PATCH SELECTION BY CATTLE - MAXIMIZATION OF INTAKE RATE IN HORIZONTALLY HETEROGENEOUS PASTURES

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.