The recognised roles of oestradiol in the bovine oestrous cycle include stimulating behavioural symptoms of oestrus, facilitating spontaneous luteolysis and initiating the ovulatory surge of LH. An important ancillary role with progesterone is to inhibit the release of gonadotrophins (Burke et al.,1996). Oestradiol administered by injection or per vaginum can perturb follicle development with consequent effects on responses to superovulation (Bo et al.,1994) and on synchrony patterns (Macmillan et al.,1993). The hypotheses which were tested in the following trial were that injecting a low dose of oestradiol benzoate (1mg ODB) at about mid-dioestrus during the cycle follow-ing first insemination: (i) would not reduce the pregnancy rate to this insemination; but (ii) would alter the pattern of returns to service; and (iii) could affect the pregnancy rate to the second insemination.

KL, Macmillan, VK Taufa, and AM Day

Proceedings of the New Zealand Society of Animal Production, Volume 57, , 237, 1997
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