Page "Horse breeding" Paragraph 32

While horses in the wild mate and foal in mid to late spring, in the case of horses domestically bred for competitive purposes, especially horse racing and various futurities, it is desirable that they be born as close to January 1 in the northern hemisphere or August 1 in the southern hemisphere as possible, so as to be at an advantage in size and maturity when competing against other horses in the same age group.

When an early foal is desired, barn managers will put the mare " under lights " by keeping the barn lights on in the winter to simulate a longer day, thus bringing the mare into estrus sooner than she would in nature.

Mares signal estrus and ovulation by urination in the presence of a stallion, raising the tail and revealing the vulva.

A stallion, approaching with a high head, will usually nicker, nip and nudge the mare, as well as sniff her urine to determine her readiness for mating.