Effects of changes in photoperiod on freezability of ram spermatozoa

The effect of photoperiod on freezability of ram spermatozoa was evaluated in ejaculates collected over 52 weekly periods from two groups of rams housed in windowless rooms maintained under either a natural light regimen corresponding to latitude 45 degrees N or its reverse. The survival of spermatozoa after freezing of 0.5-ml straws at 15 degrees C/min, storage in liquid nitrogen, and thawing in a water bath at 39 degrees C was evaluated as freeze-thaw motility percentage and rating and as a cryosurvival percentage. Freeze-thaw motility percentage was highest during the decreasing photoperiod, regardless of season. Motility percentage after freezing was positively correlated with motility percentage before freezing (r = 0.40) and ejaculate osmolality (r = 0.41), and negatively correlated with percentage of abnormal spermatozoa (r = 0.46). Cryosurvival was significantly lower during the winter and spring seasons for semen collected from rams maintained under the natural light regimen. No significant differences in cryosurvival over the year were observed in semen collected from rams maintained under the reverse light regimen. Cryosurvival was positively correlated with ejaculate osmolality. The vigor of frozen-thawed spermatozoa, assessed as motility rating, was significantly lower during the increasing photoperiod for rams exposed to the natural light regimen. However, the motility rating of spermatozoa collected from rams under the reverse light did not differ significantly.