291 Influence of feeding amino acid complexes of zinc and copper on growth performance and carcass response of finishing steers fed ractopamine hydrochloride

Abstract

Abstract A 42-d research study evaluated effects of feeding beef steers increasing levels of dietary Zn from Zn methionine (AAC-Zn; ZINPRO, Zinpro Corporation, Eden Prairie, MN) with or without supplemental Cu from Cu amino acid complex (AAC-Cu; Availa Cu, Zinpro Corp.) concurrently with ractopamine hydrochloride (RAC; Optaflexx, Elanco Animal Health, Greenfield, IN). One hundred-twenty steers (mean BW = 624 kg), ≤ 50 d from projected harvest were randomized to three pens (40 steers/pen) equipped with GrowSafe Systems (Calgary AB, Canada) feed bunk technology. Each pen was assigned to one of three treatment diets: 1) basal finishing diet + 30 mg Zn/kg DM from AAC-Zn (CON); 2) basal finishing diet + 90 mg Zn/kg DM from AAC-Zn (AAC-Zn90); or 3) AAC-Zn90 diet + 10 mg Cu/kg DM from AAC-Cu (AAC-Zn/Cu). Following allocation to treatments, cattle were acclimated to pen cohorts and GrowSafe feed bunks for 7 days. All steers were fed 300 mg RAC∙hd-1∙d-1 starting 35 d prior to harvest. Individual feed intake measurements began with RAC feeding and continued for 35 d until cattle were shipped for harvest. Carcass data were collected from each steer. Data were analyzed with individual animal as the experimental unit using PROC MIXED and PROC GLIMMIX procedures of SAS 9.4 (SAS Institute, Cary, NC). Numerically AAC-Zn90 fed cattle had heavier carcass weights than CON and AAC-Zn/Cu. Steers fed AAC-Zn90 had greater (P = 0.02) marbling scores compared to AAC-Zn/Cu. Steers fed AAC-Zn90 had lower backfat thickness (P = 0.02) and numerically greater marbling scores compared to CON. A biphasic program of feeding a lower level of AAC-Zn for the duration of the finishing phase followed by an increased rate of AAC-Zn during RAC feeding may optimize overall live and carcass response and improve total individual animal value.