Colostrum, the first milk, is rich in immunoglobulins. Immunoglobulins provide immunity to the foal against diseases. In pregnancy, there isn't any transfer of maternal antibodies across the placenta to the fetus. So newborn foals are without any natu ral protection against disease. The foal's immune system is intact when it's born, but it doesn't function until the foal is about 4-6 months old.
Foals come in contact with a number of opportunistic microorganisms in nature.
The greatest risk of death in the first week of a foal's life is septicemia. A number of broodmares that graze endophyte-infested tall fescue pastures while pregnant will be agalactic. They won't have any colostrum or milk. So the newborn foal doesn't even have an opportunity to get colostrum and acquire passive immunity. Remove pregnant mares from endophyte-infested tall fescue pastures at 300 days of gestation.