Effect of anabolic hormone exposure during the backgrounding-phase in calf-fed steers of different mature sizes

The mature size (MS) of cows in the United States is diverse, which leads to diversity in the MS of feeder cattle and hot carcass weights (HCW) of fed cattle. Cattle feeders must manage this inherent variation. Given that implants alter MS, they may be an effective tool to manage variation in HCW across groups of cattle. Two experiments were conducted to evaluate the interaction of MS and implant status in calf-fed steers. Steer calves from the same two sources were used in both experiments. Because breeding seasons were <60 d, it was assumed that weaning weight (WW) to a large extent reflected differences in MS. Smaller MS (SMS) and larger MS (LMS) steers were identified from the tails of the WW distribution. Within each MS group, steers were implanted with 20 mg estradiol benzoate and 200 mg progesterone (SS) or 14 mg estradiol benzoate and 100 mg trenbolone acetate (CH) on d 1 (Exp. 1), and nonimplanted (NI) or implanted with SS on d 2 (Exp. 2). In both experiments, a common terminal implant was used and steers were fed for 161 (Exp. 1) and 168 d (Exp. 2). Data were analyzed as a randomized complete block design with a 2 × 2 factorial treatment structure, with main effects of MS and implant. No MS × implant interactions were observed in either experiment (P ≥ 0.08). In both experiments LMS steers had heavier body weights (BW), HCW, and BW adjusted to 28% empty body fat (AFBW), greater average daily gain (ADG), dry matter intake (DMI), and lesser gain:feed (G:F) than SMS steers (P ≤ 0.02). No differences were evident in marbling score or Quality Grade distributions between SMS and LMS steers (P ≥ 0.13). In Exp. 1, no differences in growth performance, carcass traits, AFBW, or calculated frame size (FS) were observed for steers initially implanted with SS or CH (P ≥ 0.12). In Exp. 2, steers implanted initially with SS had heavier final BW, greater ADG and DMI (P ≤ 0.01), and similar G:F (P = 0.78) than NI steers. Steers initially implanted with SS had heavier HCW (P < 0.01), but no other differences in carcass characteristics were observed (P ≥ 0.23). Additionally, steers implanted with SS tended to have heavier AFBW (P = 0.07) and greater calculated FS (P = 0.05) than NI steers. Steers of different MS responded similarly to implants. Previous exposure to implants did not alter the response to the terminal implant. Estradiol increases the FS of steers; however, when similar doses of estradiol are compared, trenbolone acetate did not further increase FS (Exp. 1).