The effects of repeated injections of oestradiol benzoate (ODB) on the behavioural oestrous responses of ovariectomized ewes were studied using a pool of 144 animals. Oestradiol benzoate (15·6–421·2 μg) injected at intervals of 4 or 6 days produced a gradual decrease in the number of ewes showing induced oestrous behaviour. The response eventually fell below 10% at which time the ewes were defined to be refractory to oestrogenic stimulation of behavioural oestrus. When a continuous variable was used as a measure of response an immediate effect was observed; after the second injection of ODB the duration of oestrous behaviour decreased while the time to onset of oestrous behaviour showed a corresponding increase when compared with the first injection. However the proportion of ewes responding remained unaltered, indicating that continuous variables are a more sensitive measure of refractoriness. Treatment of refractory ewes with the progestagens progesterone, SC-9880 or Nilevar restored normal oestrous behaviour. These results are interpreted to mean that oestrogens (15–400 μg) act to increase thresholds for subsequent oestrogen stimulation of behavioural oestrus, while progestagens (1–10 mg) act to decrease this threshold.
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Online ISSN: 1479-6805
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