EXPOSURE OF EWES TO LONG-DAY PHOTOPERIODS BEFORE THE WINTER SOLSTICE CAN DISRUPT REFRACTORINESS TO SHORT DAYS

The main objective of this study was to determine if 7, 15 or 30 days of exposure to a 16 h light:8 h dark (16L:8D) photoperiod prior to the winter solstice, or 15 days starting near the solstice, would disrupt photorefractoriness to short days. In Experiment 1, two different groups of ewes were exposed to 16L:8D for a period of 7 or 15 days starting on November 23, whereas a third group was exposed to 16L:8D for a period of 15 days starting on December 23. In Experiment 2, one group of ewes was taken from outdoors and exposed to 16L:8D for 30 days starting on November 23. After these long-day treatments, the ewes were exposed to continuous 10L:14D or 8L:16D photoperiods, respectively. For both experiments, reference groups were kept outdoors under natural environmental conditions. Blood samples taken by venipuncture twice weekly were assayed for progesterone concentrations. All ewes exposed to 16L:8D stopped cycling significantly earlier (P < 0.05) than those kept outdoors. Within Experiment 1, groups of ewes treated with 16L:8D did not differ in the time interval between the start of the study and cessation of estrous cycles. Once they had stopped cycling, all ewes in Experiment 1 remained anestrus for the remainder of the study. In Experiment 2, three of the six ewes exposed to 16L:8D resumed cycles after exposure to an 8L:16D photoperiod. It was concluded that an exposure to 16L:8D for 7 or 15 days starting 1 month before the winter solstice, or for 15 days starting immediately after the solstice, is not sufficient to restore responsiveness to short days. However, 30 days of exposure to 16L:8D 1 month before the winter solstice is partially effective in restoring responsiveness to a long- to short-day transition.