When a woman is pregnant, the first recommendation a doctor makes is to begin prenatal vitamins. This advice holds true for horses as well. Pregnancy and lactation are the times of peak nutrient demand for any horse. Roughly speaking, when a mare is providing milk for her foal, her calorie and vitamin requirements have doubled, protein needs have tripled, and calcium, phosphorus, and amino acid requirements have quadrupled compared to what she needed when she was not carrying a baby.