Effects of oestrogen on the fertility of ewes at progesterone-synchronized oestrus

Abstract

Estimates were made of the endogenous oestrogen production of ewes at progesterone- synchronized oestrus. The duration of oestrous behaviour was 11.5 hr shorter at the first period of synchronized oestrus after treatment with progesterone than at the second, and it was estimated from bioassay with spayed ewes that this represented a difference in endogenous oestrogen production equivalent to 5.2 pg of oestradiol benzoate. In a subsequent field test, however, the injection of 5 ¦g of oestradiol benzoate at intervals of up to 4 days after progesterone withdrawal failed to improve fertility at the first oestrus after progesterone treatment.