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