Biological values of protein in grasshays cut in two years and at different times of the year were estimated in 9 trials with groups of 2 to 4 male Texel lambs, by different methods: 1) the method of Thomas and Mitchell: a) assuming a value of 3.438 for metabolic faecal protein or b) using values estimated by pepsin-HCI digestion of the faeces; 2) different modifications of the nitrogen balance index method.Protein-poor and moderate-protein diets were used with, respectively, 3.2-7.6 and 7.3-10.3 % crude protein in dry matter. The average biological value of all hays in the low-protein diets was 89.3 and 89.9 and in the moderate-protein rations 77.8 and 75.7 (methods 1a and 1b resp.). Method 2 gave lower values, between 44.4 and 90.6. Biological value of protein in autumn hays was lower than in spring hays. Differences in composition of protein and non-protein N and heating in the stack may be partly responsible.
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