Photoperiodism in the ewe. 3. The effects of various patterns of increasing daylength on the onset of anoestrus in Clun Forest ewes

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Abstract
SUMMARY In light-proofed buildings experiments were conducted with six groups each of ten Clun Forest ewes to determine whether the onset of anoestrus was affected by the absolute size of increase in the case of an abrupt increase in daylength, and by the rate of increase in the case of a given total increase in daylength. The mean number of days from the onset of the light treatments to the cessation of oestrus (the reaction interval) for ewes receiving gradual increases in daylength of 3·5, 7·2 or 10·9 min per day from 23 December were 79·8, 84·1 and 75·7 days respectively. The reaction interval for ewes receiving abrupt increases in daylength of 3·75, 7·75 or 11·75 hr on 23 December were 86·3, 76·5 and 61·5 days respectively. A control group of ten ewes at pasture on natural daylength had a reaction interval of 93·6 days. The results show that increasing daylength largely controls the mean date of onset of aneostrus in Clun Forest ewes. The greater the increase in daylength the sooner the onset of anoestrus occurs. The observations on vaginal smears indicate that photoperiodic changes, whilst affecting the oestrous status of the ewe, may have no effect on ovulation.