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Continuous exposure of Suffolk ewes to an equatorial photoperiod disrupts expression of the annual breeding season.

The objective was to determine if "clamping" ewes onto a 12L:12D photoperiod resulted in expression of circannual rhythms of reproductive activity. On 24 February, 1986, two groups of 6 yearling ewes each were placed in isolated adjacent photochambers under a 12L:12D photoperiod and controlled temperature. Six control ewes were kept outdoors. Blood samples taken thrice weekly were analyzed for progesterone. Data from Days 0-1056 are reported. The mean number of cycles by control and 12L:12D ewes did not differ (32.8 +/- 1.7 vs. 29.7 +/- 4.0). The ranges were 27-39 vs. 4-51, respectively. Ten 12L:12D ewes started cycling coincidentally or later than the controls, and then cycled either regularly or irregularly throughout the study. Two of the 12L:12D ewes cycled continuously. The mean number of cycles during the period 15 April-15 August (anestrus) in Years 1, 2, and 3 were 0.7, 0.7, 0.2 for controls versus 0.3, 5.1, and 4.5 for 12L:12D ewes. The mean number of cycles during the period 15 September-15 January (breeding season) in Years 1, 2, and 3 were 7.3, 7.7, and 7.3 for controls versus 2.8, 4.8, and 4.0 for 12L:12D ewes. All controls showed distinct, alternating annual periods of anestrus and ovarian cycles whereas only two 12L:12D ewes showed a similar pattern. Estrous cycles were distributed nonrandomly in all controls and in 2 ewes exposed to 12L:12D. In the 12L:12D ewes, melatonin concentrations rose immediately after the lights-off and fell immediately after on. Lengths of the luteal phases of the cycles did not differ between groups. In summary, estrous cycles of most ewes clamped on a 12L:12D photoperiod occurred throughout the year at variable intervals rather than in distinct breeding seasons.