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THE SEASONAL NATURE OF REPRODUCTIVE PHENOMENA IN THE SHEEP

Summary.: Three groups, each of twelve spayed crossbred ewes, were maintained for 12 months on three different nutritional regimens. Twice each month each group was treated in a standardized fashion with progesterone followed by three doses (10·0, 15·6, 24·4 µg) of oestradiol benzoate, and oestrous behaviour and vaginal oestrus were recorded. There was a seasonal pattern of variation in sensitivity to oestrogen which was modified by nutritional treatment. During late winter-rearly spring (August to October), loss of live weight was associated with decrease in sensitivity and maintenance or increase in live weight with increase in sensitivity. No such relationship was shown at other times of the year. Sensitivity was uniformly low, regardless of nutritional regimen, in late spring-early summer (November to January). It is concluded that nutritional status during the August to October period when entire ewes are entering into anoestrus could have an important bearing on the incidence of ovulation without oestrus ('silent heat'). During the remainder of the year, nutritional status appears relatively unimportant.