Editor’s Note: This is an excerpt from Understanding Equine Preventive Medicine by Bradford G. Bentz, VMD. 

Throughout usually the first seven to eight months of gestation, the nutritional requirements of the pregnant mare do not differ substantially from that of idle horses.  In fact, the nutritional requirements of barren and non-lactating mares may be met by feeding forage with greater than eight percent protein by dry weight and providing access to trace-mineralized salt up to the final trimester of pregnancy (last three to four months). During the last trimester and during lactation, the requirements for dietary energy, protein, calcium, and phosphorus increase significantly.  During the final trimester and during lactation, nutrient requirements may be met by feeding a forage greater than 11 percent by dry weight in protein and providing access to trace-mineralized salt fortified with calcium and phosphorous.  If a forage less than 11 percent protein is fed, the diet may be supplemented with a grain or pelleted mix as needed.

Dietary energy requirements increase progressively, 10 percent to 20 percent during gestation and 80 percent during lactation.  Inadequate energy consumption may cause a decrease in milk production and reproductive efficiency. For mares to maintain body weight during pregnancy, their weight must increase by an amount equal to the foal’s birth weight plus the weight of the placenta and fluids (about nine  percent to 12 percent of the mare’s weight).  For the 1,100-pound mare, the total gain should be between 100 and 130 pounds during pregnancy with 0.75 to 1 pound gained daily during the last 90 days of pregnancy.  Tables 1.7, 1.8, and 1.9 are meant as guidelines for the requirements during pregnancy and lactation.  A complete feed analysis and evaluation of individual nutrient needs are best performed on an individual basis.