Effects of Time of Administration of an Implant Containing 200 Mg of Trenbolone Acetate and 20 Mg of Estradiol in Finishing Heifers Given 80 Mg of Trenbolone Acetate and 8 Mg of Estradiol at Initial Processing and Fed for 181 Days

A total of 1,866 cross-bred heifers (268 ± 9 kg initial BW) were used in a 181-d finishing study to evaluate the effects of time of terminal implant administration on growth and carcass measures. All heifers received an initial implant containing 80 mg of trenbolone acetate and 8 mg of estradiol (Revalor-IH, Merck Animal Health, Madison, NJ) at initiation of the experiment. Cattle were fed in 24 pens using a randomized complete block design. There were a total of six blocks, with arrival date serving as the blocking factor. The four treatments were achieved by administration of a subsequent implant containing 200 mg of TBA and 20 mg of estradiol (Revalor-200) at 160, 120, 80 or 40 d before harvest (DBH). Marketing dates of blocks of heifers were determined at initiation of the experiment and were based on expected growth performance of the heifers using historical data for the facility. All animals that died or were removed from the study were excluded from the analysis. Dry matter intake was similar (P ≥ 0.12) between all treatments. A quadratic response to treatment was observed for G:F (P = 0.02), HCW (P = 0.03), and LM area (P = 0.01), and a tendency for a quadratic response to treatment was observed for ADG (P = 0.07) and dressing percentage (P = 0.06). For all of these variables, the greatest numeric treatment values were observed for the 120 and 80 DBH treatments. Based on an axis of symmetry analysis for the regression lines, HCW was optimized at 94 DBH and G:F was optimized at 90 DBH. In addition to these observations, linear increases in marbling score (P = 0.03) and percentage of carcasses grading USDA Prime (P = 0.02) were observed as DBH treatment decreased. These results indicate that the optimal time to give a terminal implant to heifers based on G:F and HCW is between 90 and 94 DBH. However, the earlier in the finishing period that the terminal implant was given, the greater the negative impact on intramuscular fat deposition.