DISCRIMINATION AND PATCH CHOICE BY SHEEP GRAZING GRASS-CLOVER SWARDS

1. The proposition that large herbivores make foraging choices which maximize intake rate was examined in two experiments on discrimination by sheep grazing pairs of swards (patches) varying in composition of two botanical components, grass and clover, and varying in sward surface height. 2. Patch selection was influenced by sward height and by clover content, and their effects were additive. Tall patches were preferred, and intermediate contents of clover were selected in preference to low or high contents. This would reinforce the patchy distribution of the grass and clover components of mixed swards. Sheep did not exploit local variation in clover content within patches. 3. There was no evidence that patch choice was influenced by differences in the intake rate obtained from the two alternatives at hand, although on average the highest intake rates were obtained on tall patches with intermediate clover contents, coincident with the pattern of preference. 4. During frequent switching between patches, components of the functional response such as bite depth, bite weight and intake rate were modified by patch composition and also by the composition of the alternate patch. This short-term carry-over effect must increase the discrimination difficulties on heterogeneous vegetation. 5. It is argued that pre-consumption cues signalling patch quality are of less use to sheep than the information gained about a patch during grazing, but that this is initially distorted by recent experience; and that information constraints on the maximization of long-term intake rate will occur during frequent switching within heterogeneous vegetation.