Depending on the breed of horse, you should try to keep your mare at a body condition score of a 5 or 6. The forage part of the diet is a place you could make a change to maintain weight. Use a hay of good quality, which could be an alfalfa or grass/alfalfa which would increase the mare's nutrient intake. As for the grain ration, you could increase it to 6-8 lbs per day after weaning to provide more energy for weight gain. The changes in hay and the increases in grain should allow the mare to gain weight to above a body condition score of 5.