Testes function, feedlot performance, and carcass traits were evaluated in bulls actively immunized against gonadotropin-releasing hormone (GnRH) at different ages. Bull calves were randomly assigned to one of seven treatment groups (n = 15 calves/group). Calves were unimmunized (Group 1), immunized at 1.5, 4, 7, or 12 mo of age with a GnRH-keyhole limpet hemocyanin (KLH) conjugate (Groups 2 to 5, respectively), or castrated at 4 mo of age (Groups 6 and 7). Immunized bulls did not receive a secondary, or booster, immunization. Calves in group 6 received Synovex-C at castration and Synovex-S at weaning and feedlot entry. Anti-GnRH titer was evident at slaughter in all immunized bulls. However, the final immune response of bulls immunized at 1.5 mo was significantly lower than the response of bulls immunized at later stages of development. Final scrotal circumference and testis weight in bulls immunized at 4, 7, or 12 mo of age were significantly reduced relative to unimmunized bulls. The final live weight, feedlot gain, and carcass weight of immunized and unimmunized bulls did not differ (P > .05) from the same parameters in steers implanted with Synovex. Longissimus muscle area, marbling score, and backfat thickness did not differ between immunized and unimmunized bulls. The sex class score of the carcasses of immunized bulls did not differ from the score of steer carcasses. In contrast, a significantly higher proportion of carcasses from unimmunized bulls graded as bullock carcasses. Taken together, these data indicate that a single immunization against GnRH at 4 to 12 mo of age results in significant attenuation of testicular growth in bulls. These data also demonstrate that immunization against GnRH reduces the masculinity of carcasses from bulls, but does not affect feedlot performance, longissimus muscle area, marbling score, or backfat thickness. These results suggest that single immunization with the GnRH-KLH conjugate may have practical utility as a noninvasive alternative to surgical castration in management of beef cattle.

Stress-like concentrations of cortisol increase the negative feedback potency of oestradiol in castrated male sheep. A similar cortisol-dependent response in female sheep might be expected to suppress gonadotrophin secretion and impair follicular development and ovulation. The oestrous activity of 21 female sheep was synchronized using progestogen-treated vaginal pessaries to test this hypothesis. Stress-like concentrations of cortisol (60-70 ng ml-1) were established by continuous infusion of cortisol (80 micrograms kg-1 h-1; n = 13) beginning 5 days before, and continuing for 5 days after, pessary removal. Control animals (n = 8) received a comparable volume of vehicle (50% ethanol-saline) over the 10 day infusion period. Serum concentrations of oestradiol increased progressively in control sheep during the 48 h immediately after pessary removal. This increase in serum oestradiol was blocked or significantly attenuated in sheep receiving stress-like concentrations of cortisol. Preovulatory surge-like secretion of LH was apparent in control animals 58.5 +/- 2.1 h after pessary removal. In contrast, surge-like secretion of LH was not observed during the 5 days after pessary removal in 54% (7 of 13) of sheep receiving cortisol. Moreover, the onset of the surge was significantly delayed in the cortisol-treated ewes that showed surge-like secretion of LH during the infusion period. The ability of episodic pulses of exogenous GnRH to override the anti-gonadal effect of cortisol was examined in a second study. Oestrous activity of 12 ewes was synchronized using progestogen-containing pessaries as described above. Ewes were randomly assigned to one of three treatment groups (n = 4 ewes per group). Animals received cortisol (100 micrograms kg-1 h-1; groups 1 and 2) or a comparable volume of vehicle (group 3) beginning 5 days before, and continuing for 2 days after, pessary removal. Pulses of GnRH (4 ng kg-1 h-1, i.v.; group 1) or saline (groups 2 and 3) at 1 h intervals were initiated at pessary removal and continued for 48 h. Serum concentrations of oestradiol were not significantly increased after pessary removal in sheep receiving cortisol alone. Conversely, serum concentrations of oestradiol increased progressively during the 48 h after pessary removal in control ewes and in ewes receiving cortisol and GnRH. At the end of infusion, serum concentrations of oestradiol did not differ (P > 0.05) between control (7.7 +/- 0.8 pg ml-1) ewes and ewes receiving cortisol and episodic GnRH (6.4 +/- 1.3 pg ml-1). Moreover, these values were significantly greater (P < 0.05) than the serum concentrations of oestradiol in animals receiving cortisol (1.0 +/- 0.4 pg ml-1) alone. Collectively, these data indicate stress-like concentrations of cortisol block or delay follicular development and the preovulatory surge of LH in sheep. In addition, episodic GnRH overrides cortisol-induced delay in follicular maturation.

The effect of implants containing anabolic steroid (Synovex) on testis function, feedlot performance, and carcass traits was evaluated in bulls, steers, and bulls actively immunized against gonadotropin-releasing hormone (GnRH). Ninety-one Hereford or Hereford x Red Angus bull calves were randomly assigned to a 3 x 2 factorial experiment. The treatments were: 1) unimmunized control (n = 30), 2) actively immunized against GnRH (n = 31), or 3) castrate (n = 30). The classes were 1) unimplanted (n = 45) or 2) implanted (n = 46) with Synovex. Animals in treatment Groups 2 and 3 were either immunized with a GnRH-keyhole limpet hemocyanin (KLH) conjugate or castrated at 3.6 +/- .1 mo of age. Animals assigned to Class 2 received Synovex-C at the time of initial treatment and Synovex-S at weaning and feedlot entry. Animals in treatment Group 2 received a booster immunization at feedlot entry. Anti-GnRH titer was evident at weaning and remained elevated to slaughter in all (31 of 31) immunized animals. Anti-GnRH titer was not evident in steers or unimmunized bulls. Immunization and (or) Synovex decreased (P < .05) serum concentrations of testosterone (T) and scrotal circumference (SC) at weaning. Similarly, SC, testis weight, and tissue concentrations of spermatozoa at slaughter were significantly reduced by immunization, Synovex, or the immunization/Synovex combination. Immunization, but not Synovex, reduced (P < .05) serum levels of T at slaughter. The masculinity of immunized bulls was less (P < .05) than that of control bulls.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of stress-like concentrations of cortisol (C) on the feedback potency of estradiol (E2) was assessed using 32 orchidectomized sheep (wethers) assigned at random to 1 of 4 treatment groups in a 2 x 2 factorial design. Wethers received C (3. 6 mg/50 kg per hour; groups 2 and 4) or a comparable volume of C delivery vehicle (groups 1 and 3) as a continuous infusion for 7 days. During the final 48 h of infusion, wethers received E2 (0.3 microg/50 kg/h; groups 3 and 4) or E2 delivery vehicle (groups 1 and 2). The pattern of LH secretion was assessed during a 4-h period of intensive blood collection beginning 44 h after initiation of E2 infusion. Gonadotroph responsiveness (LH secretion induced by GnRH challenge [500 ng, i.v.]) was determined 48 h after E2 delivery was begun. Although the frequency of secretory episodes of LH was not affected (p > 0.05) by infusion of C or E2 alone, LH pulse frequency was significantly decreased in wethers receiving C and E2 in combination. In contrast, neither the magnitude of basal gonadotroph responsiveness nor the extent of E2-dependent augmentation of responsiveness was significantly affected by stress-like concentrations of C. In a second experiment, the effect of C on the magnitude of E2-induced increase in pituitary concentration of GnRH receptor and GnRH receptor mRNA was assessed using 32 additional wethers. Continuous infusion of E2 for 48 h increased (p < 0.05) tissue concentrations of GnRH receptor and GnRH receptor mRNA. Concurrent delivery of C did not affect (p > 0.05) E2-induced increase in GnRH receptor mRNA but significantly reduced the magnitude of the E2-dependent increase in pituitary concentration of GnRH receptor. Collectively, these data indicate that stress-like concentrations of C enhance the negative feedback potency of E2 and reduce estrogen-dependent augmentation of the concentration of GnRH receptor in pituitary tissue.

The contraceptive effect of active immunization against GnRH was evaluated in beef heifers. Crossbred heifers were randomized by breed and weight (initial weight = 227.3 +/- 1.2 kg) and assigned to one of three treatment groups. Animals in Group IB (n = 48) were actively immunized against GnRH; heifers in Groups NB (n = 48) and NN (n = 49) did not receive the anti-GnRH vaccine. Sixteen weeks after primary immunization, bulls of proven fertility were introduced into pens containing Groups IB and NB. Bulls were maintained with heifers for 2 mo. Heifers in group NN were not intentionally exposed to fertile males. At the end of 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) immunized against GnRH. Gravid uteri were present at slaughter in 40 of 48 (83.3%) NB 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 breeding period was higher (P < .05) in IB heifers than in NN control animals. Dressing percentage and longissimus muscle area were decreased (P < .05) and marbling and quality grade were increased (P < .05) in NB heifers compared with NN control heifers. Carcass traits of IB heifers were intermediate between those of the NB and NN groups. Taken together, these data indicate that active immunization against GnRH reduces the fertility of terminal heifers. These observations suggest that immunoneutralization of GnRH may be an effective management tool that will reduce the incidence of unintended pregnancy in heifers destined for feedlots.