Effect of rumen-protected lysine on growth performance, carcass characteristics, and plasma amino acid profile in feedlot steers

The objective of this study was to evaluate growth performance, carcass characteristics, and plasma amino acid profiles of feedlot steers fed rumen-protected Lys. Forty-two Angus-cross steers (304 ± 25 kg) were blocked by weight and fed treatment diets for 180 d (growing days 0 to 55; finishing days 56 to 180): 1) Lys-deficient diet (CON; n = 12 steers), 2) Lys-adequate diet containing soybean meal (POS; n = 12 steers), or 3) Lys-deficient diet plus supplemental rumen-protected Lys (RPL; AjiPro-L; Ajinomoto Animal Nutrition North America, Eddyville, IA; n = 18 steers). Consecutive day bodyweights (BWs) were recorded to begin and end growing and finishing. Individual steer dry matter intake (DMI) was recorded. Blood was collected on days 0, 56, and 179 for analysis of physiological free amino acids. Steers were harvested on day 180 and carcass characteristics were recorded. Data were analyzed using Proc Mixed of SAS 9.4. Steer was the experimental unit and treatment was the fixed effect for all parameters. Block was a fixed effect for growth performance, feed intake, and carcass data. The day 0 value for each parameter of physiological free amino acids was used as a covariate during analysis. The CON steers had greater BW, average daily gain (ADG), and gain to feed (G:F) at the end of growing (day 56; P ≤ 0.05) vs. POS and RPL. The CON steers also had greater final BW (P = 0.04) and overall ADG (P = 0.04) than RPL, while POS was intermediate. Carcass characteristics were not different across treatments [hot carcass weight, dressing percent, ribeye area, back fat, kidney/pelvic/heart (KPH) percent, marbling, or calculated yield grade; P ≥ 0.13]. Plasma urea N was greater in POS steers on days 56 and 179 (P ≤ 0.04). Plasma Lys and Arg concentrations were greater in POS at day 56 (P ≤ 0.02); however, there was no difference among treatments for these two variables at day 179 (P ≥ 0.44). Steers in all treatments had greater DMI than predicted, causing a negative metabolizable Lys balance for all treatments during growing. Though the metabolizable Lys balance was positive for POS and RPL-fed steers during finishing, the increased metabolizable Lys in these treatments may have decreased performance if other amino acids were imbalanced due to increased intakes.