Utilization of grass silage by growing cattle: effect of a supplement containing ruminally protected amino acids

Sixteen crossbred steers (278 +/- 4.9 kg) were used to determine the influence of supplemental ruminally protected lysine and methionine on performance of growing cattle fed grass silage. During the 154-d experiment, all steers were allowed ad libitum consumption of a good-quality grass silage during the first 70 d and of a lesser-quality silage during the remaining 84 d of the trial. The steers received a supplement of .5 kg/d of barley with or without a mixture of ruminally protected amino acid (RPAA) containing 8.2 g of lysine and 2.6 g of methionine. Compared with controls, steers supplemented with RPAA showed 16.3% improved (P less than .03) ADG (.92 vs 1.07 kg/d). Dry matter intake was not affected (P greater than .50) by treatment and averaged 2.03% BW across treatments. Supplementation with RPAA improved feed/gain by 13.6% (7.88 vs 6.81 for control and RPAA treatments, respectively, P less than .01). Plasma levels of methionine, lysine, arginine, and glutamic acid were higher (P less than .05) and of histidine were lower (P less than .001) when RPAA were fed. These results indicate that feeding RPAA can improve the performance of growing steers fed grass silages of varying qualities.