Q: What is colostrum, and why is it so important for the newborn foal?

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Colostrum, the elixir of life for newborn foals, is a thick, yellowish fluid produced in the mare’s mammary glands towards the end of the pregnancy. It is rich in immunoglobulins, mainly IgG-class and some IgM-class antibodies. These are critical for immune protection of the foal, which has no natural defenses at birth due to the epitheliochorial-type placenta which prevents in utero transfer of antibodies from mare to fetus. Comprised of carbohydrates, fats, proteins and electrolytes, this important first milk is ingested by the foal through nursing immediately after birth, resulting in ‘passive transfer of immunity’ which will protect it from infectious diseases. Colostrum is only produced by the mare once per pregnancy, and is depleted and replaced by regular milk within 24 hours once the foal begins nursing.

A healthy foal should stand and nurse within one or two hours of birth, and in order to acquire sufficient antibodies should ingest two or three litres of colostrum over the next few hours. Special epithelial cells (enterocytes) in the small intestine absorb the antibodies, discharging them into lymphatic vessels, which carry them to the bloodstream. This absorption and transfer process is at its peak efficiency 6-8 hours after birth and declines rapidly; by the 24-hour mark, the epithelial gut cells change and antibody transfer can no longer take place.

If the mare loses large amounts prior to foaling, has low-quality colostrum (due to eating endophyte-infected fescue, for instance), rejects the foal, or the foal will not nurse, the passive transfer of antibodies will fail. A blood test can determine the blood IgG levels; the foal may need to be supplemented with frozen-thawed colostrum, a commercial colostrum substitute containing equine antibiodies, or fresh colostrum from a ‘nurse’ mare.

Mare owners are wise to maintain a supply of frozen colostrum, which can be safely stored for up to two years, harvested in reasonable quantities from postpartum mares without adversely affecting passive transfer to their foals.