Factors limiting the intake of feed by sheep. VII. The digestion of a medium quality roughage and the effect of post-ruminal infusion of casein on its consumption by young sheep

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Feed intake and digestion studies were conducted with sheep offered a medium quality roughage which contained, on a dry matter basis, 33.4% of acid detergent fibre and 10.3 % of crude protein. The roughage had an organic matter (OM) digestibility of 57%. The quantity of OM apparently digested in the stomach was equal to 63 % of the OM digested in the alimentary tract, and 80% of the digestion of the dietary fibre took place in the stomach. The quantity of nitrogen in forms other than ammonia that left the stomach in digesta was similar to the dietary nitrogen intake and the apparent digestibility of this digesta nitrogen in the intestines was 66%. Approximately 13 g of crude protein was digested in the intestines per 100 g digestible OM intake. The values of parameters relating to the movement of digesta through the stomach were within the ranges expected with roughage diets. The levels of essential acids in the peripheral blood plasma of sheep given the roughage ad libitum were comparatively low; the values for the total essential amino acids ranged from 85 to 105 pmoles/100 ml and varied with the time of day. The voluntary consumption of roughage by young sheep (8-9 months of age) was 66 g OM/day/(kg body eight)0.75, an intake that did not permit the animals to obtain sufficient energy to express their growth potential. Post-ruminal infusion with casein (34-39 g/day), although increasing the amount of crude protein digested in the intestines (DCP1) by about 30%, failed to increase voluntary feed consumption or rate of body growth. Accordingly voluntary feed consumption and body growth were not limited by the availability of essential amino acids at the tissue level. It was concluded that the voluntary feed consumption and rate of body growth in young sheep would not be limited by the supply of essential amino acids at the tissue level with diets providing 13 g or more of DCP1 per 100 g digestible OM-the equivalent of about 49 g DCP1/Mcal net energy.