Abstract:
In six experiments, sheep were fed ad libitum on either chopped lucerne hay, chopped wheaten straw, or a ground, pelleted lucerne hay/barley diet. On each diet either the dietary material itself, a more highly digestible material, or sawdust was introduced per fistulam into the reticulorumen in amounts equal to c. 25 % of the normal daily intake by weight. For each diet, with intraruminal additions on a single occasion only, the most accurate adjustments of oral intake on a weight or a digestible energy basis were observed when the diet itself was added. On the pellet diet, reduction of oral intake was most rapid and of greater magnitude when materials of high digestibility were introduced, though intake was also reduced when sawdust was introduced. On the chopped lucerne hay diet and on the chopped wheaten straw diet, oral intake was depressed more after the introduction of materials of lower digestibility. When pellets were introduced into animals fed on chopped wheaten straw, oral intake was at first reduced but subsequently increased, which indicated an interaction between the basal diet and the supplement. In longer-term experiments, daily introduction per fistularn of the dietary material at c. 25 % of the mean daily intake resulted in a rapid decrease in oral intake of each diet. The decreases in intake, expressed as a percentage of the weight of diet added per fistulam (response index), were 95-1 15 % (pellets), 60-80 % (lucerne hay), and 76-104% (wheaten straw), and generally became greater with time. With the pellet diet, trends in intake with time showed an increasingly greater adjustment to materials of high digestibility, and an apparent accommodation or compensation for imposition of a load of indigestible material. Such compensation for indigestible material was not apparent on the two all-roughage diets. Introduction of lucerne/barley pellets into the rumen of sheep fed on chopped wheaten straw resulted in no change, or a slight increase, in straw consumption, and an overall increase in intake of digestible dry matter. In two further experiments a bladder, expanded with water to volumes of 1,2, or 3 litres, caused significant decreases in intake of both the pellet diet and the chopped wheaten straw diet, effects being more rapid and more drastic in the case of the wheaten straw diet. Although there was subsequently a partial recovery of oral intake with the pellet diet, no such apparent compensation occurred with the wheaten straw diet. The results indicate that sensitivity to indigestible material can limit the intake of roughage even though the digesta content of the reticulorumen is not at a maximum, and confirm that important relationships exist between the nutritive value of the diet or nutritional status of the animal and the ability to accommodate indigestible material.