The immunization of ewes against steroids as a means of increasing prolificacy in a mediterranean environment

Four experiments are reported in which the reproductive performances of Merino ewes in the Mediterranean environment of the south of Western Australia were examined following active immunization of the ewes against the steroids androstenedione (AD) and oestrone (OE) using DEAE-dextran adjuvant. The immunogens and protocols used were modified between experiments as the compounds and procedures required to elicit higher, reliable performances were improved.The changes in performance following treatment with the AD immunogens relative to those recorded for untreated ewes ranged from 0.12 to 0.38 ovulations per ewe ovulating, 0 to 62.1% multiple births, ?13.9 to 37.5% lambs born, and ?24.5 to 18.8% lambs marked to ewes present at the end of lambing.The respective figures for the ewes treated with the OE immunogens were ?0.04 to 0.56 ovulations per ewe ovulating, 3.5 to 64.7% multiple births, ?21.1 to 44.4% lambs born, and ?28.3 to 55.6% lambs marked.Whereas liveweight of the ewes at joining did not have a major effect on their performance, nutrition had an important influence, with the better fed pregnant immunized ewes having more multiple births (42.4 vs. 14.0%, P<0.01). A late December joining gave better results than an early February joining.Immunization against either steroid produced consistent increases in ovulation rates but there was not always a corresponding gain in the percentage of lambs born, whilst further losses occurred to lamb marking. The lack of a consistent and large improvement in the lambing performance of steroid immunized ewes in these experiments may have been associated with a depression in the pregnancy rate of the ewes. If this problem was overcome, immunization could improve the overall reproductive performance of well fed Merino ewes in Mediterranean environments now that the AD immunogen is commercially manufactured.