Can you just increase the amounts you are feeding but basically feed the prepregnancy diet and maintain the mare’s weight in pregnancy? Yes – her amount of fat anyway. Can you get a live, full term foal? Yes. Are you risking problems? Yes, absolutely.
Your average size horse mare is about to deliver 100+ pounds of newly formed skin, bone, blood, muscle and organs along with another 150+ pounds of membranes and fluids formed for support of the developing foal. She didn’t build all that from air.
All successful species evolve with the ability to survive difficult times. If a pregnant mare is deprived of a nutrient her developing foal needs, her body will sacrifice from its own supply to support the fetus. Fat breaks down for calories, muscle for amino acids, bone and other storage sites like liver for minerals. The foal will be born alive but her weakened condition impairs fertility, immunity and her ability to produce adequate milk. There are long term effects like risk of arterial rupture from copper deficiency.
If the mare still can’t fully provide for the foal, it can be born with issues like white muscle disease from selenium deficiency, weakness and goiter in iodine deficiency, contracted tendons or OCD lesions from copper deficiency, skeletal abnormalities in vitamin A deficiency. More common than full blown deficiencies are foals born small, weak and spindly rather than the vigorous and robust foals of a properly fed mare.
In late pregnancy, calorie needs increase by 28% but protein needs by 100% and minerals 80%. A diet formulated adequate for an adult nonpregnant horse won’t cut it. Mare and Foal feeds are not necessarily the answer either. Even at 14 to 16% protein they will provide only about 1/3 of what is needed. Minerals add up better but still fall short and both the protein and mineral requirements still needed are less likely to be met because of greatly decreased hay fed when grains are given at their maximum recommended amounts.
To determine exactly what your mare needs, and how much, you should consult a professional. In general though, the answer lies in concentrated protein/mineral supplements with 25 to 30% protein, lysine 2+%, calcium 4+% and copper around 300 ppm with other minerals in correct balance.
When you see a truly robust foal you’ll know you have it right. There’s no comparison.
Eleanor Kellon, VMD