In the 10 years since it was observed that immunisation of ewes against steroid hormones resulted in an increased ovulation rate, research has provided some understanding of the mechanism of immunoneutralisation and of the factors which influence or modify the immunological and physiological response of ewes. It has also led to the development of a commercial vaccine. The mechanism of action is still uncertain although several changes in the endocrine balance of the ewe may be involved. Active immunisation against a number of steroids has been shown to increase ovulation rate and with passive immunisation, a mixture of steroid antibodies produced the best result. One possible mechanism via a reduced negative feedback on the pituitary and increased gonadotrophin release has been recorded. Effects at the ovarian level influencing follicular atresia must be also considered. The initial attempts at the practical application of this technology were unsuccessful. Although ovulation rates were increased, fewer lambs were born per ewe joined due to problems of anoestrus and anovulation. From these data the relationship between antibody titre and response was established and alterations to the composition of the antigen and treatment schedule were developed to modify the antibody response. Factors such as ewe age, level of nutrition and breed had little effect on the response. The response to changes in nutrition and live weight appear to be additive to those of immunisation, as is the response to exogenous gonadotrophins (PMSG). The timing of treatment relative to mating appears to have some effect, with intervals of less than 21 days reducing the lambing response through a high proportion of barren ewes. However there has been considerable variation in the final response indicating a major effect of early embryonic mortality. Lamb birth rank distribution has been within the predicted range and been seen mainly as an increase in twins with some higher order multiples. The mortality of lambs from treated ewes has been similar to that of lambs of the same birth rank from untreated ewes. The growth and reproductive performance of the progeny from treated ewes has been unaffected.
Proceedings of the New Zealand Society of Animal Production, Volume 45, , 163-166, 1985
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