Herbage intake in relation to availability and sward structure: grazing processes and optimal foraging.
The relationship between grazing intake, herbage availability and sward structure was investigated using grazing process models. The general model expressed the instantaneous intake rate (IIR) while grazing as the ratio of bite wt:the sum of searching and handling time per selected bite. The general model was modified to make it applicable to 3 different descriptions of the sward. Model A analysed the effects of vegetation cover, height, bulk density and herbage mass on IIR, assuming uniform distribution of herbage mass within food items. Model B explored the effect of spatial heterogeneity on IIR, using a theoretical normal function to describe the availability of bite wt. It was assumed that the animal selected a range of bite wt so as to maximize IIR. Model C proposed a method of characterizing sward heterogeneity from empirical data. Model A showed that the mechanism and magnitude of intake response to increasing herbage mass depended upon the combination of sward structural attributes that led to that increase. Similarly, intake rate at a given herbage mass depended strongly upon the spatial organization of the herbage. Models B and C show that the greater the variance of bite wt in the sward at a given mean herbage mass, the greater the max. IIR, and the narrower the range of bite wt selected. At constant bite wt variance, IIR increased asymptotically with mean herbage mass. These models may explain the considerable variation in functional response to herbage availability reported in the literature. It appears that heterogeneity in potential bite wt needs to be accounted for if realistic predictions of intake rate and sward dynamics are required.