Photoperiodism in the ewe 5. An attempt to induce sheep of three breeds to lamb every eight months by artificial daylength changes in a non-light-proofed building

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Abstract
SUMMARY An attempt to induce ewes to lamb at intervals of 8 months using high levels of artificial daylength applied in a non-light-proofed building was carried out over a period of 2½ years. Thirty each of Dorset Horn, Clun Forest and Kerry Hill ewes were used. There were two different light regimes, both involving abrupt changes in daylength, all of which occurred at daylengths in excess of natural daylength. In both treatments the artificial daylength was abruptly increased to 22 hr at conception. In one treatment there was an abrupt decrease in daylength at parturition whilst the other treatment was an abrupt decrease in daylength approximately 50 days before parturition. The size of the daylength decrease possible was determined by the natural daylength at the time the decrease was applied and at the estimated time of onset of next oestrus. The majority of ewes lambed four times in the 2½ year period, parturition occurring in March-April 1969, December 1969-January 1970, September-October 1970 and May 1971. The Clun Forest ewes produced 38, 29, 20 and 22 lambs at the four lambings, whilst the Dorset Horn ewes produced 41, 35, 26 and 25 lambs at the four lambings. Only 11 of the Kerry Hill ewes lambed throughout the whole experiment producing 3, 5,0 and 7 lambs at the four lambings. It is suggested that the changes in daylength at these high levels controlled the breeding activity of the ewes.