Herbage Intake in Relation to Availability and Sward Structure: Grazing Processes and Optimal Foraging

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Abstract
(1) The relation between grazing intake, herbage availability and sward structure is investigated using grazing process models. The general model expresses the instantaneous intake rate (IIR) while grazing as the ratio of bite weight to the sum of searching and handling time per selected bite. (2) The general model is modified to make it applicable to three different descriptions of the sward. Model A analyses the effects of vegetation cover, height, bulk density, and herbage mass on IIR, assuming uniform distribution of herbage mass within food items. Model B explores the effect of spatial heterogeneity on IIR, using a theoretical normal function to describe the availability of bite weights. It is assumed that the animal selects a range of bite weights so as to maximize IIR. Model C proposes a method of characterizing sward heterogeneity from empirical data. The model is applied to field data. (3) Model A shows that the mechanism and magnitude of intake response to increasing herbage mass depends upon the combination of sward structural attributes that led to that increase. Similarly, intake rate at a given herbage mass depends strongly upon the spatial organization of the herbage. (4) Models B and C show that the greater the variance of bite weight in the sward at a given mean herbage mass, the greater the maximum IIR, and the narrower the range of bite weights selected. At constant bite weight variance, IIR increases asymptotically with mean herbage mass. (5) These models may explain the considerable variation in functional response to herbage availability reported in the literature. It appears that heterogeneity in potential bite weight needs to be accounted for if realistic predictions of intake rate and sward dynamics are required.