Roberto A. Distel

Patch selection by cattle: maximization of intake rate in horizontally heterogeneous pastures☆

Abstract The role of various perceptual cues on selectivity patterns of cattle grazing heterogeneous swards were studied in a controlled experiment. Patches of different height and/or bulk density, representing similar or dissimilar potential instantaneous intake rate (IR), were created in a ryegrass ( Lolium perenne L.) pasture to study foraging responses of cattle ( Bos taurus ) to heterogeneity in patch structure. The experiment involved three heterogeneity treatments with two patch types each. One patch type (short-dense) was present in all three treatments. In all treatments, cattle encountered and entered both patch types available with the same frequency. In contrast, average residence time and total number of bites removed were greater in the patch type that allowed greater instantaneous intake rate. Patch utilization and residence time agreed with qualitative predictions of a rate-maximizing model. Residence time in short-dense patches was shortest in the treatment that allowed the greatest intake rate, and longest in the one that allowed the lowest intake rate. Short-dense patches were relatively avoided when the alternative patch was tall and dense, but they were preferred when the alternative patch was short and sparse. The pattern of relative preference for the different patches ruled out vegetation height, density and herbage mass as cues for the allocation of residence time. Steers consistently spent more time in the patches that allowed greater bite weights and instantaneous intake rate while at the patch. Relative preference of patches exhibited a pattern of overmatching, under the assumption that herbage intake was the reward to grazing behavior. Overmatching indicates that behavior was allocated to patches more than proportionally to the relative rewards. However, the degree of overmatching was less than predicted by maximization of intake rate. Grazing behavior and residence time did not respond to repeated short-term exposures to a treatment within the same day.

Decomposition of and nutrient dynamics in leaf litter and roots of Poa ligularis and Stipa gyneriodes

Abstract A field experiment was conducted in a temperate semi-arid grassland of central Argentina to determine mass loss and P and N concentrations of decaying leaf litter and roots of Poa ligularis (high-quality litter) and Stipa gynerioides (poor-quality litter). Leaf litter and roots of each species were incubated on sites dominated by P. ligularis and on sites dominated by S. gynerioides , to separate species from site effects. The experiment lasted up to 21 months. Mass loss of leaf litter and roots was higher ( p P. ligularis than in S. gynerioides . Mass loss of leaf litters were higher ( p P. ligularis sites, whereas mass loss of roots were similar ( p >0.05) on both type of sites. Both species showed similar pattern of P and N dynamics in the leaf litter and roots, but net mineralization was higher ( p P. ligularis than in S. gynerioides .

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.

Inter- and Intraspecific Variations in Production of Spines and Phenols in Prosopis caldenia and Prosopis flexuosa

In central Argentina the leguminous tree Prosopis caldenia is more abundant in sites of relatively high productivity (lowlands), whereas P. flexuosa is more abundant in sites of relatively low productivity (slopes and uplands). Based upon current antiherbivore defense theory, we predicted: (1) a higher investment in defenses in P. flexuosa than in P. caldenia, and (2) that limitations in resources would result in an increase of the defenses in both species. Our approach for testing these predictions was to estimate leaf phenol concentration and spinescence in adults (field study) and seedlings (greenhouse study) of both species growing at different levels of resource availability. In adult plants, the concentration of phenols was higher (P < 0.01) in P. flexuosa than in P. caldenia, but the opposite relationship was observed in seedlings. The amount of biomass invested in spines was similar (P > 0.10) in both species, whereas spine density was higher (P < 0.05) in P. caldenia than in P. flexuosa. In both species, limitations in resources did not result (P > 0.05) in increases in leaf phenol concentration, amount of biomass invested in spines, spine length, or spine density, except for the increase (P < 0.05) of spine density in seedlings of P. caldenia at low water and nutrient availability. In general, our results did not support current hypotheses on the production of antiherbivore defenses. It is argued that factors such as herbivore behavior (e.g., habitat selection, trampling, branch breakage) and alternative sinks for carbon (e.g., N2 fixation, carbohydrate reserves), in addition to resource availability in evolutionary and ecological time, should be considered for a more complete understanding of the inter- and intraspecific variations in the production of both physical and chemical antiherbivore defenses.

Relationships between early experience to dietary diversity, acceptance of novel flavors, and open field behavior in sheep

This study determined whether early experiences by sheep to monotonous or diverse diets influence: (1) plasmatic profiles of cortisol, a hormone involved in stress responses by mammals, before and after an ACTH challenge, (2) the readiness to eat new foods in a new environment, (3) general fearfulness and response to separation--as measured by the open field test (OFT) and stress induced hyperthermia (SIH)--and (4) the link between (2) and (3). Thirty, 2-mo-old lambs were randomly assigned to 3 treatments (10 lambs/treatment). Lambs in one treatment (Diversity--DV) received in successive periods of exposure all possible 4-way choice combinations of 2 foods high in energy and 2 foods high in protein from an array of 6 foods: 3 high in energy (beet pulp, oat grain, and a mix of grape pomace:milo [40:60]) and 3 high in protein (soybean meal, alfalfa, corn gluten meal). Lambs in another treatment (DV+T) received the same exposure described for DV but two phytochemicals, oxalic acid (1.5%) and quebracho tannins (10%) were randomly added within any period of exposure to foods high in energy or to foods high in protein. Lambs in the third treatment (Monotony--MO) received a monotonous balanced ration containing all 6 foods fed to the other groups. After exposure, lambs were offered a choice of the aforementioned 6 foods (DV; DV+T) or the monotonous diet (MO). Lambs were intravenously injected with ACTH 1 h after food presentation, and sampled at 1, 2, and 3 h post feeding for determinations of plasma cortisol concentrations. Reluctance to eat novel flavored foods (onion-, coconut- and cinnamon-flavored wheat bran), open field behavior, and SIH was assessed in all treatments. Lambs in MO showed greater concentrations of plasma cortisol 1 h after food presentation than lambs in the DV or DV+T treatments (P=0.04). However, the difference was small and no differences among treatments were detected after an ACTH challenge (P>0.1). Lambs in DV consumed more onion-flavored wheat bran than lambs in MO (P=0.05). Lambs in DV also showed a greater cumulative consumption of novel flavors on d 2 than lambs in MO (treatment×day; P=0.01). Lambs in DV showed lower increase in rectal temperature (P=0.07) than lambs in MO. Only lambs in DV exhibited a positive relationship between consumption of cinnamon-flavored wheat bran and attempts of escape (R(2)=0.58; P=0.02). Our results suggest that exposure to diverse foods early in life may be less stressful than exposure to monotonous rations, as measured by plasma cortisol concentrations after food ingestion, and by changes in rectal temperature after exposure to the OFT. Lambs exposed to diverse diets early in life may also increase the initial acceptance of new flavors in novel environments relative to lambs exposed early in life to monotonous diets.

Early experience with diverse foods increases intake of nonfamiliar flavors and feeds in sheep.

This study determined whether early experiences by sheep with monotonous or diverse diets influence intake of unfamiliar flavors and feeds later in life. Thirty 2-mo-old lambs were randomly assigned to 3 treatment diets (n = 10): diverse (DIV), diverse with plant toxins (DIV+T), and monotonous (MON). Lambs in DIV received in 9 successive periods of exposure 4-way choice combinations of 2 foods high in energy and 2 foods high in protein from an array of 6 foods: 3 high in energy [beet pulp, oat grain, and a mix of milo:grape pomace (60:40)] and 3 high in digestible protein (DP) (soybean meal, alfalfa, corn gluten meal). Lambs in DIV+T received the same exposure as DIV, but 2 plant toxins, oxalic acid (1.5%) and quebracho tannins (10%), were randomly added to 2 of the feeds in each of the choice combinations. Lambs in MON received a monotonous balanced diet, made with a mixture of all 6 feeds detailed before. All treatments received their feed in 4 separate buckets. During exposure, treatments did not differ in total daily DMI (P = 0.31), but daily intake of ME was less (P < 0.02) and daily intake of DP was greater (P < 0.03) for lambs in DIV and DIV+T than for lambs in MON. Treatments did not differ in ADG or G:F (P > 0.05). After exposure, lambs were offered a familiar feed (wheat bran) containing novel flavors (maple, garlic, or bitter) and 2-way choices of novel feeds (fescue hay vs. corn distillers grains, rice vs. calf manna, and green peas vs. rolled oats). Intake of maple-flavored wheat bran tended (P = 0.08) to be greater for lambs in DIV than for lambs in DIV+T and MON. Intake of bitter-flavored and garlic-flavored wheat bran were greater (P = 0.03 and P = 0.04, respectively) for lambs in DIV and DIV+T than for lambs in MON. During 2-way choice trials, lambs in DIV, but not in DIV+T, showed greater intakes of fescue hay (P = 0.05) and rice (P = 0.04) than lambs in MON. Intake of green peas was greater (P = 0.03) for lambs in DIV and DIV+T than for lambs in MON. At the end of testing, lambs in DIV but not in DIV+T showed greater ADG than lambs in MON (P = 0.05). Thus, early exposure to diverse foods enhanced acceptance of novel flavors relative to early exposure to a monotonous ration. Early experience with diverse feeds plus plant toxins led to a less diverse diet than early experience with diverse feeds. Early exposure to diverse feeds may be beneficial in production systems that require rapid acceptance and high intake of unfamiliar feeds.

Role of early experience in the development of preference for low-quality food in sheep

Domestic ruminant selectivity induces floristic changes in pasturelands, risking sustainability and limiting the subsequent availability of susceptible plant species. Development of preferences for species of lower nutritional quality may help to overcome those problems. In this study, we tested the hypothesis that early experience of sheep with a low-quality food (LQF) in a nutritional enriched context increases preference for LQF in adulthood. We predicted a higher proportional consumption of LQF in experienced lambs (EL) than in inexperienced lambs (IL) in choice situations involving LQF and alternative foods. Additionally, we determined intake of LQF by EL and IL at different levels of high-quality food (HQF) availability. From 60 to 210 days of age, EL were fed in separated feed bunks mature oat hay (LQF) simultaneously with sunflower meal (SM) and corn grain (CG), whereas IL were fed alfalfa hay (HQF) simultaneously with SM and CG. After exposure, EL and IL were offered LQF in free choice situations involving alternative foods, and also at five levels of HQF availability (100%, 75%, 50%, 25% and 0% of ad libitum intake). Proportional consumption of LQF was lower or similar in EL than IL. Intake of LQF was also lower or similar in EL than IL at all levels of HQF availability, except when the LQF was the only food available. Our results did not support the hypothesis that early experience with a LQF in a nutritional enriched context increases preference for LQF in adulthood. On the contrary, experience with LQF diminished subsequent preference for LQF in adulthood. It is proposed that, in the conditions of our study, continuous comparison between the LQF and the high-quality supplements (CG and SM) during the early exposure period lead to devaluation of LQF by EL through a simultaneous negative contrast effect.

Stemflow and Soil Water Redistribution in Intact and Disturbed Plants of Larrea divaricata in Southern Argentina

The interception of rainfall by the shrub canopy and its redistribution via stemflow to deep soil layers represents an important source of deep-soil-water. Therefore, reductions in stemflow caused by shrub canopy disturbance should be reflected in less moisture in the deeper layers of the soil. The objectives of this study were to determine (1) stemflow, (2) soil moisture next to the root crown at three depths (10–25 cm; 25–45 cm; 45–75 cm), and (3) predawn plant water potential in intact and sprout regrowth after mechanical clearing of Larrea divaricata in southern Argentina. Stemflow was collected and measured through funnels placed at the plant crowns. Soil moisture was measured by time-domain reflectometry and water potential with a pressure chamber. Stemflow was lower (P ≤ 0.01) in disturbed (3.6% precipitation) than in intact (7.2% precipitation) plants. Soil moisture next to the plant crown was similar or lower (P ≤ 0.05) in disturbed compared to intact plants, particularly at deep soil layers. Variations of predawn water potential in intact plants were accounted for (P ≤ 0.05) by variations in soil water content at the 45–75 cm depth, whereas variations of predawn water potential in disturbed plants were accounted for (P ≤ 0.05) by variations in soil water content at both the 45–75 cm depth and 10–25 cm soil depth. Sprouts of L. divaricata after mechanical clearing showed reduced stemflow, similar or lower soil moisture next to the root crown at different soil depths, and stronger association of predawn water potential with shallow water, compared to intact plants of the same species.

Leaf water potential trends in three grasses native to semiarid Argentina.

The natural grazing land of the semiarid region of central Argentina is subject to long and intense periods of drought during the hot season. The aim of the present study was to analyze the pattern of leaf water status for the cool-season grassesPiptochaetium napostaense and Stipa tenuis and the warm-season grass Pappophorum subbulbosum, which are important forage species in this grazing area. Leaf water potential, soil water potential, and relative humidity were measured during 1983 and 1984. At soil water potentials above -0.5 MPa, the leaf water potential of the 3 species showed high values at 0800, after nocturnal rehydratation, and minimum values at 1400, coincident with maximum atmospheric water demand. P. subbulbosum showed a greater capacity to withstand the drop in water potential at 1400 than did the cool-season species. During periods of low soil water potential the range of daily variation in leaf water potential was reduced in the 3 species because of marked decreases in the maximum and minimum values. Leaf water potential in the cool-season species correlated better with the deficit of atmospheric vapor pressure than with soil water potential. Leaf water potential of P. subbulbosum on the other hand showed a higher correlation with soil water potential than with atmospheric vapor pressure deficit.

Effects of supplementing endophyte-infected tall fescue with sainfoin and polyethylene glycol on the physiology and ingestive behavior of sheep

Tannins in sainfoin (Onobrychis viciifolia) may bind to alkaloids in endophyte-infected tall fescue [E+; Lolium arundinaceum (Schreb.) Darbysh.] and attenuate toxicosis. If so, supplementing E+ with sainfoin will increase use of E+ by sheep, and polyethylene glycol (PEG)-a polymer that selectively binds to tannins-will reduce such response. To test these predictions, thirty-six 2-mo-old lambs were randomly assigned to 3 treatments (12 lambs/treatment). During exposure, all lambs were individually penned and fed E+ supplemented with beet pulp (CTRL), fresh-cut sainfoin and beet pulp (SAIN), or fresh-cut sainfoin plus PEG mixed in beet pulp (SAIN+PEG). Feed intake was measured daily. Rectal temperatures and jugular blood samples were taken at the beginning and end of exposure. After exposure, all lambs were offered choices between endophyte-free tall fescue (E-) and orchardgrass, and preference for E- was assessed. Then, all lambs were allowed to graze a choice of E+ and sainfoin or a monoculture of E+. The foraging behavior of lambs was recorded. When sainfoin was in mid-vegetative stage, lambs in SAIN ingested more E+ than lambs in CTRL (P = 0.05), but no differences were detected between lambs in SAIN+PEG and CTRL (P = 0.12). Sainfoin supplementation improved some physiological parameters indicative of fescue toxicosis. Lambs in SAIN had lower rectal temperatures (P = 0.02), greater numbers of leukocytes (P < 0.001) and lymphocytes (P = 0.03), and greater plasmatic concentrations of globulin (P = 0.009) and prolactin (P = 0.019) than lambs in CTRL. Some of these differences were offset by the SAIN+PEG treatment. When lambs were offered choices between E- and orchardgrass, only lambs in SAIN had greater intake of E- than lambs in CTRL (P < 0.001). When lambs were allowed to graze a choice of E+ and sainfoin, all treatments used E+ to the same extent (P > 0.05). On the other hand, when they grazed on a monoculture of E+, lambs in SAIN+PEG showed greater acceptance of E+ than lambs in SAIN or in CTRL (P < 0.05). In summary, sainfoin supplementation alleviated several of the classic signs of fescue toxicosis and increased intake of endophyte-infected tall fescue. Tannins in sainfoin partially accounted for this benefit since feeding a polymer that selectively binds to tannins (PEG) attenuated some these responses. However, sainfoin supplementation during initial exposure to E+ did not lead to an increased preference for E+ during grazing.