There are no secret formulas or potions to increase mare fertility. It is important to have the mare enter the breeding season with a body condition score of five or greater (see the learning module on body condition scoring in horses) and fed a balanced diet. There is little evidence to suggest that protein, vitamins or mineral supplements will improve conception rates if mares are fed a balanced diet. Of course, a deficiency of any of several nutrients such as protein, phosphorus, selenium, iodine or vitamin A can reduce fertility.