Effects of Ractopamine Hydrochloride on Lysine Utilization By Growing Cattle

Two separate sets of 6 ruminally cannulated Holstein steers (240 kg) were used to study effects of ractopamine on lysine utilization. Steers were housed in metabolism crates and limit-fed 2.52 kg DM/d of a diet (10.1% CP) containing 82% soybean hulls twice daily. Treatments were arranged as a 2 x 3 factorial with 2 levels of ractopamine-HCl (0 and 200 mg/d) and 3 levels of lysine (0, 6, and 15 g/d) continuously infused abomasally. For each replicate of 6 steers, a completely randomized design was used for main-plot treatments (ractopamine-HCl; 0 vs. 200 mg/d). Each steer received subplot treatments (lysine; 0, 6, or 15 g/d) in random sequence, such that each replicate contained concurrent 3 x 3 Latin squares, with each Latin square representing either 0 or 200 mg/d ractopamine-HCl. All steers were continuously infused ruminally with 200 g/d acetic acid, 200 g/d propionic acid, and 50 g/d butyric acid and abomasally with 300 g/d glucose to increase energy supply. Urea (10 g/d) was infused ruminally to ensure adequate ruminal ammonia. Steers were abomasally infused with excess of all essential AA except lysine to ensure lysine was solely limiting. Periods were 6 d with N retention measured on d 3 to 6 and longissimus biopsies and blood collected on d 6. Lysine increased (P < 0.01) N retention for both control steers (21.3, 28.6, 33.9 g/d) and ractopamine-fed steers (25.5, 33.2, 38.9 g/d); ractopamine tended (P = 0.07) to increase N retention. Lysine linearly decreased (P < 0.01) plasma urea (2.83, 2.04, 1.88 mM), leucine, and valine, but increased (P < 0.01) plasma lysine (34, 51, 76 mM), arginine, and alanine. Ractopamine decreased (P < 0.05) plasma lysine and arginine concentrations. In longissimus, ractopamine increased (P < 0.05) phosphorylation of RPS6 on Ser-235 and Ser-244, and tended to increase (P = 0.08) phosphorylation of AKT on Thr-308 but not Ser-473. Abundance of MAFbx was decreased (P = 0.01) and MURF1 was numerically decreased (P = 0.14) by ractopamine. Lysine supplementation did not affect phosphorylation of RPS6 or AKT or abundance of MAFbx or MURF1. The efficiency of lysine use between 0 and 6 g/d supplemental lysine was 49% for control and 51% for ractopamine-supplemented steers. N retention tended to be improved by ractopamine when lysine supply was limiting, but incremental efficiencies of lysine utilization were not affected by ractopamine, suggesting that ractopamine reduced maintenance requirements for lysine.