The effect in ewes of source and level of dietary protein on milk yield, and the relationship between the intestinal supply of non-ammonia nitrogen and the production of milk protein

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Abstract
Individually penned Finnish Landrace × Dorset Horn ewes in early lactation, and each suckling two lambs, were used to test the effects on milk yield, milk composition and the concentrations of some plasma constituents of supplementing a basal diet with either urea, groundnut, soya bean, linseed, fish, meat and bone, or blood meal. The basal diet contained 94 g crude protein and 10 MJ of metabolizable energy per kg dry matter and supplied daily 0·3 MJ of metabolizable energy per kg body weight. Except for urea, which was tested at an inclusion rate that increased the protein (nitrogen × 6·25) content of the basal diet by 43 g/kg, the remaining sources were tested at three levels, corresponding to increases in protein content of 34,60 and 86 g/kg. Daily milk yields were 1·92 and 2·08 kg for ewes given the basal diet and the basal diet supplemented with urea. For the high inclusion rates of each protein source the following daily yields were obtained (kg): groundnut, 2·26; soya bean, 2·45; meat and bone, 2·49; linseed, 2·68; fish, 2·84; and blood meal, 2·91. The daily yields of true protein in milk were (g): basal diet, 76; basal diet plus urea, 80; groundnut, 104; soya bean, 107; meat and bone, 105; linseed, 112; fish, 136; and blood meal, 125. Plasma concentrations of free fatty acids did not appear to support the hypothesis that the milk yield response to protein is accomplished solely by increases in tissue energy loss.The increment in the amount of non-ammonia nitrogen reaching the abomasum as a result of the low levels of inclusion of the protein sources in the basal diet was used for the production of true protein-nitrogen in milk with an efficiency of 0·58.

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