Maternal Age, Parity and Nursing Status at Fertilization Affects Postpartum Lactation Up to Weaning in Horses

•In primiparous mares, milk production is reduced but milk is richer in fatty acids.•Immunoglobulin G concentrations are increased in colostrum of primiparous mares.•The saturated/polyunsaturated fatty acid ratio is increased in old mares' colostrum.•Sodium concentrations are increased in both primiparous and old mares' milk.•Milk production is reduced in late lactation when mares were nursing at breeding.