Colostrum is produced by the mare and is the first milk she produces when she gives birth. She only produces this milk for up to 24 hours after giving birth. The milk appears different from that which she will produce for the rest of her lactation period, in that it is yellow, thick, and sticky.

As the mare approaches her foaling date, her udder will fill with this colostrum. As she begins to produce milk, it will drip off the ends of her teats. This is the so-called ‘waxing up’ that you look for just prior to foaling. ‘Waxing up’ is a normal part of the process, but some mares, unfortunately, leak large quantities of the colostrum at this stage, which can reduce the availability of this all-important ‘first milk’ for the foal. In these instances, the vet should be called and replacement colostrum may need to be sourced for the arrival of the foal.

Colostrum is so essential as it is full of immunoglobulins (antibodies) that the mare’s mammary gland has taken specifically from her bloodstream. These immunoglobulins help to keep the foal healthy and help him to fight infection until his own immune system is adequately functioning at around eight to 10 weeks of age.

This is essential as foals, unlike human babies, are not born with any circulating antibodies from the dam’s bloodstream. The foal in the uterus has a bloodstream that is separated from the mother’s bloodstream by a placenta that, in the horse, has six layers. These layers do not allow the immunoglobulins to cross, so the foal is born without any circulating immunoglobulins to fight infections.

The foal needs to drink and absorb the colostrum within the first 12-24 hours of life as the immunoglobulins within the milk can be absorbed by the foal’s gastrointestinal tract only during the first 24 hours of life. After this time the foal’s intestinal tract cannot absorb the larger immunoglobulin molecules. This means that if the horse does not drink colostrum in the first 24 hours of life, his immune system will be compromised for at least the first couple of months of life until his own immune system is better developed.