Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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Dr. Getty
Hi Becky,
Congratuations! I would suggest you switch to the diet that you feed your geldings. As long as she's getting good quality grass hay 24/7, she'll be fine with these ingredients. I would offer her a third meal, assuming you're feeding her two meals per day now, of alfalfa, either as hay or as pellets, and an additional 2 lbs of beet pulp. And, you could also add an additional 1/2 cup of flaxseed meal.
This is an estimate, since I don't know you mare's size. But for the average 1000 lb mare who is not overweight, she will need the additional feed to provide her with the nutrients she needs for her foal.
A supplement that is designed for pregnant mares is very important, both during pregnancy and after the foal is born. I recommend Mare/Foal IIP. And after the foal is born, you will need to continue feeding her plenty -- even more than during her pregnancy -- since lactation uses a large amount of calories.
If you don't already have a foaling guide, I also suggest The Complete Foaling Manual to help you know what to expect and care for your mare and the newborn.
Happy Foaling!
Dr. Getty 
Where are you from? Bayfield, CO
Becky:
Wondered if you can get a couple of bales of alfalfa to try the mare on that? I tried alfalfa pellets with mine, and they acted like it was poison or something, but they LOVE alfalfa hay. Guess the pellets didn't LOOK like forage. 
Nancy
Where are you from? Tennessee
How did you locate this forum? google search
Hi Becky,
As Nancy suggested, alfalfa hay would be worthwhile adding to the diet to help with weight gain. I frankly, have never seen a horse that didn't like alfalfa hay (pellets? yes -- but not the hay).
Add a weight gain product, as well, to the diet. You can mix it with a small amount oats, just to provide a "carrier" for the supplements. I recommend Hard Keeper.
Keep me posted!
Dr. Getty
Where are you from? Bayfield, CO