Effects of heat stress and delayed implanting on feedlot heifers

One hundred sixty-eight Angus and Angus crossbred heifers (black hair coats) were used to evaluate the effects of shade and implant strategy during heat stress conditions in West Texas. Treatments were arranged factorially and included: (1) shade with an initial Revalor H (Intervet, Millsboro, DE) implant on d 0 (SI); (2) shade with a delayed Revalor H implant on d 56 (SDI); (3) no shade with an initial Revalor H implant on d 0 (NSI); and (4) no shade with a delayed Revalor H implant on d 56 (NSDI). Heifer performance, carcass traits, behavior, and respiration rates were measured. Final BW was greater (shade x implant interaction, P < 0.002) for SI cattle than for SDI, NSI, and NSDI cattle, which did not differ (P > 0.11) in final BW. Final adjusted (constant dressing percent) BW was greater (P < 0.05) for SI cattle than for SDI, NSI, and NSDI cattle. The NSDI heifers had a greater (P < 0.05) final adjusted BW than SDI and NSI heifers, which did not differ from each other. Overall, average daily gain (ADG) was affected by a shade x implant strategy interaction (P < 0.06), such that SI heifers had consistently greater (P < 0.05) ADG than heifers in the other three treatments. Similarly, final adjusted ADG was affected by a shade x implant strategy interaction (P < 0.01), in which SI heifers differed (P < 0.05) from the other three treatments, which did not differ from each other (P > 0.09). Overall, delayed implanting seemed to negatively affect ADG for heifers provided with shade. Overall intake (DMI) and feed:gain ratio (F:G) did not show a shade x implant strategy interaction. Providing shade for cattle did not significantly (P > 0.28) change overall DMI compared with nonshaded cattle. Providing an initial implant resulted in a 2.4% increase (P < 0.05) in DMI compared with delaying the implant to d 56. Providing shade to the heifers did not effect F:G (P > 0.80). Heifers receiving an initial implant did not (P > 0.31) improve F:G ratio compared with heifers receiving the delayed implant strategy. Hot carcass weight (HCW) was effected by a shade x implant strategy interaction (P < 0.0023), with SI heifers having a greater (P < 0.05) HCW than SDI, NSI, and NSDI heifers, and the NDSI heifers having a greater (P < 0.01) HCW than SDI and NSI heifers. Dressing percent did not differ among treatments, nor was there a shade x implant interaction (P < 0.50). A shade x implant interaction (P < 0.07) was noted for fat thickness, in which NSI heifers had a greater (P < 0.05) fat thickness than the SI and SDI heifers and tended to have a greater (P < 0.10) fat thickness than NSDI heifers. Kidney, pelvic, and heart fat (KPH) tended (P < 0.06) to be greater for shaded than for nonshaded heifers, and USDA yield grade tended to be less (P < 0.09) for heifers that received a delayed implant than for initially implanted heifers. Neither shade nor implant strategy affected (P > 0.10) the distribution of USDA quality grades. Shaded heifers generally showed increased (P < 0.01) lying and decreased (P < 0.01) standing behavior. Respiration rates were greater (P < 0.05) for nonshaded than for shaded heifers. Overall, feedlot performance was best when shade was provided to heifers that received an implant at initial processing; however, shade did not benefit heifers implanted after 56 d on feed