The effect of light on the breeding season, gestation, and birth weight of merino sheep

Peppin strain Merino ewes, when running continuously with vasectomized rams, exhibit a definite seasonality in the incidence of oestrus. Although less decisively separated into seasons of quiescence and activity, the oestrous pattern resembles that of the British breeds of sheep. The pattern may be reversed by artificially reversing the seasonal trend of daylight duration. This is presumptive evidence that the natural breeding season of Merinos is regulated by seasonal fluctuations in the length of day. Though there was no evidence of it in this experiment, some Merinos are believed to commence their breeding season before the days begin to shorten. This is not necessarily irreconcilable with photoperiodic control, and such animals may represent a variant genetic type. Pregnant ewes, subjected during winter-time to a summer level of light, produced lambs of normal birth weight following a gestation of normal length. This finding helps to isolate summer heat as the factor responsible for the low birth weight sometimes reported of lambs produced out of season