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Effects of active immunization against a synthetic peptide sequence of the inhibin -subunit on plasma gonadotrophin concentrations, ovulation rate and lambing rate in ewes

Summary. Thirty adult Mule (Blue-faced Leicester x Swaledale) ewes were actively immunized against a synthetically produced peptide corresponding to the N-terminus of the -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

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