Determination of minimum light treatment required for photostimulation of winter anoestrous mares

Classical photostimulation of winter anoestrous mares consists of a light treatment with 14.5-16.0 h white light (100 lux), starting near the time of the winter solstice and ending around the time of the summer solstice. Cyclicity is obtained typically after about 70 days of treatment. The aim of the present study was to establish the minimum number of days of treatment, the lowest light intensity and the duration of light required per day to advance the onset of ovarian activity in winter anoestrous mares. In Expt 1 it was demonstrated that a 35 day exposure to a photoperiod (14.5 h light: 9.5 h dark, 100 lux) starting at about the winter solstice is sufficient to advance the onset of the breeding season of Pony mares undergoing winter anoestrus. In Expt 2 it was established that a light of only 3 lux intensity inhibits an increase in melatonin concentrations in the evening after the usual time of switching the lights off. In Expt 3, a low light intensity (10 lux) produced with a 25 W white incandescent bulb was sufficient for photostimulation of the mares. In Expts 4 and 5, 1 h light (10 lux) during the photosensitive phase for 35 days advanced the onset of the breeding season in only half of the mares treated.