Courtesy of Penn State Extension
The mare in late gestation differs nutritionally from the open mare. Her nutrient requirements are slightly higher because she is maintaining her body and supplying nutrients to a rapidly growing fetus.
Body Condition and Energy Needs
The mare requires an additional 3 to 4 mega calorie of digestible energy above maintenance requirements. This increased energy need can be met by feeding more of the same grain mix she was consuming when open and during early gestation. A 1,200-pound mare will need about 2 to 3 pounds of additional grain mix per day when she enters late gestation to meet her increased energy requirement. It is important that she remain in fleshy condition if she is to be rebred following foaling. The added nutritional stress brought on by early lactation can cause mares in a marginal condition to drop in condition to the point of affecting rebreeding efficiency. Also, many pastures are in their lowest nutritive value this time of year. Close inspection of mare condition is necessary so grain and hay levels can be adjusted accordingly.
The gestating mare has a slightly increased need for protein above her requirement in the open state. Crude protein requirements will increase about 1/2 of a pound when mares are in late gestation. This increased need above maintenance is usually met when feeding more grain mix to supply adequate energy, so a higher percent protein grain mix is usually not necessary.
Calcium and Phosphorus Needs
As with other nutrients, the amount of calcium and phosphorus required during late gestation is increased above the amounts needed in the open state. These mares require approximately 20 more grams of calcium and phosphorus than when in an open state. As with protein, these amounts are usually more than adequately met when increasing the amount of grain mix for energy purposes
The major vitamin of concern during late gestation is vitamin A. Vitamin A requirements are doubled when mares are in late gestation and lactation. Most commercially prepared grain mixes have sufficient levels of added vitamin A to adequately meet requirements; however, many producers routinely feed a vitamin premix to gestating mares consuming feeds of questionable vitamin levels. It is important that the premix contain about a 9 to 1 ratio of Vitamin A to Vitamin D, a 6 to 1 ratio of Vitamin D to Vitamin E and be fed at levels recommended on the label. Overfeeding of several of the vitamins can cause irreversible damage to the mare and fetus.