In April and May 1983 60 Merino x Romney ewes of known Booroola genotype (30 F+, 30++) were immunised against androstenedione. An equal number of contemporary ewes of the same breed type and Booroola genotype were run together as an untreated control. Ewes were joined with vasectomised rams and ovulation rates were measure 4 weeks after the booster injection and thereafter at 3 successive 17-day intervals. The immunised F+ ewes showed an initial ovulation rate response of 1.62 compared with F+ controls and the response had decreased to 0.17 at the fourth observation 51 days later. The immunised ++ ewes showed an initial response in ovulation rate of 0.29 which increased to 0.61 at the second observation and decreased to 0.12 at the fourth observation. The experiment was repeated with the same sheep in 1984. Half of the ewes immunised in 1983 were given a booster injection in December 1983 (early group) and the other half were boosted in May 1984 (late group). Control ewes remained untreated. The early F+ immunised ewes had an ovulation rate response of 1.93 in February and 6 months later in August the response was still 0.90. The early ++ immunised had an initial response of 0.33 which increased to 0.40 at the second observation and decreased to 0.9 in August. Late F+ and ++ immunised ewes had initial responses of 0.71 and 0.38 which declined to 0.52 and 0.05 respectively 51 days later. These results all show a consistently higher response to immunisation in F+ ewes. The F+ ewes immunised in December before the onset of the breeding season showed the highest initial response and 8 months after the booster vaccination a significant response was still evident. F+ ewes immunised during the breeding season (booster injection in May) showed a lower initial response and a lower subsequent response than F+ ewes immunised before the breeding season. The duration of the response could not be determined in these ewes because of the short period from boosting until the end of the season.

CA, Morris, NG Cullen, PA Carson, and GA Morley

Proceedings of the New Zealand Society of Animal Production, Volume 45, , 167-170, 1985
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