PSVI-7 Effects of feeding chromium- and zinc- amino acid complexes on growth performance and carcass characteristics of finishing bulls fed zilpaterol hydrochloride for the last 30 days in the feedlot

Abstract

Abstract A 2 x 2 factorial study was conducted to evaluate main and interaction effects of feeding amino acid complexed sources of Zn (Zn-AAC; 40 mg /kg DM from Availa® Zn; Zinpro Corp., Eden Prairie, MN) and Cr (chromium methionine; Cr-Met; 0.20 mg / kg DM from Availa® Cr; Zinpro Corp., Eden Prairie, MN) fed in combination or separately, compared to an iso-Zn level Control (CON) group that provided Zn from ZnSO4 (80 mg Zn∙kg-1DM) with no supplemental source of Cr. Treatments were replicated into 4 pens with 5 finishing Bos taurus x Bos indicus bullocks/pen (80 bullocks; BW= 421 kg). Treatment diets were initiated 28 d prior to starting zilpaterol hydrochloride (ZIL; 15 mg/kg BW) with ZIL fed daily for 33 d followed by a 4 d withdrawal before harvest. During the ZIL-feeding Zn-ACC x Cr-Met interactions were present for ADG and Gain:Feed (G:F) where Zn-ACC improved ADG (P≤ 0.05) and Cr-Met improved G:F vs. CON. Effects may have been mediated by differences in DM intake (DMI), where DMI was numerically increased by Zn-ACC and decreased (P ≤ 0.01) by Cr-Met. Overall, feeding Cr-Met moderated DMI and improved G:F (P ≤ 0.05). Measures of carcass composition were not affected by Zn-ACC or Cr-Met. Hot carcass weight was increased (P ≤ 0.05) by feeding Zn-ACC and tended to increase with Cr-Met. Daily carcass gain increased from feeding either Zn-ACC or Cr-Met during the ZIL phase and overall study. Feeding Zn-ACC was a primary driver during the ZIL period and overall trial for increasing growth rate, with little or no influence on DM intake while feeding Cr-Met moderated DM intake to improve dietary and energetic efficiency. Balancing the feeding levels of Zn-ACC and Cr-Met will be key to optimizing the overall biological response and net return.