We reported previously that active immunization of heifers using a synthetic peptide-based inhibin vaccine (bIα(1–29)Tyr³⁰) can enhance ovarian follicular development and ovulation rate during spontaneous oestrous cycles. To extend this study, we investigated the effect of inhibin immunization more closely by monitoring plasma hormone profiles and ovarian activity in bIα(1–29)Tyr³⁰-immunized and control (ovalbumin-immunized) heifers (n = 6 per group) over three consecutive oestrous cycles, which were synchronized and shortened by administering a PGF_2α analogue at intervals of 14 days. Blood samples were collected at 2–8 h intervals for 40 days and the ovaries were examined daily using ultrasonography. Repeated-measures anova showed that inhibin immunization significantly increased plasma FSH concentration (by 52% overall; P < 0.01) and ovulation rate (by 58%; P < 0.01). Both immunized and control heifers showed the same cyclic pattern of plasma FSH (treatment × time interaction; not significant), indicating that the increase in plasma FSH was sustained throughout the cycle. Immunization did not affect the concentration or pattern of secretion of LH, oestradiol or progesterone and had no influence on the timing of the LH surge or ovulation after PG injection. While inhibin immunization increased the number of 'large' (i.e. growing to ≥ 10 mm diameter) follicles that developed during both the preovulatory (by 90%, P< 0.02) and postovulatory (by 190%, P< 0.01) period, there was no difference between groups in the temporal pattern of growth or regression of large follicles or of corpora lutea. These observations confirm a physiological role for ovarian inhibin as a component of the ovarian feedback mechanism controlling FSH secretion in heifers, and support the hypothesis that active immunization of heifers against inhibin enhances ovarian follicular development and ovulation rate by promoting a sustained increase in pituitary FSH secretion.

Summary. Thirty adult Mule (Blue-faced Leicester × Swaledale) ewes were actively immunized against a synthetically produced peptide corresponding to the N-terminus of the α-subunit of bovine inhibin conjugated to tuberculin purified protein derivative (PPD). Primary immunization in the late anoestrous period was followed by two booster injections at 5 week intervals. Control groups were either not immunized (n = 15) or received PPD only (n = 15). Ten days after the second booster, oestrus was synchronized using progestagen sponges and ovulation rate was assessed by laparoscopy on days 9–10 of the cycle. Blood samples were taken at the time of each immunization and immediately before laparoscopy. Ewes were mated with fertile rams in midNovember and the resulting conception, pregnancy and lambing rates monitored. All inhibin-immunized ewes generated antibodies that bound ¹²⁵I-labelled native bovine inhibin (M _r 32 000), and their plasma follicle-stimulating hormone (FSH) concentrations after the second booster were significantly higher than the preimmunization values (30%; P < 0·001) and the corresponding value in the controls (25%; P < 0·025). Inhibin immunization was associated with a 90% increase in ovulation rate (P < 0·005) and had no adverse effect on conception rate (100%), pregnancy rate (100%) or length of gestation (146 days). However, only a 37% increase (P < 0·05) in lambing rate was recorded for inhibin-immunized ewes, indicating a higher incidence of wastage of ova, or embryos, or both, in these ewes. As a consequence of the increased litter size, mean live birthweight of lambs was significantly lower (18%; P < 0·001) in the inhibin-immunized group and a higher proportion of the lambs born to inhibinimmunized ewes (15·4% compared with 0·02% in controls) were stillborn. Stillborn lambs weighed considerably less (38%; P < 0·001) than viable lambs. In terms of the number of viable lambs produced, there was no significant difference between the inhibin-immunized and control group. Although this study confirms the effectiveness of inhibin immunization using a synthetic peptide-based vaccine as a reliable method for increasing ovulation rate in sheep, the results indicate the limitations of this technique for further increasing viable litter size in 'improved' breeds of relatively high inherent prolificacy.

Keywords: inhibin; immunization; sheep; ovulation; litter size; FSH

The aim of this study was to determine whether supplementary treatment with recombinant bovine growth hormone(rbGH) can enhance the ovulatory response of ewes to inhibin immunization. Crossbred ewes (n = 20) were actively immunized against bovine inhibin a1–29 peptide conjugate while 20 ewes served as controls. Oestrus was synchronized using progestagen sponges and ewes were allocated to four groups: control ewes (n = 10); control ewes given rbGH (n = 10); inhibin-immunized ewes (n = 10) and inhibin-immunized ewes given rbGH (n = 10). A single s.c. dose of rbGH (50 mg) was given 7 days before sponge removal. Blood was collected for measurement of inhibin antibody titre, and concentrations of insulin-like growth factor I (IGF-I), FSH, oestradiol and progesterone. Ovulation, pregnancy and lambing rates were also recorded. All inhibin-immunized ewes produced antibodies that bound¹²⁵I-labelled (32 kDa) inhibin. The concentration of FSH in the plasma of the ewes after the second booster inhibin immunization was higher than that in control ewes (P < 0.005). Treatment with rbGH promoted a 2–3-fold increase in plasma concentration of IGF-I (P < 0.001); the response was less (P < 0.01) in immunized compared with control ewes. Treatment with rbGH alone had no significant effect on the concentration of FSH or oestradiol or on ovulation rate or litter size. Overall, inhibin-immunized ewes had higher mean FSH concentrations (P < 0.002), higher preovulatory oestradiol surges (P <0.05) and higher progesterone concentrations in the luteal phase (P < 0.0001). Treatment with rbGH reduced the effects of immunization on FSH (P < 0.01) and progesterone (P < 0.02) concentrations. Immunized ewes showed a threefold increase in ovulation rate (P < 0.001) and a 1.8-fold increase in litter size (P <0.05) compared with control ewes. In immunized ewes given rbGH, ovulation rate was increased by a factor of 2.2 and litter size by a factor of 1.8. In conclusion, these data do not support the hypothesis that supplementary treatment of ewes with rbGH to raise plasma IGF-I concentrations (and presumably intraovarian IGF-I) can enhance the ovulatory response to inhibin immunization.