Response of ovariectomized ewes to injection of oestradiol-17 at different times of the year

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Abstract
Long-term ovariectomized Clun Forest ewes were challenged with a range of doses (12.5-50.0 .mu.g/injection) of estradiol benzoate every 2 mo. from March to Nov. All treatments induced a biphasic pattern of change in LH [luteinizing hormone] concentrations, consisting of an initial depression in concentrations followed by an LH peak, similar to a preovulatory LH surge. The positive feedback response to 12.5 .mu.g estradiol was significantly lower than that after the 2 higher dose levels, but the magnitude of the response showed no significant seasonal variation. A seasonal change in responsiveness to positive feedback is unlikely to contribute to the absence of ovulation during seasonal anestrus.