Effect of Feeding Melengestrol Acetate, Monensin, and Tylosin on Performance, Carcass Measurements, and Liver Abscesses of Feedlot Heifers

Approximately 4,000 yearling heifers (initial BW = 306 ± 1.9 kg) were randomly allotted into 4 treatment groups to determine the effect of melengestrol acetate (MGA) on feedlot performance and carcass parameters. Treatments included a diet containing monensin (Rumensin) and tylosin (Tylan; RT) fed during the entire feeding period; RT plus MGA (RTM) also fed during the entire feeding period; RT withdrawn 35 d preslaughter (RTwd); and RTM withdrawn 35 d preslaughter (RTMwd). All cattle were implanted at arrival with Revalor-IH and reimplanted with Revalor-200, and were fed a standard feedlot finishing diet (that met or exceeded 1996 NRC requirements) for approximately 150 d. Final BW, ADG, hot carcass weight (HCW), DMI, and percentage of cattle grading Choice were greater (P < 0.05) for RTM than for all other treatments. Withdrawal of MGA (RTMwd) significantly (P < 0.01) decreased ADG, HCW, 12th-rib fat thickness, calculated YG, and percentage of carcasses grading Choice, but significantly increased the percentage of dark-cutting carcasses, LM area, and YG 1, 2, or 3 carcasses compared with RTM. There were no significant differences (except for dressing percentage) between RT and RTwd. Heifers continuously fed MGA increased in ADG, HCW, and carcass quality traits of economic importance compared with other treatments. Withdrawal of MGA, Rumensin, and Tylan 35 d preslaughter adversely affected carcass weight and carcass quality traits of economic importance when compared with a feeding program including MGA, Rumensin, and Tylan.