When an average size mare delivers she will have produced a 100+ lb foal, an 11 pound placenta, and as much as 16 pounds of fluid. She has also greatly increased the size and thickness of her uterus and blood volume increased about 30% during pregnancy. The raw materials to build these things didn’t come from thin air.
By the halfway point, the foal is about the size of a Beagle, all organs formed, and rapid growth begins
Providing adequate calories is the easy part. Rapidly dividing cells also have critical needs for amino acids, vitamins and minerals which they must obtain from the placenta. It’s true that the dam will rob her own body tissues if necessary to provide for the fetus (not that this is a very smart management tactic!). It’s also true that the dam cannot provide something she herself does not have. If she starts the pregnancy with low body reserves and her diet is not adequate, the foal will be short-changed and the mare become even more deficient.
Extreme deficiencies result in things like White Muscle Disease and goiter with hypothyroidism in foals. More insidious effects include a higher risk for developmental orthopedic disease like OCD and contractures. Chronic copper deficiency has been linked to uterine artery rupture in mares.
Advice on feeding pregnant mares used to be no special attention to nutrition until the last trimester. The latest (2007) NRC recommendations begin to allow for increased nutrients in the 5th month but since there are still gaps in the research, there are also gaps in their recommendations. For example, they don’t allow for any increase in zinc or manganese but obviously foals have those essential minerals in their bodies.
Good quality grass hay or pasture should be the bulk of the pregnant mare’s diet. In fact, a hay with 10 to 11% protein and digestible energy (calories) of 0.9 Mcal/lb can meet calorie and protein requirements throughout pregnancy. Even in the last month of pregnancy the mare would only need to consume a little over 2% of her nonpregnant body weight to meet her needs. For every 1% below 10% in the protein, the mare needs 45 grams of supplemental protein per 10 lbs of hay. For example, if a 9% protein hay and she’s eating 20 lbs, she needs 2 x 45 = 90 grams of supplemental protein. A common range for protein in good quality grass hay is 8 to 12%.
If you don’t know the protein level in your mare’s hay, it’s wise to supplement. “High” (14%) feeds won’t help because they have 2.5 to 3 times more calories than hay but not 2.5 to 3 times more protein so you feed a lot less. Choose a supplement with a blend of vegetable and whey sources, guaranteed levels of lysine and methionine. If you assume 8% protein, a mare eating 20 lbs/day will need 180 g of protein = 450 g of a 40% protein supplement (1 pound).
You may want to meet part of your extra protein needs with a combination protein and mineral supplement. As a rule of thumb, the pregnant mare will need double her baseline mineral intake at the time of greatest demand so look for a supplement with at least 5% calcium and 225 mg copper per 1 lb serving. A pound of it will provide about 112 g of protein if 25% protein.
Do not stop your mare’s usual mineral supplements when she is pregnant. You still need to have her eating a balanced diet base. The above supplementation is for the additional needs of pregnancy. Compared to what is already invested, this is cheap insurance. A breeding farm client of mine once described foals from mares managed this way as “robust”. How many 1 week-old foals look like this?