Studies of protein requirements of ruminants

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Abstract
Nitrogen balance trials were made with cattle of 2 African breeds given 4 levels of protein. For each breed a total of 12 steers, aged 1, 2 and 3 years, was used. Energy intake was adequate to support considerable weight gains. There were no appreciable differences between the breeds in apparent digestibility of N and efficiency of N utilization. All the animals were in positive N balance when the diets, which had crude protein contents of 5, 7, 9 and 11%, were given, which indicated that the cattle had a low maintenance requirement for protein. The maintenance requirement of the steers for apparently digestible N in the six breed-age groups was approximately one-third of that advocated by Brody (1945) and of the value used in many feeding standards. The daily requirement of the cattle for apparently digestible N was related to metabolic body size (W0.73) by the equation N (mg) = 197 W0. kg/73 The maintenance requirement for apparently digestible N was on average 2*96 times the endogenous N output of the cattle. The probable reasons for the low protein requirements for maintenance obtained and their applicability to field conditions over a long period are discussed.