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  • Author: D. R. Gifford x
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M. J. D'Occhio, D. R. Gifford, C. R. Earl, T. Weatherly and W. von Rechenberg

Summary. Post-partum acyclic beef cows received continuous long-term treatment with GnRH (200 or 400 ng/kg body wt/h) or the GnRH agonist buserelin (5·5 or 11 ng/kg body wt/h) using s.c. osmotic minipumps which were designed to remain active for 28 days. All treatments increased circulating LH concentrations whereas FSH remained unchanged. Ovulation and corpus luteum (CL) formation as judged by progesterone concentrations ≥ 1 ng/ml occurred in 0/5 control, 4/5 200 ng GnRH, 4/4 400 ng GnRH, 4/5 5·5 ng buserelin and 3/5 11 ng buserelin cows. The outstanding features of the progesterone profiles were the synchrony, both within and across groups, in values ≥ 1 ng/ml around Day 6, and the fact that most CL were short-lived (4–6 days). Only 3 cows, one each from the 400 ng GnRH, 5·5 ng buserelin and 11 ng buserelin groups, showed evidence of extended CL function. Cows failed to show a second ovulation which was anticipated around Day 10 and this could have been due to insufficient FSH to stimulate early follicular development, or the absence of an endogenously driven LH surge. The highest LH concentrations for the respective groups were observed on Days 2 and 6 and by Day 10 LH was declining, although concentrations did remain higher than in controls up to Day 20. The fact that LH remained elevated in treated cows, but CL were nevertheless short-lived, suggested that endogenous LH secretion in post-partum cows following a GnRH-induced ovulation is not limiting to CL function, and that other factors cause early demise of the first CL. The doses of GnRH used were not directly limiting to CL function since in a second trial extended CL were observed when GnRH was preceded by progesterone.

Keywords: cow; post partum; continuous GnRH; ovary

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M. J. D'Occhio, D. R. Gifford, R. M. Hoskinson, T. Weatherly and B. P. Setchell

Summary. Groups of heifer calves received a primary immunization against androstenedione (Group A; N = 11) or oestradiol-17β (Group E; N = 10) at 3 months of age and booster injections on 5 occasions at 2- to 3-month intervals. Controls (Group C, N = 11) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group A (1126 ± 261; reciprocal of titre) and Group E (10 357 ± 4067) heifers. In Groups A and E there was a general decline in the respective peak antibody titres after successive booster injections. From 3 to 9 months of age mean plasma concentrations of LH were higher (P < 0·05) in Group E heifers (0·89 ± 0·08 ng/ml) than in Group C (0·46 ± 0·03 ng/ml) and Group A (0·59 ± 0·05 ng/ml) heifers which did not differ from one another. There were no differences between groups in plasma FSH concentrations. At 10 months of age the LH response to exogenous LHRH was of higher (P < 0·05) amplitude for heifers in Group E (2·59 ± 0·56 ng/ml) than for those in Groups C (0·61 ± 0·07 ng/ml) and A (1·04 ± 0·22 ng/ml). Elevated plasma progesterone concentrations at 5 months of age were shown by 2 heifers in Group C, 10 in Group A, and 6 in Group E. From 8 to 14 months of age a consistently higher proportion of Group A heifers exhibited elevated progesterone compared with Group C and Group E heifers. After ovarian synchronization and booster injection at 15 months of age a corpus luteum was present in 2 heifers in Group C, 7 in Group A and none in Group E. The ovaries of Group A heifers were different from those of Groups C and E and were characterized by greater numbers of 2–4 mm follicles. It is concluded that active immunization against gonadal steroids influences both LH secretion and ovarian function in prepubertal heifers. Early increases in ovarian activity in androstenedione-immunized heifers are maintained after puberty and may therefore confer some lifetime reproductive advantages.

Keywords: heifer; gonadal steroid immunization; gonadotrophins; puberty; ovary

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M. J. D'Occhio, D. R. Gifford, R. M. Hoskinson, T. Weatherly, P. F. Flavel, P. E. Mattner and B. P. Setchell

Summary. Groups of bull calves received a primary immunization against testosterone (Group T; N = 7) or oestradiol-17β (Group E; N = 9) at 3 months of age and booster injections on four occasions at ∼2 month intervals. Controls (Group C, N = 7) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group T (730 ± 231; reciprocal of titre) and Group E (12 205 ± 4366) bulls; however, peak antibody titres generally declined with successive booster injections. Mean plasma concentrations of LH, FSH and testosterone during the period from 3 to 10 months of age were higher (P < 0·05) in Group T bulls than in Groups C and E. Group T bulls had larger testes compared with controls from 6 months of age onwards. At castration at 14 months of age, testes of Group T bulls were heavier (P < 0·05) than those of Groups C and E (179 ± 13, 145 ± 8 and 147 ± 6 g, respectively). At 10 months of age, there were no differences among treatment groups in LH responses to LHRH, but the testosterone responses were greater (P < 0·05) in bulls in Group T (26·2 ± 4·9 ng/ml) and Group E (16·6 ± 1·8 ng/ml) compared with those in Group C (6·9 ± 0·6 ng/ml). Testosterone responses to hCG determined at 13 months of age were also greater (P < 0·05) in Groups T and E relative to controls. At 14 months of age daily sperm production rates per bull (× 10−9) were higher (P < 0·10) in Group T bulls (2·2 ± 0·1) than those in Groups C (1·6±0·2) and E (1·6±0·1). These results indicate that early immunity against testosterone is associated with increased gonadotrophin secretion and accelerated growth of the testes in prepubertal bulls. Also, chronic immunity against testosterone or oestradiol-17β enhances the steroidogenic response of bull testes to gonadotrophic stimulation. If the above responses observed in young bulls are shown to be sustained, then immunity against gonadal steroids early in life may confer some reproductive advantage in mature animals.