FOOD-INTAKE AND DIET SELECTION IN SHEEP - THE EFFECT OF MANIPULATING THE RATES OF DIGESTION OF CARBOHYDRATES AND PROTEIN OF THE FOODS OFFERED AS A CHOICE

An experiment was designed to investigate whether the degree of synchr ony between the rates of digestion of carbohydrates and N of foods off ered as a choice would have an effect, through their consequences, on the short- and long-term diet selection of sheep. Four foods (R(L), R( H), S-L and S-H> with the same high metabolizable energy, and similar high metabolizable protein contents were made into pellets. Foods R(L) and R(H) were based on a rapidly fermentable carbohydrate source and foods S-L and S-H on a slowly fermentable carbohydrate source; within each source one food (R(L) or S-L) had a low, and the other (R(H) or S -H a high, rumen-degradable protein (RDP) content. The foods within a carbohydrate source were offered either singly or as a choice (R(L)/R( H) or S-L/S-H) to eleven rumen-fistulated mature sheep. The design was two 3 x 3 Latin squares (replicated once) with 5-week periods; square s consisted of two single foods and their respective choice. Weeks 1, 3 and 5 were considered to be controls, and weeks 2 and 4 used for rum en infusions of either urea or fructose infused over 4 h (10.00-14.00 hours). Food intake (FI) and diet selections (DS) were recorded daily and every 2 h (08.00-16.00 hours) on days 2-5 of each week; rumen pH a nd NH3 concentrations were also measured during these time intervals o f day 5. As expected, feeding treatment affected significantly the rum en measurements: rumen NH3 concentrations were higher on foods R(H) an d S-H, and rumen pH lowest on R(L). Daily FI was lowest on treatments S-L and choice S-L/S-H. The mean daily proportion of the low-RDP food in the selected diet was lower when the carbohydrate source was rapidl y (choice R(L)/R(H)) rather than slowly fermentable (choice S-L/S-H); this was consistent with the experimental hypothesis. Short-term infus ions affected further rumen variables (in the expected directions), ir respective of feeding treatment. However, DS over the 4 h infusion per iod were unaffected; these short-term DS were consistent with the ones selected over the longer term (daily). The results suggest that the l ong-term (daily) diet selection of sheep may be affected by the degree of synchrony of energy and protein to the rumen. The fact that diet s elections were not altered further by short-term manipulations of thes e supplies might reflect inadequacies of the methodology (infusions) a dopted here.