Horses can utilize large amounts of ________ in their diet.

Horses are designed to consume substantial amounts of roughage, which primarily comes from forage such as hay and grass. Their digestive system is uniquely adapted to process fibrous plant material, allowing for efficient nutrient extraction from these sources. Roughage is essential for maintaining proper gut health, preventing issues like colic, and supporting normal digestive function.

The structure of a horse's digestive system, particularly the large cecum and colon, facilitates the fermentation of roughage by beneficial microorganisms, enabling the breakdown of cellulose and other complex carbohydrates. This process not only provides essential nutrients, such as vitamins and minerals, but also supports the overall health and well-being of the horse.

In summary, the high roughage requirement in a horse's diet is crucial for its digestive health and overall nutritional needs, making it the best answer to the question posed.