A Meta-Analysis Evaluation of Feeding Melengestrol Acetate to Feedlot Heifers Implanted with Estradiol, Trenbolone Acetate, or the Combination of Estradiol and Trenbolone Acetate

Mixed models were used to study feeding melengestrol acetate (MGA) to feedlot heifers implanted with an estrogenic compound (EST), trenbolone acetate (TBA), or the combination of EST and TBA. One hundred one treatment means from 18 research trials were included in the analyses. Estrogenic implants increased DMI (p < 0.01) and LM area (P = 0.08). Estrogenic implant or MGA had no effect on percentage of carcasses grading low Choice or greater. Heifers implanted with TBA had greater (p < 0.05) final BW, ADG, hot carcass weight (HCW), and lower feed-togain ratio (FG) and YG. Trenbolone acetate tended to reduce the percentage YG 4 and 5 carcasses (P = 0.14) and percentage of carcasses grading low Choice or greater (P = 0.15). Interactions between MGA and EST were important (p < 0.01) for final BW, ADG, FG, and HCW, suggesting that the effect of MGA on these measurements depended upon EST implant. For heifers not implanted with EST, MGA improved (p < 0.01) FG and increased final BW, ADG, and HCW. For heifers implanted with EST, MGA had no effect on performance. Heifers fed MGA had greater (P = 0.05) DMI, increased (p < 0.01) fat depth, YG, and percentage YG 4 and 5 carcasses, and decreased (p < 0.01) LM area, regardless of implant status. In the absence of EST, MGA is effective in improving performance. However, when EST implants are used, there appears to be little performance benefit from feeding MGA.