Refractoriness to inductive day lengths terminates the breeding season of the Suffolk ewe.

The ambient photoperiod was recorded over an 8-yr period at the beginning and end of the breeding season in a flock of Suffolk ewes maintained outdoors. The transition into the breeding season in this short-day breeder occurred at a day length which was much longer (14.0 h, Sept. 3 +/- 5 days) than that at the onset of anestrus (11.5 h, Feb. 15 +/- 3 days). This produced a marked asymmetry of the annual cycles of photoperiod and reproduction. In an attempt to explain this asymmetry, we determined if ewes enter anestrus because increasing day lengths curtail breeding or because ewes lose the ability to respond to prevailing short day lengths (i.e., become photorefractory). On the winter solstice, 3 groups of 6 ovariectomized ewes bearing s.c. Silastic implants of estradiol were placed on different day length treatments: 1) natural environment; 2) artificial day length which simulated natural photoperiod; or 3) artificial day length equivalent to that of the winter solstice (10L). Transition into anestrus was determined from the precipitous drop in serum luteininzing hormone (LH) concentrations. The breeding season was not prolonged in ewes held on the winter solstice day length; LH did not remain elevated beyond the time that it plummeted in the other groups. These findings lead to the conclusion that Suffolk ewes normally cease breeding, not because they are actively inhibited by increasing day lengths, but because they become photorefractory. This can account, at least in part, for the asymmetry between the annual cycle of photoperiod and reproduction in this short-day breeder.