Exploring the nutritional basis of preference and diet selection by sheep

Efficient livestock production from pasture requires an understanding of preference and selection by grazing animals. Selective grazing influences diet not just in the short-term but more importantly, in the long-term through its affect on pasture composition. This paper summarises techniques developed to explore the nutritional basis for preference in complementary indoor and field studies. Indoors, sheep in metabolism crates were offered ryegrass and white clover in separate feed bins. Voluntary intake and eating behaviour were recorded, and following a period of free choice to establish preference, an intra-ruminal infusion of ammonia was given when sheep ate clover. We hypothesised that sheep given additional ammonia with clover would alter selection toward grass as a means of reducing the input of this metabolite from the rumen to the blood stream. When offered a choice, sheep selected a diet comprising 70–85% clover and 15–30% grass. Infusion of ammonia did not affect the proportion of grass or clover selected, but it reduced feed intake by about 15%. In field studies, each of 12 rumen-fistulated sheep was fitted with a harness carrying a remotecontrolled infusion pump and a pack of either water or a urea solution for infusion during grazing. Urea, as a rumen-ammonia precursor, was infused only during periods of eating. Sheep were offered either grass alone or clover alone and infused with urea or water over 4 days to create an association between eating that species and elevated ammonia load. The sheep were then offered a choice between adjacent monocultures of ryegrass and clover for 2 days to test preference and were infused only when the background species was selected. The sheep from the clover background responded to elevated rumen ammonia. They reduced grazing time on both clover alone (275 mins grazing/day when infused with urea vs 325 mins/day for water controls), and when they had a choice between grass and clover (425 vs 490 mins/day for urea infused and control sheep, respectively). These techniques provide novel opportunities for identifying the roles of plant nutrients in preference and ways these might be manipulated to control diet selection and intake. This paper describes the methods used, and gives preliminary results of trials with clover and grass using ammonia compounds.