Fat-added grain mixes provide more energy and cause horses to eat less feed. Both of those are helpful in these situations. While it's possible for mares to store body fat that can be used for rapid fetal growth and later for lactation, mares don't store much protein. The amount and quality of protein are both important. Feed a 16-percent protein grain mix in late pregnancy.
Protein quality relates to amino acids, lysine being the most important.
Alfalfa hay, soybean meal and milk by-products are good sources of lysine. Pregnant mares may need more lysine than recommended. Recent research showed that mares may need twice the recommended level of lysine in early pregnancy. With rapid fetal growth in the last trimester and impending lactation, the lysine level fed in late pregnancy may be more critical than previously thought. Mares that won't foal for 120 or more days give you a broader window to improve their nutritional status if they don't have a BCS of 5.5-7.5.