Many mares who are weaning foals will be very thin as they have used a lot of their energy in milk production. Dr. Kathleen Crandall from Kentucky Equine Research indicates that these mares need to be conditioned properly to prepare for pregnancy and lactation in the next year. Lots of broodmare owners will decrease the mare's nutrition at the time of weaning to help lessen the mare's milk production. However, there is no scientific evidence to support the practice of decreasing nutrition to decrease or stop milk production.
Nonetheless, when energy reduction is performed gradually, such as slowly decreasing concentrates being fed to the mare over a 2-week period, no ill effects were noted with mares that were in moderate or above-moderate body condition. The foal should continue to have access to concentrates even if the mare's supply of concentrates is reduced. Again, this is with mares in good flesh, but feeding of thin mares is much different. Milk production continues because of the stimulus of nursing and after weaning, milk production decreases. If you attempt to milk out these mares, you will only provide a stimulus for further milk production so it is advisable to just monitor these mares for pain or heat that could indicate mastitis. And while withholding feed could decrease milk production, this should not be done with thin mares, but you should not have to increase calories either as once the foal is weaned, all calories can be used for weight gain instead of feeding the foal. If weight gain does not occur in a couple of months, fat can be safely added to the diet as a source of extra calories.