Mares Transitioning to the Breeding Season

Horses are long day seasonal breeders, with both males and females showing seasonal variability in reproductive function but it is much more obvious for the mares. Most mares have suppressed ovarian activity which begins in the fall and ends in early spring but they do not immediate begin normal cycles.  The interval between normal cycling and cessation of cycling is called the transition period and lasts approximately 6 to 10 weeks.


 Cycling resumes under the influence of increasing day length

Here in the continental United States, the transition period usually begins between March and April but may be delayed by older age, low body weight, very cold weather or poor nutrition. An increasing plane of nutrition, such as grazing spring pastures, seems to hasten cycling and fertility.

During the transition period there will be 2 to 5 “waves” of developing follicles which are either resorbed or remain as large follicles which fail to ovulate. Most mares show no outward sign of cycling or receptivity to the stallion during the transition phase. Others may show estrus but at irregular intervals that are either longer or shorter than normal.

There is really no treatment available or necessary for the transition period in most mares. If you would like to breed your mare, an experienced reproductive veterinarian will be able to follow the follicles by palpation and/or ultrasound to know when your mare is truly ovulating and ready to be bred.

Mares that typically show wide swings in behavior with their cycles will begin to show this pattern during the transition period. It may even be exaggerated when follicles reach a large size but fail to ovulate (anovulatory).

Some mares with metabolic syndrome have the most difficulty of all. The large anovulatory follicles can cause severe colicy pain.  Insulin may rise and is not controlled well by diet. Crests typically enlarge and harden at the same time.  Some mares have mammary development and may lactate. The mare can even progress to becoming laminitic.

Tincture of time will resolve any transition period issues for most mares. However, when irregular cycling, pain, behavior swings and anovulatory follicles persist Vitex agnus-castus can help support the natural transition to a normal hormonal balance.

Vitex agnus-castus (Chastetree, Chasteberry) fruits/berries act like adaptogens for the reproductive system, helping the mare’s body maintain normal function in both the brain and the ovaries. Unlike common hormonal drugs, it does not interfere with function but assists her to progress to natural, balanced activity. Studies have shown Chasteberry extracts help maintain normal levels of dopamine and the hormones LH and FSH.

The transition period is a temporary interval of disarray during which the mare’s hormones progress to a more orderly normal cycle. Mares that remain highly symptomatic or do not appear to be leveling out can be helped to achieve homeostasis with Vitex agnus-castus. Using a 5X extract allows you to feed a much smaller volume. Most average size horses will respond to a dose of 3 to 6 grams.

Eleanor Kellon, VMD



About Dr. Kellon

Graduate of University of Pennsylvania Veterinary School. Owner of Equine Nutritional Solutions,, industry and private nutritional consultations, online nutritional courses. Staff Veterinary Expert at Uckele Health and Nutrition.
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4 Responses to Mares Transitioning to the Breeding Season

  1. Karla Stanley says:

    Where does one get Vitex agnus-castus 5X extract? Thank you!

  2. Jennifer Moncur says:

    Does feeding Chasteberry to a mare already on pergolide for diagnosed Cushings disease have an impact on medication requirements? Do you think it would augment or interact with the medication? Thanks 🙂

    • Dr. Kellon says:

      Theoretically there could be an interaction but by clinical appearance this does not seem to be a concern. Best course would be to recheck ACTH a few weeks after starting it.

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