Factors limiting the intake of feed by sheep. V. Feed intake and the productive performance of the ruminant lamb in relation to the quantity of crude protein digested in the intestines

Abstract

The voluntary feed consumption (VFC), liveweight increase, and wool growth of lambs, initially 9-10 weeks old and weighing 22 kg, were measured over 6 weeks when pelleted diets of differing protein content were offered. The crude protein contents of the diets, on a dry matter basis, were 11.7% (LP diet), 15.8% (MP diet), and 19.1 % (HP diet). The digestion of the HP and LP diets was studied, with use of markers, in similar lambs prepared with stomach fistulae. VFC, expressed as grams dry matter/day/kg body weight0.75, was 82 (SE�3) with the LP diet, 94�3 with the MP diet, and 97�4 with the HP diet. The dietary crude protein intakes were 110, 178, and 220 g/day with the LP, MP, and HP diets respectively and the corresponding quantities of crude protein (6.25 x nitrogen in forms other than ammonia) apparently digested in the intestines (DCP,) were 75, 134, and 179 g/day. The LP and HP diets provided about 12 and 23 g DCP1/lOO g digestible organic matter respectively; the value for the MP diet was estimated to be 18 g DCP1/100 g. The rates of liveweight increase were 153 � 16, 240�9, and 267�12 g/day with the LP, MP, and HP diets respectively. Wool production was the same with the MP and HP diets but lower with the LP diet. The level of intake of DCP1 and the DCP1/digestible organic matter ratio had little effect on the gross chemical composition of the lambs, as estimated indirectly from tritiated water space. The levels of essential amino acids in peripheral blood plasma were generally in the order HP diet > MP diet > LP diet. Digesta flow from the rumen and abomasum, rumen volume, ruminal volatile fatty acid concentration, and pH of ruminal digesta were similar with the LP and HP diets. It was concluded that a level of about 18 g DCP1/100 g digestible organic matter was adequate for the expression of maximum VFC and that concentrations in excess of this would have little enhancing effect on liveweight gain or wool production under conditions of ad libitum feeding.