Spring is a transition period during which the mare's reproductive system must gear up for mating and pregnancy. In the spring transition, the mare's reproductive system is unpredictable, giving owners ulcers and sleepless nights. During spring transition, mares don't ovulate. Mares cannot become pregnant until they ovulate, which normally doesn't occur until about April 7. Mating the mare before this time is a waste of time and money.
Artificial lights can fool the mare into thinking it's spring.
But to be effective you have to put them under lighting at least by Dec. 1. Hormone programs may aid in shortening the transition period. But only your veterinarian or a reproductive equine specialist should administer such programs. This isn't a bad plan. Most foals are born in April and May anyway.