The Effect of Photoperiod and Melatonin on Reproductive Performance of Anestrous Ewes

Seasonal fertility has been known to exist in the sheep industry for hundreds of years. The inability to control this seasonality has restricted year-round lamb production. Only recently has man attempted to alter this unfertile period of the year, through the use of different stimuli. The ewe produces the hormone melatonin in response to a dark photoperiod. In the fall, when the daily dark phase increases, one sees an increase in duration of the ewe\u27s serum melatonin level. It is this increased melatonin during the fall that has been hypothesized to signal the onset of estrus in the ewe. Artificial photoperiod or melatonin, either fed or implanted during the summer, has been used in an attempt to duplicate the increase in serum melatonin levels normally present in the fall. As melatonin levels increase, the ewe will gradually move from deep anestrus to a fertile breeding period. If successful, a producer may use this as a method to more efficiently utilize labor and facilities by spreading out the normal breeding season. It may also be possible to take advantage of out-of-season lamb markets, increasing the value of those lambs produced. Because some aspects regulating melatonin can now be explained, it is becoming possible to develop a system in which melatonin administered exogenously can alter reproduction. Point of administration (during the photorefractory period), dosage, and nightly duration all are critical factors to consider in determining the response to melatonin treatment. Melatonin either fed, implanted or injected has proven to be adequate in raising and extending plasmal levels of melatonin, similar to those seen during short photoperiods of the breeding season. Arendt et al. concluded that raising plasma melatonin levels similar in duration to those found in the winter, during the summer, stimulated the onset of estrous activity two to eight weeks before that of untreated controls kept in normal daylight. Though many questions remain as to the mechanisms of how melatonin signals the changing seasons, little question remains as to its usefulness