Effect of nitrogen and energy supplements on the growth of cattle grazing oats or Rhodes grass

Abstract

In two experiments the effects of feeding high-nitrogen and high-energy-low-nitrogen supplements on the growth rate of grazing cattle were measured. In one experiment supplements of either 1 or 2 kg of rolled sorghum grain (0.96 per cent N) or 1 kg of soybean meal (8.2 per cent N) per head per day were fed to steers about 18 months of age grazing oats. In the other experiment twin and non-twin weaners (mean age of 13 months) were fed supplements of either molasses plus minerals or molasses plus minerals and urea ad lib. while grazing a winter dormant Rhodes grass pasture. In both experiments the supplemented and unsupplemented groups were randomly allocated to the grazing areas at weekly intervals. Growth rate of the steers in the first experiment was unaffected by either supplement, and it was concluded that the unsupplemented steers reduced their intake of forage by an amount similar to the digestible energy supplied by the supplements. In the second experiment both supplements increased the rate of body weight gain of the weaners, but the error variances were such that significance was reached only for the twins. The growth rate of the twin weaners fed the supplement of molasses plus minerals and urea was significantly (P<0.01) greater than that of the unsupplemented weaners and was also significantly (P<0.05) greater than the growth rate of those given the same amount of molasses plus minerals but without urea. Twins given the molasses plus minerals supplement had a significantly (P<0.05) greater rate of body weight gain than the unsupplemented twin weaners. Calculated returns from feeding grain or soybean meal supplements to steers grazing oats were negative. For weaners grazing Rhodes grass, the molasses plus minerals supplements gave an increase in body weight gain which, on current values, was considerably less than the cost of the molasses, whereas the molasses plus minerals and urea supplement gave a return which was greater than the cost of the molasses plus urea.