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It is likely that your mare's are being fed adequately without the grain. Grain's add carbohydrates the body, evolutionarily, when horses roamed the plains, they did not have access to high carbohydrate concentrated rations and were maintained on grass hay. I would not recommend giving grain to the mares to increase the foal weights. The grain energy will be distributed to the mare as well as the foal. It may also result in faster growing foals in utero - this will lead to increased problems with crooked legs and contracted tendons.
Without seeing the mare's and foals, I cannot give an opinion on your particular mare's regarding their exact weight but in general, foal's usually come out fairly gaunt and ribby and quickly catch up off the mare's milk. If your mare's are having problems coming into milk, it may be possible that your mare could have some hormonal imbalances. Given the area you are located and the type of winter, there is a possibility that your mares are being affected by something environmental. I would recommend having a veterinarian out to examine the broodmare herd and ensure that everything looks ok first hand rather than pouring money into grain when that might not be the problem.
Hope this helps. All the best