Utilization by sheep of casein administered per duodenum at different levels of roughage intake

Abstract

The apparent digestibility of casein infused per duodenum at two levels (providing 3 or 6 g nitrogen per day) was between 90 and 105%. The efficiency of retention of casein nitrogen was investigated at three levels of roughage intake and the effects of level of nitrogen and level of roughage upon nitrogen retention in wool and in tissues other than wool were studied. Casein administration per duodenum resulted in an increased rate of digestion of cotton threads suspended in the reticulorumen, which indicated effective recycling of nitrogen. Actual rates of digestion were lower at higher roughage intakes. Possible reasons for this are discussed. At each level of roughage intake, casein administration led to increased nitrogen retention, the partial efficiency of retention of the added nitrogen being 56-85% for the first 3 g of casein nitrogen and 22-32% for the second 3 g. At a given level of casein, retention and partial efficiency of retention were increased by increased roughage intake. At low levels of roughage intake, the increase in wool nitrogen retention as a response to casein supplements was as great as at higher levels of roughage intake. Wool nitrogen retention was increased by 30%, from about 1.0 to 1.3 g/day, for the first 3 g of casein nitrogen, with a 10% partial efficiency of incorporation of added nitrogen into wool. The second 3 g of casein nitrogen led to a smaller additional increment in wool nitrogen retention, with less than 5% partial efficiency of incorporation of added nitrogen into wool. Increases in the level of roughage intake did improve the partial efficiency of retention of nitrogen in tissues other than wool, from 50 to 75% for the first 3 g of casein nitrogen, and from 15 to 30% for the second 3 g. This effect was not accompanied by any decrease in the partial efficiency of incorporation of administered nitrogen into wool.