The first milk produced by mares after foaling is colostrum which should be ingested by the foal during the first few hours of its life. In the first month of lactation the 500kg mare will produce milk at up to 16 kg per day, rising to a peak of 18kg/Day between the 5th and 12th weeks of lactation. The foal of a 500kg mare should weigh around 50kg at birth and will consume 8 Liters of milk on day one rising to 12-15 Liters at a week old. At one month after foaling this milk will contain about 20g protein, 70g Lactose sugar, 13g fat, 0.9g Calcium and 0.6g Phosphorous per kilo as well as all the minerals, electrolytes, and trace elements necessary for the foals nutrient requirements. By one month of age a Thoroughbred foal will have doubled its birthweight, almost all of the increase having come from milk.
Producing milk places the highest demands for nutrients encountered at any stage of life and the mare must either receive these in her diet or will access them from bodily reserves. When dietary intake fails to meet these increased demands, the mare will draw down her reserves from fat, muscle, and bone, and lose condition. Often this may coincide with the time of re-breeding when optimal fertility requires the mare be in good body condition and ideally on a rising plane of nutrition. For these reasons provision of adequate nutritional support, including adequate energy and good quality protein, minerals and trace elements is particularly important to the lactating mare.