An Analysis of Forage Preference Indices

Abstract

Of those models currently used to describe the preference of animals for various plants under given conditions, all have serious shortcomings for purposes of accurately explaining the data, in the regression sense. When five equations, based in various ways on preference and availability, were used to estimate diets of cattle and sheep, no clear advantage of one expression over another could be found. All models tested with the sheep data resulted in increased predicted sums of squares compared with total sums of squares. In contrast, models tested with the cattle data showed some reduction in unexplained variation in diet estimates during the entire year, spring, and summer, but not during fall. This improvement was probably because the cattle pastures were more homogeneous than the sheep pastures and species were aggregated. The best model was Ratio 4 (preference-availability) but it requires a complex and expensive parameter estimation technique. It was concluded that sampling problems combine with inadequacies of the preference indexes to prevent accurate representation of the concept of diet preference. It was also concluded that sampling problems arise when the fecal, rumen fistula, or esophageal fistual techniques are used to estimate diets. A technique for adjusting these techniques to make them suitable for predicting diets was described. Further investigations into animal behavior are needed to determine variables which affect what the animal perceives as being desirable in relation to what is available.