The effects of active immunization against GnRH on testicular development,feedlot performance, and carcass characteristics of beef bulls

The objective was to determine the effects of a recombinant fusion protein anti-GnRH vaccine on testicular development, feedlot performance, and carca ss quality of beef bulls. Crossbred beef bulls (n = 58, average weight 306 kg, 9 mo of age), were randomly allocated to two groups and received either an anti-GnRH vaccine (GnRH) or placebo (Control) by intramuscular injectio n on d 0, 56, and 112. There were group effects (P < 0.01; as a percentage of Control) on testicular weight (53%), daily sperm production (40%), and e pididymal sperm reserves (16%). There were group x time interactions (P < 0 .0001) for scrotal circumference and serum testosterone concentrations; at slaughter, bulls in the GnRH group had a smaller (P < 0.05) scrotal circumf erence (28.3 vs 33.9 cm) and lower (P < 0.05) serum testosterone concentrat ions (2.2 vs 8.6 ng/mL) than those in the Control group. Average daily gain , feed intake, and feed efficiency were not different between treatments du ring the backgrounding phase (d 0 to 84). During the finishing phase (d 98 to 182), ADG was greater (P < 0.05) for bulls in the Control group (1.69 vs 1.42 kg/d), as was carcass weight (6.9%; P < 0.01). However, GnRH bulls ha d numerically better feed efficiency (6.12 vs 7.08 kg DMI/kg gain; P < 0.23 ) and shear force values for ribeye that were 16% lower (P < 0.14) than Con trol bulls, warranting further investigation. Vaccinating bulls against GnR H suppressed testicular function, with growth and carcass characteristics s imilar to that expected with steers.