Effect of ruminally protected lysine on growth and nitrogen balance of grain-finished beef cattle fed diets of decreasing protein concentration.

Two experiments were conducted to determine the effect of ruminally protected lysine (AjiPro-L) supplementation on performance and nitrogen (N) balance of feedlot cattle fed decreasing concentrations of dietary protein. Diets contained (DM basis) 68.3% to 72.2% corn, 10% grass silage, 5.5% supplement, and soybean co-products (meal, hulls). Treatments consisted of a standard protein control diet (14.4% CP, CON), a reduced crude protein (CP) diet supplemented with 0.163% AjiPro-L (12.4% CP, RCP1), and a reduced CP diet supplemented with 0.315% AjiPro-L (11.6% CP, RCP2). In experiment one, 90 steers were allotted by weight (352 ± 13.8 kg) to 1 of 3 treatments to determine the effect of diets on performance and carcass characteristics. Steers were slaughtered at a target BW of 642 kg. In experiment two, 6 heifers (393 ± 10.9 kg) were allotted to the same diets in a replicated 3 × 3 Latin square design (28-d periods) to determine the effect of diets on N balance. Urine and feces were collected and quantified daily during the last 4 days of each period using a urinary catheter and fecal bag. Statistical analyses were conducted using the GLIMMIX procedure of SAS. Decreasing CP and adding lysine to high grain feedlot diets did not affect weight, average daily gain, dry matter intake, gain:feed, or days on feed (