Feeding horses has long been considered an art passed down from generation to generation. However, recent advances are rapidly making horse nutrition a more precise science. Research in horse nutrition has provided valuable information on the nutrient needs of all classes of horses.
In feeding broodmares, you must consider several important factors:
nutrient requirements of the particular class of horse being fed,
physiological events involved in pregnancy,
nutrient content of the feed,
physiological limitations to certain rations.
Equine Section, Department of Animal Sciences, University of Kentucky, "Nutrition of the Broodmare" (1988). Agriculture and Natural Resources Publications. Paper 21.