Factors limiting the intake of feed by sheep. VIII.* The roughage requirement of the ruminant lamb fed on concentrate diets based on wheat

Abstract

The voluntary feed consumption (VFC) and body weight gain of lambs initially 12 weeks old and weighing 22 kg were measured when diets based on whole wheat grain and varying in straw content were offered. The basal diet comprised, on an organic matter (OM) basis, about 82 parts whole wheat grain and 18 parts protein concentrate; ground wheaten straw was added to provide diets with 2%, 7% or 13% straw. Daily VFC, expressed in relation to body weight in kg0.75 (BW0.75), increased from 85.6 with 2% straw through 90.4 with 7% straw to 94.2 with 13% straw. The corresponding OM digesti bilities were calculated to range from 84.8 to 77.6%. However, the intake of digestible OM and the rate of body weight gain were not affected by the straw content of the diet, the respective mean values for all lambs being 73g/day BW0.75 and 293 g/day. Lambs initially 25 weeks old and weighing 38 kg were offered a diet (2% straw) based on wheat in which the wheat was presented either as whole grains or in ground and pelleted form. With the whole grain diet the daily rates of OM intake, digestible OM intake and body weight gain were 75.6 g/ BW0.75, 64.7 and 254 g respectively; the corresponding values with the ground grain diet were about 20% lower. Studies involving the slaughter of animals showed that a large proportion of the wheat grain consumed reached the rumen in the form of whole grains and these contributed significantly to the dry matter of the ruminal digesta. The rate of ruminal digestion of whole wheat grains was slow (37% in 96 hr) when the sheep were given a diet based on wheat. By contrast, when a roughage diet was consumed the rate was high (81% in 96 hr). Fractured grain was rapidly digested in the rumen (67% in 20 hr). The mean residence time of water-soluble marker in the rumen and the rate of digesta flow from the rumen were greater in fistulated sheep given a whole grain diet than with a ground grain diet. The diets also differed with respect to the concentration of potassium in rumen liquor, the rumen potassium pool size and the rate of flow of sodium and volatile fatty acids to the omasum. It was concluded that the roughage requirement of the ruminant lamb fed on concentrate diets based on whole wheat did not exceed the equivalent of 2 g ground straw per 100 g diet. The possibility of a higher roughage requirement with ground wheat diets is suggested. ___________________ *Part VII, Aust. J. Agric. Res., 24: 387 (1973).