Studies of protein requirements of ruminants

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Abstract
Long-term feeding trials (16 weeks) were carried out with eighteen 2-year old cattle of each of 2 African breeds given 3 levels of dietary protein combined with varying amounts of digestible energy. Live-weight gain was significantly affected by level of intake of digestible crude protein (DCP) and total digestible nutrients (TDN) and also by a highly significant interaction between these dietary components. The steers made appreciable gains in body-weight when given amounts of DCP and TDN similar to those recommended by Brody (1945) for maintenance. Protein requirements could be defined only when the amounts of energy provided by the diet were known. There was a significant difference in performance between the 2 breeds. Mashona steers either gained weight more rapidly, or lost weight less rapidly, than the Africander steers at all levels of intake of DCP and TDN. The difference was most marked at the highest and lowest levels of feeding. As intake of protein increased, the estimated amount of TDN required by the steers for maintenance declined. The calculated TDN requirements of Africander and Mashona steers for maintenance when "adequate" DCP was given were approximately 83 and 73% respectively of Brody''s (1945) standard. The body-weight losses of steers receiving the lowest level of energy were appreciable in the 1st few weeks of the trial and the rate of loss then progressively decreased. This behaviour may indicate an "adaptation" by the steers to low levels of energy intake.