The role of protein supplements in nutrition of young grazing cattle and their subsequent productivity

The effects of supplements of protein meal or sorghum grain given to young cattle for 140 days during their first winter and spring post-weaning, were recorded over 560 days for cattle grazing on low quality pastures in subtropical Australia. The supplements were offered to steers in four treatment groups, replicated three times, at the following rates per head to late spring: (a) mineral mix (M) + cotton-seed hulls, 90g/day (control), (b) M + protein meal, 600 g/day (protein daily), (c) as for (b) but at 2·1 kg/3·5 days (protein twice-weekly), or (d) M + sorghum grain, 560 g/day (sorghum). For the next 130 days, to late summer, only supplement (c) was continued while the remaining three groups were not supplemented. None of the supplements was offered for the final 290 days of the experiment when the steers grazed an improved pasture. At the end of the first spring, the live weight of the steers supplemented daily and twice-weekly with protein was respectively 229 and 216 kg. These were significantly (p < 0·01) heavier than the sorghum-supplemented (174 kg) or control steers (160 kg). At the end of the first summer, the steers given supplements of protein meal twiceweekly, or daily, remained significantly heavier (265, 250 kg respectively) than sorghum-supplemented or control steers (209, 198 kg). By the second summer, the steers previously given protein supplements were still heavier than the steers from the control group (357 v. 317 kg, respectively). In steers which were offered the protein supplement during winter and spring, samples of blood taken at the end of spring had lower (P < 0·01) plasma ratios of glycine to branched-chain amino acids (valine, leucine and isoleucine) and a lower ratio of methionine: valine, than in the steers which were not offered the supplement. These ratios were significantly (P < 0·05) correlated with live-weight change over the preceding 140 days. On this basis it was considered that the improved growth of the steers offered the protein supplements was associated with an increased availability of specific essential amino acids, which may have initiated in the steers an increased appetite for pasture D.M