Summary. Two groups of ovary-intact ewes were placed in separate photochambers on the day of the vernal equinox (VE). One group was exposed to a 16 h light:8 h dark (16L:8D) photoperiod and the other to 8L:16D. On the day of the summer solstice (SS) and at 90–91-day intervals thereafter [autumnal equinox (AE), winter solstice (WS), VE and SS], each group was changed to the opposite photoperiod. The latent period between each change and either onset or cessation of cycles, as determined by measuring blood progesterone concentrations, was recorded. The latent period between change to 8L:16D and onset of cycles was shortest after the exposure at AE and longest after exposure at WS (P < 0·001). The latent period after AE was shorter (P < 0·001) than after VE. The correlations were small between ambient temperature and interval to onset of cycles. The latent period to cessation of cycles in response to 16L:8D was shorter after SS exposure than after WS exposure (P < 0·01), but other differences were not significant. There was a strong (r = –0·94, P < 0·05) negative correlation between interval to cessation of cycles and ambient temperature. Cessation of cycles in response to 16L:8D occurred more rapidly (P < 0·001) than onset in response to 8L:16D. These results show that responsiveness to the inductive effects of photoperiod varies significantly with time of the sidereal year.
Keywords: sheep; seasonal breeding; photoperiod; circannual rhythm; progesterone