The active immunization of sheep against oestrone, andostenedione or testosterone to increase twinning.

Procedures have been developed to produce consistent increases in ovulation rate in Merino ewes by immunization against the steroids, androstenedione, testosterone and oestrone. The procedures were based on experiments in the field over the period 1978-1981 using groups of 50 ewes for each treatment. The method uses selected steroid-human serum albumin conjugates, together with DEAEdextran adjuvant, administered subcutaneously in two treatments, three weeks apart. Joining with rams then commences three to five weeks later. With Merino ewes, increases in ovulation rate of 0.2 to 0.4 can be obtained whilst maintaining full oestrus. This results in increases in lambing rate of about 20%, essentially confined to twins.