Photoperiodism in the ewe: 2. The effects of various patterns of decreasing daylength on the onset of oestrus in clun forest ewes

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Abstract
SUMMARY In six light-proofed buildings experiments were conducted with Clun Forest ewes to determine whether the date of first oestrus was affected by the absolute size of reduction in the case of an abrupt decrease in daylength and by the rate of reduction in the case of a given total decrease in daylength. The mean number of days from the onset of the light treatments to first oestrus (the reaction interval) for ewes receiving gradual decreases in daylength of 3·5, 7·2 or 10·9 min/day from 1 July were 66·4, 53·7 or 33·6 days, respectively. The reaction interval for ewes receiving abrupt decreases in daylength of 3·75, 7·75 or 11·75 hr on 1 July were 59·5, 44·8 or 33·6 days, respectively. A control group of ewes at pasture on natural daylength had a mean reaction interval of 66·2 days. It is concluded that the various light treatments applied significantly affected the date of onset of oestrus in Clun Forest ewes.