Horses use ________ for their main energy supply.

Horses primarily rely on carbohydrates as their main energy supply. Carbohydrates are easily digestible compounds that can be found in grains, forage, and other plant-based feedstuffs. When horses consume these carbohydrates, their bodies break them down into simpler sugars, which are then utilized for energy. This energy is crucial for supporting various activities, including maintaining health, sustained work, and overall performance.

Fats and proteins are also essential for a horse's diet, but they serve different purposes. Fats provide a dense source of energy and are beneficial for increasing caloric intake without increasing the bulk of feed, whereas proteins are important for building and repairing tissues, not for energy in the same primary role as carbohydrates. Vitamins play a vital role in various metabolic functions and overall health but do not supply energy. Thus, carbohydrates are the primary source of energy for horses, underscoring why this choice is the correct answer.