PREGNANCY STATUS AND FEEDLOT PERFORMANCE OF BEEF HEIFERS ACTIVELY IMMUNIZED AGAINST GONADOTROPIN-RELEASING-HORMONE

The contraceptive effect of active immunization against GnRH was evalu ated in beef heifers. Crossbred heifers were randomized by breed and w eight (initial weight = 227.3 +/- 1.2 kg) and assigned to one of three treatment groups. Animals in Group IB (n = 48) were actively immunize d against GnRH; heifers in Groups NE (n = 48) and NN (n = 49) did not receive the anti-GnRH vaccine. Sixteen weeks after primary immunizatio n, bulls of proven fertility were introduced into pens containing Grou ps IB and NE. Bulls were maintained with heifers for 2 mo. Heifers in group NN were not intentionally exposed to fertile males. At the end o f the breeding period, heifers received Synovex H implants and entered a commercial feedlot. Heifers were slaughtered after 116 d of feedlot confinement. Anti-GnRH titer was evident in all heifers (48 of 48) im munized against GnRH. Gravid uteri were present at slaughter in 40 of 48 (83.3 %) NE heifers. In contrast, only four (8.3 %) IB and two (4.1 %) NN heifers carried gravid uteri at slaughter. Although ADG did not differ between groups during feedlot confinement, ADG during the bree ding period was higher (P < .05) in IB heifers than in NN control anim als. Dressing percentage and longissimus muscle area were decreased (P < .05) and marbling and quality grade were increased (P < .05) in NE heifers compared with NN control heifers. Carcass traits of IB heifers were intermediate between those of the NE and NN groups. Taken togeth er, these data indicate that active immunization against GnRH reduces the fertility of terminal heifers. These observations suggest that imm unoneutralization of GnRH may be an effective management tool that wil l reduce the incidence of unintended pregnancy in heifers destined for feedlots.