You need to remember that they will require an increased feed allowance right thru the first 3 months after foaling also. It takes a tremendous amount to produce milk and maintain body weight.
Generally my mares are consuming 2-3X their personal before mommyhood rations during lactation. So last trimester it increases, after birthing it will generally need to increase a little more, then stabilize, then gradually decrease.
I break this into 3 feedings, normally, to keep their systems in good balance. Of course, with those who have great pasture, it can be adjusted as needed for their body condition. Most often the pastures are not good enough to cut back much (if an early in yr foal), so just plan on the increased feed until the foal is eating well, etc.