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The National Research Council lists the daily protein requirements of medium-frame and large-frame steers weighing 770 lb and gaining 2.2 lb daily as 1.7 and 1.8 lb, respectively; comparable values at 1,100 lb are 1.9 and 2.1 lb. These amounts of protein are expected to be provided for medium-frame steers weighing 880 lb and gaining weight at 2.2 lb daily by a diet containing 10.3% protein; the corresponding value for large-frame steers is 10.2% protein. Medium- and large-frame steers weighing 1,100 lb and gaining weight at 2.2 lb daily require 9.5% protein in the diet.
The zinc requirement of steers is not well defined; an estimated requirement of 30 ppm has been derived from experiments with calves and sheep. Corn grown at U.S. Meat Animal Research Center contains 20 to 30 ppm zinc, a level that may be marginal for finishing beef cattle.
The known involvement of zinc in protein metabolism and the greater growth rate of large-frame than of medium-frame growing-finishing beef cattle suggests the possibility of an interaction of dietary zinc and protein levels and cattle frame size with respect to weight gain and feed utilization. The purpose of this experiment was to test this possibility.