How degree of selectivity modifies foraging behaviour of dry ewes on reproductive compared to vegetative award structure

This study was conducted to investigate the effect of sward phenological stage in interaction with the degree of selectivity for green leaves on foraging behaviour of dry ewes. We tested the hypothesis that dry matter (DM) intake per bite, intake rate and intake were determined by the amount of preferred food items (green leaves) per unit area, and that the relationships did not vary with the phenological stage of the sward nor the degree of selectivity of the animal. Two sward structures were compared: V = vegetative, and R = reproductive (fully emerged inflorescence stage, 15% reproductive tillers). They were compared under rotational grazing by 11 ewes, with 6 ewes of variable degree of selectivity studied in detail. Measurements of foraging behaviour were made during the progressive grazing down of the paddocks. Green leaf mass per ha (GLM) was the variable most strongly correlated with intake per bite in both sward structures, but inclusion of the variable stem mass per ha (STEM) in the model resulted in slight, but significant improvement in the prediction of intake per bite. The coefficient of the variable STEM in the multiple regression varied among animals (P < 0.05) and was related to faecal nitrogen concentration, an indicator of animal's degree of selectivity. Time per bite increased linearly with intake per bite, the increase being greater on the reproductive than on the vegetative sward structure. Consequently, parameters of the predictive models of intake rate and digestible intake rate varied with sward phenological stage. On the reproductive sward, slope of the regression of time per bite on intake per bite and parameters of the predictive models of intake rate were linearly related to the animal's degree of selectivity. These results emphasize the importance of inter-individual variation in the trade-off between quality of the diet selected and intake rate. For a given green leaf mass, grazing time was higher on R than V. Grazing time, daily dry matter (DM) intake and digestible daily DM intake were not related to the degree of selectivity of the animals. (C) 1998 Elsevier Science B.V.