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

Abstract 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 low crude protein (CP) diet supplemented with 0.163% AjiPro-L (12.4% CP, LCP1), and a low CP diet supplemented with 0.315% AjiPro-L (11.6% CP, LCP2). 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 (P ≥ 0.11) and did not affect blood concentrations of total protein, urea N, alanine aminotransferase, or triglyceride (P ≥ 0.54). Diet did not affect hot carcass weight, fat depth, longissimus muscle area, KPH%, or marbling score (P ≥ 0.22). Steers fed LCP2 had a lower dressing percentage compared to steers fed CON and LCP1 (P = 0.01). Steers fed LCP1 and LCP2 tended produced carcasses with a lower yield grade (P = 0.10) compared with steers fed the CON diet. Heifers fed LCP2 excreted less total N (P = 0.04) and tended to excrete less urinary N (P = 0.07) compared with heifers fed CON. Total and urinary N excretion did not differ for heifers fed LCP1 compared to heifers fed CON or LCP2 (P ≥ 0.10). The percentage of N excreted in feces was greater for heifers fed LCP2 compared with heifers fed CON or LCP1 (P = 0.04). In conclusion, decreasing dietary CP to 11.6% in high grain feedlot diets and adding lysine to meet amino acid requirements decreased N excretion, improved carcass leanness, and did not alter animal performance.