Effects of immunization against GnRH, melengestrol acetate, and a trenbolene acetate/estradiol implant on growth and carcass characteristics of beef heifers

A 2 x 3 factorial experiment was conducted to determine the effects of an i mplant (trenbolene acetate/estradiol or no implant) and method of estrus su ppression (immunization against GnRH, melengestrol acetate, or no suppressi on) on growth performance and carcass characteristics of heifers fed for sl aughter, At the start of a 21-d feed adaption phase. crossbred beef heifers (n = 144, 390 +/- 2.8 kg) were given their first dose of an anti-GnRH vacc ine or started on melengestrol acetate (MGA), Thereafter, heifers were fed a high-concentrate diet (78% barley grain) for 84 d (Days 0 to 83), receive d implants on Day 0, a second vaccination on Day 21, and were slaughtered o n Days 84 or 85. Implanting increased average daily gain (1.72 vs 1.50 kg/d , P < 0.01), feed efficiency (6.02 vs 6.75 kg dry matter intake/kg gain, P < 0.01), preslaughter weight (532 vs 513 kg, P < 0.01), carcass weight(301 vs 289 kg, P <less than> 0.01), and ribeye area (88.6 vs 85.9 cm(2), P < 0. 05), but had no affect (P <greater than> 0.05) on dry matter intake, grade fat thickness, marbling score, or lean yield. Compared to heifers fed MGA, those immunized against GnRH had a greater ribeye area(90.0 vs 84.6 cm(2)) and lean yield (63 vs 61%), and had thinner grade fat (7.5 vs 8.6 mm; P < 0 .05 for each). Furthermore, immunized heifers had lower (P <less than> 0.00 1) plasma progesterone concentrations than control heifers on Days 42, 63 a nd 83. Heifers fed MGA had less estrus mounting activity (P < 0.05) and low er plasma progesterone concentrations (P < 0.001) than the remaining heifer s. Method of estrus suppression did not affect (P > 0.05) preslaughter weig ht, average daily gain, dry matter intake, feed efficiency, carcass weight, or marbling score. In conclusion, implanting significantly increased growt h performance and preslaughter and carcass weights. Compared to heifers fed MGA, immunization against GnRH significantly increased ribeye area and lea n yield, and reduced grade fat thickness. (C) 2001 by Elsevier Science Inc.