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Lipids are basically fats. The primary source of lipids in horses diets are oils. Fats typically are a low portion of the diet because they are high in digestible energy. They typically 2x the energy of carbohydrates. Most lipids are digested in the Small intestine by enzymes called lipases.
Oil is added to a horses diet to increase the calorie and energy content of the food. Hard working horses often have it added to give them slow release energy.
Grass and other forage have relatively low concentrations of lipids as do cereal grains.