Vitex agnus-castus, or Chaste Tree Berries are used to treat reproductive-cycle abnormalities in people and animals. It's an adaptogenic herb, meaning the compounds help normalise imbalances in the reproductive hormones.
 
Mares experiencing unusually short intervals between cycles or prolonged periods of behavioral estrus may respond well to it. Vitex will not stop your mare from cycling.
 
It can also be useful in regulating insulin in horses with equine metabolic syndrome.
 
Use 10g or 1 teaspoon per day.