor. During a post-trial preference test, heifers previously restricted to straw in one flavor for 5 d preferred straw in alternative flavors, whereas heifers fed straw in all three flavors preferred unflavored straw. Changes in preference were stronger for heifers fed untreated compared with ammoniated straw. Collectively, our results suggest that palatability, as evidenced by changes in preference and intake, is dynamic and depends on a food's flavor and nutritional quality and an animal's recent experiences with the food.

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.

Roles of novelty, generalization, and postingestive feedback in the recognition of foods by lambs

Ruminants select foods higher in nutrients and lower in toxins than the average of what is available whether fed in confinement or foraging on pastures or wild lands. Our objective was to better understand how sheep learn to select the most nutritious foods when they concurrently ingest different foods. We hypothesized that novelty and generalization are two ways sheep discriminate among foods. The first experiment determined whether lambs offered two foods in a meal (one a novel-flavored food and the other a familiar food) acquired a preference for the novel-flavored food following an infusion of energy (starch) into the rumen. Lambs did not increase preference for the novel-flavored food when the amount of starch infused (150 g) was held constant (P > 0.05). However, when the amount of starch infused was made directly proportional to the amount of novel-flavored food ingested, lambs quickly formed a preference for the novel-flavored food (P < 0.001). The second experiment determined whether lambs generalized preferences from familiar to novel foods. Lambs fed coconut-flavored milo grain for 51 d subsequently preferred coconut-flavored straw to plain straw (P < 0.001). Conversely, lambs that were not fed coconut-flavored milo grain preferred plain to coconut-flavored straw (P < 0.001). Lambs infused with a source of energy (starch, 100 g/[lamb x d]) consumed more coconut-flavored straw and less plain straw than lambs that had no infusions or lambs infused with a toxin (LiCl, 100 mg/kg BW; P < 0.05). Lambs infused once with LiCl had the lowest preference for coconut-flavored straw (P < 0.05). Lambs conditioned with starch had the highest intakes of coconut-flavored straw, even after starch infusions were suspended (P < 0.1). When straw was replaced by an energy-dense food (wheat grain), all lambs equally preferred coconut-flavored grain to plain grain (P < 0.001). Collectively, our results suggest that lambs learned to prefer a novel-flavored food when the amount of energy reward was contingent on the amount of novel-flavored food ingested, and that they generalized from familiar to unfamiliar foods based on common flavor cues. Once generalization occurred, postingestive feedback from nutrients and toxins calibrated preference according to the food's utility.

Lambs fed protein or energy imbalanced diets forage in locations and on foods that rectify imbalances

Ruminants eat a variety of foods from different locations in the environment. While water, cover, social interactions, and predators are all likely to influence choice of foraging location, differences in macronutrient content among forages may also cause ruminants to forage in different locations even during a meal. We hypothesized that lambs forage at locations containing foods that complement their basal diet and meet their nutritional needs. Based on this hypothesis, we predicted that lambs (n=12) fed a basal diet low in protein and high in energy would forage where a high-protein food (Food P) was located, and that lambs (n=12) fed a basal diet low in energy and high in protein would forage where a high-energy food (Food E) was located. Food P was a ground mixture of blood meal (50%), grape pomace (30%), and alfalfa (20%) that contained 47% crude protein (CP) and 2.211 Mcal/kg digestible energy (DE). Food E was a ground mixture of cornstarch (50%), grape pomace (30%), and rolled barley (20%) that contained 6% CP and 3.07 Mcal/kg DE. Food P provided 212 g CP/Mcal DE, whereas Food E provided 20 g CP/Mcal DE. Lambs growing at a moderate rate require 179 g CP and 3.95 Mcal DE. During Trial 1, we determined if lambs foraged to correct a nutrient imbalance, and if they preferred a variety of foods (Foods P and E) to only one food at a location (Food P or E). During Trial 2, we determined if nutrient-imbalanced lambs foraged in the location with the food that corrected the imbalance when the location of the foods changed daily. During Trial 3, lambs were offered familiar foods (Foods P and E) at the location furthest - and novel foods (wheat and soybean meal) at the location nearest - the shelter of their pen. During all three trials, lambs foraged most at the location with the food that contained the highest concentration of the macronutrient lacking in their basal diet, but they always ate some of both foods. Lambs did not feed exclusively at the location with a variety of foods (P and E). Rather, they fed at the location nearest the shelter that contained the macronutrient lacking in their diet. As availability of the food with the needed macronutrient declined in one location, lambs moved to the nearest location that had food with the needed macronutrient. When food that complemented their basal diet was moved to a different location, lambs foraged in the new location. Collectively, these results show that lambs challenged by imbalances in energy or protein selected foods and foraging locations that complemented the nutrient content of their macronutrient imbalanced basal diets.

Preferences for foods varying in macronutrients and tannins by lambs supplemented with polyethylene glycol

Supplemental polyethylene glycol (PEG) increases intake when animals eat foods high in tannins, but little is known about how PEG affects preference for foods that vary in concentrations of macronutrients and tannin. We investigated how varying macronutrients and tannins (commercially available extracts from quebracho trees) affected food intake, and we assessed the degree to which PEG (MW 3350) affected intake of tannin-rich foods by sheep. From 0715 to 1800 daily, lambs were offered diets that varied in concentrations of macronutrients: high energy/low protein (75% barley/25% alfalfa), medium energy/medium protein (35% barley/65% alfalfa), and low energy/high protein (100% alfalfa). Preference for these diets was determined in the absence of tannin, and then, in Trials 1 to 3, tannin was added in increasing concentrations (from 5 to 20%) to the diets with high and medium levels of energy. In Trial 4, tannin (10%) also was added to the low-energy diet. Lambs were supplemented with either 50 g of PEG mixed with 50 g of ground barley or 50 g of ground barley alone from 0700 to 1715 daily; lambs always consumed all of these supplements. In the absence of added tannins, all lambs preferred high energy/low protein > medium energy/medium protein > low energy/high protein. As tannin levels increased, preference for the high- and medium-energy foods decreased, and all lambs preferred foods that were lower in tannins and higher in protein. Lambs supplemented with PEG ate more macronutrients and tannins than unsupplemented lambs, and the effect became increasingly apparent as tannin levels increased from Trials 1 to 4. We conclude that the effectiveness of supplemental PEG may be low if alternative forages are equal or superior in nutritional quality and contain fewer metabolites with adverse effects. In such cases, animals would likely prefer alternatives to high-tannin foods.

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.

Preference for flavored wheat straw by lambs conditioned with intraruminal infusions of starch administered at different times after straw ingestion

We hypothesized that the development and extinction of preferences for flavored foods paired with infusions of starch depends on the delay between food ingestion and starch administration and on the frequency of starch administration. To test this hypothesis, we divided 24 lambs into three groups of 8 lambs/group. On even days, half of the lambs were offered chopped wheat straw flavored with a distinctive flavor, whereas the other half received straw with a different flavor. Starch (12% of the daily DE required by the lambs) was infused into the rumen immediately after lambs began to eat straw in Group 1 (150 g starch), immediately after lambs began to eat straw (75 g) and 30 min later (75 g) in Group 2, and when straw refusals were collected 1 h after lambs began to eat straw in Group 3 (150 g). On odd days, the flavors were switched and infusions of starch were replaced with water. After 8 d of conditioning, starch infusions were suspended, and lambs were offered a choice of wheat straw with the two distinctive flavors. All lambs acquired strong preferences for flavored straw paired with starch (P< .001), but the delay between straw ingestion and starch infusion affected preferences (P<.05). Lambs that received starch 1 h after beginning to eat straw (Group 3) had lower preferences than lambs that received starch immediately after beginning to eat straw (Groups 1 and 2). Lambs that received two starch infusions (Group 2) did not show higher preferences for flavored straw than lambs that received only one starch infusion immediately after beginning to eat (Group 1). Lambs in our study retained preferences for at least 17 d when eating straw was no longer reinforced with starch (P<.001). However, preferences were less persistent for lambs that received starch immediately after beginning to eat straw (Group 1) than for lambs that received starch in two pulses (Group 2) or lambs that received starch 1 h after beginning to eat straw (Group 3). Collectively, our results support the hypothesis that the delay between food ingestion and starch administration affects food preferences of lambs. These findings suggest that manipulating the chemical characteristics of foods, which affects the kinetics of forage degradation in the rumen, can increase food preferences in ruminants.

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.

Variation in food selection among lambs: effects of basal diet and foods offered in a meal

In studies of behavior and nutrition, we typically determine nutritional needs and formulate diets for the average member of the herd, not for specific individuals within a herd. Nevertheless, variation among individuals could affect productivity of the group if the diet diverges too much from what individuals at the extremes prefer to eat. Thus, it is important to determine the degree to which individuals within a group vary in their food preferences when offered alternatives. Our first objective was to determine the degree to which lambs differed in preference for foods high in energy (barley) or protein (alfalfa) (Exp. 1). When we offered lambs barley and alfalfa for ad libitum consumption for 25 d, individuals varied in the amounts of barley (range: 221 to 991 g/d) and alfalfa (range: 51 to 558 g/d) they consumed (P < .0001). At one extreme, individuals preferred a diet of 6% alfalfa and 94% barley; at the other extreme, individuals preferred a diet of 70% alfalfa and 30% barley. Our second objective was to determine whether lambs from Exp. 1 compensated, when fed a basal diet that was lower in alfalfa than they preferred, by ingesting foods higher in alfalfa when offered a meal (Exp. 2). Lambs were ranked according to the percentage of alfalfa (range from 6 to 70%) and barley (range from 94 to 30%) they ate during Exp. 1 and then assigned alternately to two treatments: 1) basal diet with similar proportions of alfalfa and barley consumed ad libitum (preferred diet) or 2) basal diet with 10% less alfalfa than consumed ad libitum (low-alfalfa diet). We then conducted three trials in which lambs fed the different basal diets were offered a meal for 15 min/d for 2 d of two foods that differed in barley and alfalfa. During Trial 1, when we offered barley and alfalfa, lambs in both groups preferred barley (138 g) to alfalfa (46 g) (P < .05). During Trial 2, when the test foods (barley and alfalfa) were diluted with grape pomace (20%), lambs fed the preferred basal diet ate more barley (116 vs 64 g) and less alfalfa (48 vs 87 g) than lambs fed the low-alfalfa basal diet (P < .05). During Trial 3, when we offered a food high in barley (80% barley and 20% pomace) and a food high in alfalfa (70% alfalfa, 14% cornstarch, and 16% pomace), lambs fed the preferred basal diet ate more of the high-barley food (124 vs 73 g) and less of the high-alfalfa food (45 vs 98 g) than lambs fed the low-alfalfa basal diet (P < .05). Collectively, these results illustrate that lambs varied greatly in their preferences for foods that differ in energy (barley) and protein (alfalfa), and that when their preferred basal diet was altered, lambs compensated by ingesting food that complemented their basal diet during a daily meal. The addition of grape pomace in Trials 2 and 3 reduced the protein content of the high-barley and high-alfalfa foods such that the high-barley food was only marginally adequate to meet needs compared with the high-alfalfa food. Lambs fed the low-alfalfa basal diet compensated by eating more of the high-alfalfa food than lambs fed the preferred basal diet.

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.

A rule of thumb in mammalian herbivores?

In two experiments on appetitive learning we conditioned lambs, Ovis aries, to particular concentrations of a flavour by mixing the flavour with an energy-rich food that complemented their energy-poor diet. The lambs were subsequently offered energy-rich food with five different concentrations of the flavour (the concentration to which they were conditioned, two higher concentrations, and two lower concentrations). At these tests, the lambs consistently preferred the weaker flavours. This finding stands in contrast to earlier results on generalization gradients. In a third experiment, similarly designed to the other two, we tested for effects of a strong flavour on the behaviour of lambs when they were offered a novel nutritious food. Half of the lambs were offered unadulterated wheat, and the others strongly flavoured wheat. We found that the flavour in itself was initially aversive. We propose that the lambs' avoidance of foods with strong flavours may be an expression of a rule of thumb of the type 'given a choice, avoid food with strong flavours'. Such a rule could be part of a risk-averse foraging strategy displayed by mammalian herbivores, and which could be of particular importance when they encounter unfamiliar foods. Copyright 1998 The Association for the Study of Animal Behaviour

Eating barley too frequently or in excess decreases lambs' preference for barley but sodium bicarbonate and lasalocid attenuate the response

We conducted experiments to determine whether preference for barley was affected when lambs ate various amounts of barley and whether lambs ate more barley when it contained lasalocid and sodium bicarbonate (NaHCO3), both of which attenuate acidosis. In Exp. 1, lambs were assigned to two treatments (six lambs/treatment). For 2 d, lambs in two treatments were offered either 400 or 1,200 g of rolled barley from 0600 to 0700 as a preload meal. A preference ratio [PR = barley ingested/(total amount of alfalfa + barley ingested)] was calculated based on lambs' intake when offered a choice of 200 g each of rolled barley and alfalfa pellets hourly from 0700 to 1100. After the preload meal, lambs in Treatment 1 (400 g preload) showed equal preference for barley (.52) and alfalfa (.48) for 4 h on d 1 (P > .05); their preference for barley was less after the meal of barley on d 1 (.52) than on d 2 (.72), but their preference for barley declined between h 3 (.81) and 4 (.55) of d 2 (P = .11). Lambs in Treatment 2 (1,200 g preload) showed a low preference for barley on d 1 (.29) and 2 (.19) (P < .001). In Exp. 2, lambs were assigned to four treatments (six lambs/treatment): 1) rolled barley + NaHCO3 (2%) + lasalocid (33 ppm); 2) rolled barley + NaHCO3 (2%); 3) rolled barley + lasalocid (33 ppm); or 4) rolled barley. Intake of barley by lambs offered NaHCO3 + lasalocid (Treatment 1) was greater (P = .07) than that by lambs offered NaHCO3 (Treatment 2), whereas intake by lambs offered lasalocid (Treatment 3) was similar (P > .05) to that by controls. We conclude that eating barley too frequently or in excess caused a decrease in lambs' preference for barley and that NaHCO3 and lasalocid attenuated the aversion.

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.

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.

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.

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.

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 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.

Food preference and acceptance of novel foods by lambs depend on the composition of the basal diet

Ruminants eat a variety of foods, varying in toxins and nutrients, but no hypotheses adequately explain this behavior. We offer an explanation, one which encompasses avoidance of toxins and acquisition of nutrients. A key concept in this hypothesis is aversion, the decrease in preference for food just eaten as a result of sensory input (taste, odor, texture, i.e., a food's flavor) and postingestive effects (of toxins and nutrients on chemo-, osmo-, and mechano-receptors) unique to each food. On the basis of this hypothesis, we predicted lambs would prefer familiar and novel foods that complemented the macronutrient composition of their basal diet. To assess the validity of this prediction, we fed 10 lambs in each of three treatments different levels of ground barley (high in energy) and alfalfa (high in protein) as a basal diet. We then offered them daily a meal of three ground foods differing in proportions of barley and alfalfa (familiar foods) or wheat and rabbit pellets (novel foods). We found that lambs fed a basal diet high in energy (barley) preferred food lower in energy and higher in protein (alfalfa); those fed a diet high in alfalfa preferred food high in barley (P < .01). In addition, the higher the barley or alfalfa content of the basal diet, the greater the acceptance of novel foods high in alfalfa (i.e., rabbit pellets) or grain (i.e., wheat), respectively (P < .01). All lambs preferred foods high in wheat to rabbit pellets or alfalfa (P < 0.01), evidently because wheat is high in energy and it differs in flavor from barley, which was eaten repeatedly as part of the basal diet. On the basis of these results, we contend that lambs preferred familiar and novel foods that complemented the flavors and macro-nutrient contents of their basal diet.

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.

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.

Effects of energy source and food flavor on conditioned preferences in sheep

Livestock and range managers would have a powerful tool to direct utilization and modify plant communities if animals could be conditioned to eat specific foods or plants. We attempted to condition preferences for a low-quality forage through nutrient loading. Sheep were fed licorice or orange-flavored straw pellets then were gavaged with glucose or propionate (.381 Mcal, which amounted to approximately 13% of the daily maintenance energy requirement) or water. Four groups of ewes (n = 4) were arranged in a cross-blocked design such that each group received a unique energy/flavor combination: 1) propionate+licorice, 2) propionate+orange, 3) glucose+licorice, or 4) glucose+orange. On alternate days, each group received the other flavor plus water to create an internal control. At the end of 8 d of conditioning, preference for the two flavors was measured by two-choice preference tests. A second trial was conducted for 4 d in which the energy level was doubled to .762 Mcal. Low energy levels of either glucose or propionate did not create significant preferences. Propionate at the low-level caused satiety but at the high level conditioned an aversion to both flavors. This high level of propionate apparently caused malaise that was then associated with the taste of the flavors. The high level of glucose conditioned a preference. The high glucose treatment increased rumen microbial mass, the nutrients of which would have been absorbed in the lower gastrointestinal tract, and may have indirectly provided the positive nutrient feedback required to form a preference. There was a flavor preference for orange that was independent of the energy supplements.

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).

Socially induced food avoidance in lambs: direct or indirect maternal influence?

Food avoidance can be directly motivated in the sense that an animal refrains from eating a food because of an aversion to it, or indirectly motivated in the sense that an animal ingests little of one item because it prefers another. We studied whether the reluctance of a lamb to eat a shrub (Cercocarpus montanus) its mother avoided resulted from a socially induced aversion to C. montanus or from a socially induced preference for an alternative shrub (Amelanchier alnifolia). Each lamb (n = 6) in the treatment group was exposed with its mother to either A. alnifolia or C. montanus for 5 min, followed by 5 min of exposure to the other food for 5 d. Mothers in the treatment group avoided C. montanus because its ingestion had previously been paired with lithium chloride, but they readily ate A. alnifolia. Each lamb in the control group (n = 6) was exposed with its mother to only A. alnifolia. Following weaning, lambs in both groups strongly preferred A. alnifolia to C. montanus when offered a choice between the two species (P < .05), but they readily ate C. montanus when only C. montanus was offered. Accordingly, the data support the hypothesis that the low consumption of C. montanus occurred as a result of a socially induced preference for A. alnifolia, not as a result of a socially induced aversion to C. montanus. If food avoidance had been directly mediated, lambs whose mother avoided C. montanus would have completely avoided the shrub during testing, as in the case when acquired food aversions are directly mediated by toxins.