A. J. Parsons

Diet Preference of Sheep: Effects of Recent Diet, Physiological State and Species Abundance

The preference of sheep for two temperate grassland species, ryegrass and white clover, was tested by releasing flocks (of three ewes) onto swards (0.25 ha) that contained adjacent monocultures of grass and clover, and observing their intake behaviour over 6 days. The test paddocks contained either 20, 50, or 80% clover by ground area to distinguish partial preference from indifference. To test whether recent diet affected preference, separate groups of dry (non-pregnant, non-lactating) ewes grazed prior to testing on one of three diet «back-grounds»: an all-grass sward, an all-clover sward, or a 50:50 grass/clover sward by area. To consider the effects of physiological state on preference, a further group of lactating ewes, also from a 50:50 grass/clover «background», were tested

Metabolic Profiles of Lolium perenne Are Differentially Affected by Nitrogen Supply, Carbohydrate Content, and Fungal Endophyte Infection

Lolium perenne cultivars differing in their capacity to accumulate water soluble carbohydrates (WSCs) were infected with three strains of fungal Neotyphodium lolii endophytes or left uninfected. The endophyte strains differed in their alkaloid profiles. Plants were grown at two different levels of nitrogen (N) supply in a controlled environment. Metabolic profiles of blades were analyzed using a variety of analytical methods. A total of 66 response variables were subjected to a principle components analysis and factor rotation. The first three rotated factors (46% of the total variance) were subsequently analyzed by analysis of variance. At high N supply nitrogenous compounds, organic acids and lipids were increased; WSCs, chlorogenic acid (CGA), and fibers were decreased. The high-sugar cultivar ‘AberDove’ had reduced levels of nitrate, most minor amino acids, sulfur, and fibers compared to the control cultivar ‘Fennema’, whereas WSCs, CGA, and methionine were increased. In plants infected with endophytes, nitrate, several amino acids, and, magnesium were decreased; WSCs, lipids, some organic acids, and CGA were increased. Regrowth of blades was stimulated at high N, and there was a significant endophyte × cultivar interaction on regrowth. Mannitol, a fungal specific sugar alcohol, was significantly correlated with fungal biomass. Our findings suggest that effects of endophytes on metabolic profiles of L. perenne can be considerable, depending on host plant characteristics and nutrient supply, and we propose that a shift in carbon/N ratios and in secondary metabolite production as seen in our study is likely to have impacts on herbivore responses.

A mechanistic model of some physical determinants of intake rate and diet selection in a two-species temperate grassland sward

1. A model is described which provides a mechanistic explanation of the functional response of intake rate to herbage availability [Leaf Area Index (LAI), height or mass] in large herbivores. 2. When grazing non-selectively (between plant species) rates of encounter (cf. search times) are unlikely to be limiting even at low herbage LAI in cool-temperate pastures. Bite rate and intake rate are limited by handling time. Even at low herbage LAI, the fixed time cost associated with opening and closing the jaw to prehend a bite limits the capacity of animals to compensate for low bite mass by increasing bite rate and so restricts intake rate. 3. The model proposes that differences in the time taken to masticate a unit bite mass are necessary to explain the major differences in intake rate between large-mounthed animals (e.g. cattle) and sheep grazing from the same sward

Not all sheep prefer clover : diet selection revisited

Several previous studies have shown that the proportion of clover in sheep diets was greater than the proportion of clover in the mixed species swards being grazed. Various hypotheses have been put forward to explain this observation. One widely held hypothesis is that sheep prefer (actively select) clover to grass. This hypothesis was tested by offering 12 sheep, six that had been recently grazing perennial ryegrass and six that had recently been grazing white clover, the choice between foraging on grass or clover provided as turves. It was found that, rather than prefer clover, the sheep preferred the opposite species to the one they had previously grazed. From these observations, alternative hypotheses are proposed to explain the disparity in composition of the diet relative to the swards observed in previous studies, and the evidence for a constant preference for clover and its basis in grazing behaviour is reconsidered.

Uptake, cycling and fate of nitrogen in grass-clover swards continuously grazed by sheep

Components of the N cycle were studied at Hurley, UK, in 1985-87. In grass-clover (Lolium perenne and Trifolium repens) swards, grazed at three intensities, low total inputs of N were associated with low outputs and losses of N. Nevertheless, the flows (intake and excretion) of N through animals were substantial and gave rise, at the higher intensities of grazing, to an acceptably high agricultural output per hectare. This was considered evidence of a fast and efficient recycling of N between plants, animals and soil (...)

How predictable are aphid population responses to elevated CO2

Experiments investigating the population responses of aphids to CO2 enrichment have yielded results suggesting that aphid populations will be both larger under elevated CO2 and that they will be smaller under elevated CO2. Most studies have failed to reject the null hypothesis of no difference in population sizes due to atmospheric CO2 concentration. This diversity of results has led some investigators to conclude that aphid responses are not general, and that every aphid-plant interaction may be unique and unpredictable a priori. We use a single, general, mathematical model to consider the population responses of cereal aphids to grass grown under different CO2 concentrations. The model shows that it is possible to explain any of the three observed results: larger populations, smaller populations, or no difference, and that which of these three outcomes arises may depend critically on the interaction between aphid nitrogen requirements and the nitrogen fertility of the soil. The model also shows that the qualitative results will depend on how sensitive the aphid species is to increases in its own density. Past studies have shown that aphids increase their production of winged offspring in response to increasing aphid density. The model predicts that, in general, aphid species that have lower nitrogen requirements and that are less sensitive to their own density will be more likely to have larger populations in elevated CO2 compared to ambient CO2. Differences between aphid species (and clones) in their nitrogen requirements and the strength of their density-dependent response have not been widely reported in the literature. Also, the nitrogen fertility of the soil has rarely been manipulated in experiments on aphid responses to rising CO2 levels. The model suggests that the diversity of population responses of aphids may be both understandable and predictable in the context of such an interaction.

The effects of group size on grazing time in sheep

Abstract Voluntary intake of herbage by grazing sheep ( Ovis aries L.) is the result of several interacting behaviours. In gregarious animals, such as sheep, the social environment may strongly influence some of the components of ingestive behaviour that control food intake. This study investigated the effects of group size on grazing time, one of the main components of ingestive behaviour governing daily herbage intake. Grazing time was recorded for focal sheep within groups, ranging in size from one to 15 over periods of 24 h. The animals were continuously stocked on a perennial ryegrass pasture ( Lolium perenne L. cv Parcour) maintained at a sward surface height of 6 cm. Groups were established 1 week prior to recording the behaviour of a focal animal in each grazing group. Recordings were made on three occasions for 24 h, with a period of 24 h between each recording. It was found that sheep in smaller groups spent less time grazing than sheep in larger groups. There was no relationship between group size and intra-meal intervals, prehension biting rate or number of meals, but animals in groups of one and two tended to have shorter meals than those in larger groups. There was indirect evidence, from measurements of changes in sward surface height, to suggest that intake of herbage was also reduced for animals grazing in groups of less than four. It was concluded that a minimum flock size of three sheep, but preferably four, is required for studies of grazing behaviour.

The use of spatial memory by grazing animals to locate food patches in spatially heterogeneous environments: an example with sheep

Abstract The ability of sheep to remember the spatial location of food patches was examined in two experiments. In the first experiment, patches (plastic bowls) were laid out on a grass field and some of the patches were randomly assigned to contain food (sheep pellets). Individual sheep were released into the field on a number of occasions and on each occasion the number of patches visited until all the patches that contained food had been located was recorded. Sheep were tested at three levels of complexity: one food patch among eight total patches laid out in a horizontal array; two food patches among 16 total patches laid out in a 8 × 2 grid; four food patches among 32 total patches laid out in a 8 × 4 grid. All patches were 5 m apart. Twenty sheep were tested in the one in eight level and ten sheep in the two more complex levels. At all levels of complexity, sheep returned to the patches that contained food, 12 h after first exposure, at a level better than that expected if they had searched at random. After six exposures to the distribution of patches, at 12 h intervals, sheep visited the food patches almost exclusively. The performance of the sheep did not decline when the time between successive visits was increased to 72 h. In the second experiment, individual sheep searched for one patch that contained food among eight total patches laid in a horizontal array on a grass field. Ten animals were tested with a cue (turf of white clover) located behind the food patch, and ten animals without. A wire mesh cage was placed over the top of the cue so that it could be seen and smelled, but not eaten. Sheep learned the location of the food patch with and without cues, but learned faster when a cue was present. When the location of the cue and food patch was switched randomly between trials, the sheep used spatial memory first and went to where the food patch had previously been located, but on finding no food used associations between the cue and food patch to go to the new food patch location. The ability to use spatial memory and associations between cues and rewards would help sheep to increase the rate at which they encounter preferred food patches, and to exploit spatial heterogeneity in the food resource better.

Intake and behaviour responses by sheep, in different physiological states, when grazing monocultures of grass or white clover

Abstract Monocultures of ryegrass (G; Lolium perenne cultivar ‘S23’) and white clover (C; Trifolium repens cultivar ‘Huia’) were maintained at sward surface heights (SSH) of 3 or 6 cm by continuous variable stocking with core groups of sheep ( Ovis aries L.) in different physiological states (physiology). Each group comprised four lactating ewes (L), and their twin lambs, plus two non-lactating, non-pregnant ewes (D). There were two plots each of G6 and C6 plus one plot each of G3 and C3. Each plot was 0.33 ha and was irrigated to maintain soil moisture deficit at −1 . The experiment ran from May to October. All lambs were weaned at 15 weeks of age on 8 August and removed from the experiment. Prior to weaning (spring/summer) measurements of ingestive behaviour were made on five occasions, using an automatic system to record jaw movements and by weighing animals before and after a 1 h period of grazing, to estimate herbage intake rate. One measurement of ingestive behaviour was made after weaning (autumn) on previously-lactating ewes and D ewes. Effects of physiology and herbage species on grazing behaviour were compared by regressing mean values for the groups of ewes on SSH. Generally ewes had higher intake rates of clover than grass and CL ewes had greater daily DM intakes than GL ewes (+ 0.56 kg). This gave rise to higher lamb growth rates (366, 312, 284 and 252 g day −1 , for treatments CL6, GL6, CL3 and GL3, respectively). It is suggested that sheep can eat clover faster than grass because less time is required to prehend and masticate a bite of a given mass (handling time) for clover than grass. However, ewes markedly altered their grazing time in relation to their physiological state and intake rate of herbage, and thus relationships between sward state and bite mass and intake rate, cannot be used to predict daily intake without an understanding of factors that control grazing time.

Optimal Diet Selection by a Generalist Grazing Herbivore

1. We develop a stochastic dynamic programming model of grazing behaviour for a generalist mammalian herbivore. The model considers that behaviour depends upon three state variables: stored energy, digestible gut fill and indigestible gut fill. When the plant community comprises two alternative species, the animal must choose between five alternative behaviours: grazing species i, grazing species j, grazing whichever species it encounters, resting or ruminating. 2. We use the model to distinguish diet preference and diet selection. Diet preference is the diet selected by the animal when it is operating under a minimum of environmental constrains and diet selection refers to the way in which environmental constraints modify the animals diet preference. 3. Although the model can be used for any mammal grazing in any plant community, we demonstrate solutions derived from parameter values relevant to sheep grazing a grass-clover plant community. 4. The model demonstrates that diet preference may depend on the relative intake rates of the two alternative plant species. Furthermore, preference may depend on the absolute intake rates at which the relative comparison is made. The model demonstrates that the optimal diet should have a temporal pattern across the day and that it may be sensitive to predation hazard. The model also predicts total daily intake. 5. We use the model to demonstrate that the complex patterns of diet preference are further modified when considering total abundance of species in the community (e.g. cover). 6. We explain how the model is heuristic in pointing out reasons why the literature on diet selection in this system, and in herbivores more generally, is equivocal on what is the basis of selection and preference