Its the nutrition of the hay, along with any suppliments that counts more than the amount shes eating. Have you had your hay analized ? Do her teeth need to be done ? If shes walking around not too interested in her feed, thats a " heads up " for me that she might have an ulcer or teeth issues. She should be increasing her intake by around 30 % in her gestation. Then while shes lactating she also has specific nutritional requirements.

Dr. Clair Thunis will help you put together a nutritional plan for both mare and baby if you want. Contact me for her contact information. She can look at what you are feeding now and what she might need.