One would assume that you have checked the mare's teeth and other general health to ensure there are no underlying problems that may be causing the weight loss. Lactation is a major nutrient drain on the mare. If you did not know about the pregnancy, the mare may have been in reasonable body condition for a non-pregnant animal but lower than is reasonable for a pregnant mare. If you have pasture, you may want to turn the mare out and allow her to graze in addition to the hay and concentrate you are feeding. For the mare you describe, you would want to feed her 1.5% - 1.75% of her body weight in good quality hay. An alfalfa grass hay that has a high nutrient value is something to consider. In addition to the hay, 1% of her body weight in concentrate may be required.
