Some of those who must still practice patience are owners of pregnant broodmares. With a gestation length of 335340 days, they just have to wait until the appointed time. The answer is yes. Horse owners use artificial light to trick nonpregnant mares into an earlier estrous than normal each spring.
Mares are long day breeders.
They used 16 hours of daylight and 8 hours of darkness in a group of pregnant broodmares starting December 1 until they foaled in the spring. A control group of mares remained under normal daylight hours only. Extending the number of daylight hours during the last trimester of pregnancy shortened the length of gestation by 10 days. The birth weight, height, heart girth as well as length and circumference of the cannon bone of these early foals were no different from those born later. Reproductive factors, such as time from foaling to ovulation, start of estrus and subsequent conception rates, were not affected by the use of artificial light in pregnancy.