Effect of Repeated Annual Multi-steroid Immunisation on the Reproductive Performance of Fine-wool Merino Sheep

In fine-wool Merino ewes, immunisation against a mixture of steroids was compared with androstenedione immunisation alone and with no treatment to evaluate its long term effectiveness in improving reproductive performance by increasing fecundity. Using 2 prototype multi-steroid vaccines in field trials conducted for 4 consecutive years, gains in lambing percentage were consistently obtained, with 1 of the 2 vaccines proving slightly superior. Little variation occurred in the immune responses obtained with these mixtures and there was no reduction in the percentage of ewes joined. Gains in fecundity were not always achieved with androstenedione immunisation, although it can be equally effective as multi-steroid immunisation in some seasons.