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 i n a ryegrass (Lolium perenne L.) pasture to study foraging responses o f cattle (Bos taurus) to heterogeneity in patch structure. The experim ent 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 availa ble with the same frequency. In contrast, average residence time and t otal number of bites removed were greater in the patch type that allow ed greater instantaneous intake rate. Patch utilization and residence time agreed with qualitative predictions of a rate-maximizing model. R esidence time in short-dense patches was shortest in the treatment tha t allowed the greatest intake rate, and longest in the one that allowe d 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 relat ive 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 great er bite weights and instantaneous intake rate while at the patch. Rela tive 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 th an proportionally to the relative rewards. However, the degree of over matching was less than predicted by maximization of intake rate, Grazi ng behavior and residence time did not respond to repeated short-term exposures to a treatment within the same day.