FEEDLOT PERFORMANCE, CARCASS CHARACTERISTICS, HORMONES, AND METABOLITES IN STEERS ACTIVELY IMMUNIZED AGAINST GROWTH HORMONE-RELEASING FACTOR

Large-framed Simmental and Charolais steers were actively immunized ag ainst growth hormone-releasing factor (GRF) to evaluate the effect on growth, carcass characteristics (especially intramuscular fat depositi on), and concentrations of somatotropin- (ST) and IGF-1. Primary immun izations of 1.5 mg of GRF-(1-29)-Gly-Gly-Cys-NH2 conjugated to 1.5 mg of human serum albumin (GRFi, n = 12) or 1.5 mg of human serum albumin (HSAi, n = 12) were given at approximately 10 mo of age. Booster immu nizations of .5 mg of the appropriate antigen were given at d 49 and 1 25. Weights of steers administered GRFi were less (P < .05) than those given HSAi at 126 d (34.6 kg) or at 262 d (48.2 kg) after treatment. Carcass weights were 28.2 kg less (P < .01) for GRFi than for HSAi ste ers. Dry matter intake was not affected by immunization treatment, whe reas feed efficiency was reduced in GRFi steers. Marbling scores were higher (P < .05) for HSAi than for GRFi steers but similar percentages (83.3) of both treatments graded Low Choice or higher. Rib sections o f GRFi steers contained more fat (31.2 vs 25.0%) and less lean (63.3 v s 68.4%) than those of HSAi steers (P < .05). A breed x treatment inte raction was observed for percentage of fat within the trimmed longissi mus muscle (P < .05); percentage of fat was similar for Charolais and Simmental steers when immunized against HSAi but was higher for Simmen tal than for Charolais when immunized against GRFi. Reduced serum ST, IGF-I, insulin, and plasma glucose concentrations and higher urea nitr ogen values were observed in GRFi compared with HSAi steers.