Influence of photoperiod on the pregnant and postpartum mare.

This research investigates the effect of photoperiod (length of daylight exposure) on the gestation length, foal development, and postpartum reproductive performance of mares, and the change in plasma progesterone and luteinizing hormone levels during different stages of pregnancy and postpartum.

Introduction and Objectives

• The study was conducted over two years and used 32 mares of predominantly Quarter Horse breeding, along with their foals.
• The primary focus was to evaluate the influence of an extended photoperiod regimen (16 hours of light) on the duration of gestation, foal development, and postpartum reproduction.
• The study also aimed to observe changes in plasma progesterone concentrations during the third trimester of gestation and plasma luteinizing hormone (LH) levels from the time the foal was born until the next ovulation.

Results

• Mares exposed to longer hours of daylight had an averagely shorter gestation period by ten days compared to control mares exposed to standard daylight hours.
• Foals born to mares exposed to a longer photoperiod had higher birth weights, but the size, based on body measurements, was not significantly affected by the photoperiod.
• The interval between giving birth to one foal and the 1st ovulation and between the onset of estrus to ovulation were not significantly affected by the extended photoperiod.
• On the other hand, progestogen levels rose from prepartum weeks 4 and 5 to reach peak values one week before foaling, and this was observed in both groups.
• The LH profiles from birth until ovulation were similar for both mare groups.
• However, in the 7 days before the 1st ovulation, the average plasma LH values were significantly higher in mares exposed to extended daylight compared to control mares.

Conclusion

• The results suggest that extending the photoperiod can influence the length of gestation and potentially the birth weight of the foal, indicating major implications for breeding strategy. It also impacts the hormonal changes in mares both pre and post-foaling.
• However, it does not significantly alter the size of the foal, the interval from the onset of estrus to ovulation, or the period between giving birth and next ovulation.
• The similarities and differences in hormone profile for control and experimental mares need further research to fully understand the biological processes involved.