A broodmare who is brooding may appear relaxed and contented on the outside as her midsection expands. But inside, her growing foal is making ever-increasing demands on her body, which will only dramatically increase after she gives birth.

The early stages of pregnancy, when the fetus is the size of a walnut, aren’t particularly strenuous for the average horse. Yet that situation radically changes as the mare enters her fifth month of gestation. From then on, the unborn foal’s growth accelerates — and with it, the mare’s nutritional requirements.

Weight Gain Needed

Over the course of a healthy pregnancy, a mare’s weight should increase by an amount equal to the weight of the foal at birth plus the weight of the placenta and the uterine fluids — generally 9% to 12% of the mare’s original weight. A 1,100-pound mare, for example, should gain 100 to 130 pounds (45 to 60 kilograms) during the course of her gestation, with two-thirds of that total weight gain coming in the final 3 months (averaging 0.75 to 1 pound of weight gain per day during that time period).

In the last 110 days of pregnancy, a broodmare’s energy needs will progressively increase by 10% to 20%. She’ll need almost twice the amount of calcium and phosphorus in her diet that she would normally require, and her need for protein will inch up to about 1.3 times…