Third, colostrum is protective. IgG contains antibodies against diseases. Foals are born without any natural protection to disease. From IgG foals obtain passive immunity against diseases until their own immune systems becomes functional when they're 4-6 months old. This is one reason we recommend that you place broodmares in the area they'll foal about a month ahead of foaling.
Mares produce antibodies against diseases common to that environment.
It's also why mares should receive immunization against normal diseases such as tetanus, Eastern and Western Encephalomyelitis and influenza about 30 days before foaling. The mare produces colostrum in the last two weeks prior to foaling. The foal's digestive tract only absorbs the large immunoglobulin molecules for about the first 12 hours after birth. Once colostrum wanes, the mare produces normal milk. And mares can produce a lot of milk.