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-mouthed animals (e.g. cattle) and sheep grazing from the same sward. 4. When grazing selectively (between plant species) search times increase substantially and may come to limit intake rate as total herbage availability (both plant species) declines, or as the fractional cover of the more desirable species declines below about 20%. The selective grazing strategies and optimal diets, those which provide maximum net daily energy balance and/or 'value' to the grazing animal, are identified for animals grazing from plant species that differ in both the relative vertical availability (height, mass) and relative horizontal availability (fractional cover) as well as the total availability of herbage. 5. A decrease in the relative horizontal availability of a species reduces selectivity, but only to the point of indifference. A decrease in the relative vertical availability of a species may reverse species preferences.