Onset of Seasonal Anestrus in the Ewe2

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Abstract
An attempt was made to elucidate the temporal changes in concentration of several circulating hormones in six Western Whiteface ewes during the last six estrous cycles before, and during the first 52 days of seasonal anestrus. Jugular blood samples were taken every 4 days, except for more frequent bleeding around the time of expected estrus. Ewes were checked for estrus with vasectomized rams three times daily around the time of expected estrus and twice daily at all other times. Cervical mucous smears were taken during estrus. Blood samples were analyzed for luteinizing hormone (LH), progesterone and 17β-estradiol by radioimmunoassay. The length of the behavioral estrous period immediately before the onset of anestrus was normal in two cases (24 to 36 hr) and short in four cases (<8 hr). Estrous cycle lengths were normal, as were the changes in cervical mucus crystallization patterns at estrus. The temporal changes in jugular plasma hormone concentrations appeared to be typical of the normal cyclic ewe except at the last period of behavioral estrus when, although all ewes experienced a rise in concentration of 17β-estradiol, in three ewes this was followed by a subnormal rise in concentration of LH (<1.2 ng/ml). Subsequently, plasma concentrations of 17β-estradiol and LH remained basal. Following the last period of behavioral estrus, all six ewes experienced a period of elevated plasma concentration of progesterone typical of a normal luteal phase; subsequently, concentrations of progesterone remained basal. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.

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