PSVII-15 Interaction of supplemental rumen-protected lysine and methionine, and ractopamine-hydrochloride on cattle growth performance and carcass characteristics of finishing calf-fed Holstein steers

Abstract The objective of the current study was to investigate the effects of supplemental rumen-protected lysine and methionine (SMART) and ractopamine-hydrochloride (RAC) on cattle growth performance and carcass characteristics of finishing calf-fed Holstein steers. Calf-fed Holstein steer calves [n = 96; body weight (BW) = 466 ± 29 kg] were used to evaluate the interaction of ractopamine and supplemental methionine and lysine on growth performance and carcass characteristics. Steers were blocked by initial off-truck BW and assigned within BW grouping to 16 pens (6 steers/pen). From d 1 to 67, all steers received the basal steam-flaked corn-based finishing diet. From d 68 to harvest (d 103), steers received experimental diets consisting of two levels of RAC (0 vs. 300 mg/hd/d; Optaflexx, Elanco US, Inc., Greenfield, IN) and two levels of supplemental SMART (0 vs. 0.67% DM basis, Smartamine ML Adisseo, Alpharetta, GA). There was an interaction (P = 0.04) between RAC and SMART supplementation, where cattle fed the combination of both supplements had an increase in dry matter intake (DMI) compared with cattle fed either RAC or SMART. However, there was no interaction (P ≥ 0.17) of treatments in other growth performance or carcass characteristic variables evaluated in the study. Carcass adjusted average daily gain (ADG) and energy efficiency of the diet increased (P ≤ 0.02) on cattle supplemented with SMART. Moreover, carcass-adjusted final BW and feed efficiency, as well as energy efficiency, increased (P ≤ 0.05) on cattle supplemented with RAC. However, neither SMART nor RAC supplementation affected (P ≥ 0.13) cattle DMI. Both SMART and RAC supplementation increased (P ≤ 0.03) carcass weight, with no effect (P ≥ 0.17) in other carcass characteristics. In conclusion, both SMART and RAC supplementation enhances cattle growth performance with no detrimental effects on carcass characteristics of calf-fed Holstein steers.