Levels of prolactin are highest in summer, the horse's natural mating time. Rising levels of prolactin promote the growth and development of ovarian follicles. Mares gaining weight come into estrus and ovulate about a month earlier than those losing weight. If fat mares are placed on a low-energy ration and lose weight, reproductive activity could be lowered. Protein quality may aid mares to become pregnant.
Protein quality refers to the amounts of dietary amino acids that make up protein.
Lactating broodmares fed higher levels of the amino acid, lysine, gained more weight and had foals that were larger at 6 months old. When green grass appears, some broodmare owners stop feeding hay and grain. This could be a mistake for older mares and mares below a BCS of 5 that are grazing pastures below average in quality. Make sure that potential broodmares on pasture have adequate energy and quality protein intake. If in doubt, feed a grain mix designed for yearlings.