Usually, mares will develop a noticeable bag three to six weeks prior to foaling. Mares foaling for the first time may have less development. This is especially true for mares with past performances as 'heavy milkers'. Veterinarians have available several therapies that can assist in preventing milk letdown problems, as well as reversing milk letdown problems prior to foaling.
Colostrum fills a mare's mammary glands about two to three days prior to foaling.
Colostrum oozes from the teat in most mares, forming beads of wax-like material at each teat orifice. Records can help managers recognize what is normal for a specific broodmare in this regard, although there may be variability from year to year. Generally, wax remains for a day and falls away. Thereafter, drips or streams of colostrum being discharged from teats may be observed. Freeman said small amounts of streaming or dripping colostrum should not be considered a problem.