Horse Side Vet Guide ®

Equine Health Resource

Irregular Heat Cycle, Spring Transition

Synonyms: Transitional Mare

Mares are seasonal breeders, meaning that they only show estrus cycles during the spring, summer and early fall. During the winter months during the short photo-period (hours of daylight) most mares in the Northern Hemisphere are not cycling. When palpated on rectal exam, their ovaries are small and there are not any obvious eggs (follicles) forming on their surface.

When the days lengthen, hormones cause the ovaries to enlarge and the mare to return to cyclic heat periods. The first heat cycles of the year (February through early April) are often irregular. Some may not result in normal ovulation. For this reason, mares may be difficult to breed during the early part of the breeding season.

Various treatments are used to encourage mares to have normal heat cycles at this time, but it is important to first know what you are trying to accomplish.


  • Do you have any protocols that may normalize the mare's reproductive cycle in the early part of the breeding season?

    Mares can be put under artificial light in the preceding Fall to cause them to cycle early in the season. There are a variety of other methods of hormonal manipulation that may improve the situation.

    Helpful Outside ResourcesCredible Equine Health Information on the Internet

    Author: Doug Thal DVM Dipl. ABVP


    We're not around right now. But you can send us an email and we'll get back to you, asap.