The horse is a non-ruminant herbivore. And like all herbivores, it can fill most of its daily energy requirements by eating plant fiber. In fact, at pasture, the horse spends more than 70% of its time grazing, as much as 18 hours a day. Fiber is a desirable energy source for the horse, as are starch (grain) and fat (vegetable fat). What's more, fiber is indispensable to the horse’s digestive balance. Indeed, to stay healthy, the horse must have at least 1% of its body weight in long stem fiber.
A minimum that must be met while it is often necessary to add grains, commercial feeds, or vegetable oil to the diets of horses with higher calorie needs (lactating mare, growing foal, horse in training), fiber remains the most important ingredient in any equine diet. Generally, it supplies enough energy for the horse's basic needs, such as breathing, digesting, grazing, walking, and sleeping. Apart from the young 18-month-old horse in training, every horse should have at least 50% of its daily ration in fiber to maintain optimum digestive health. For most adult horses, this percentage can be increased considerably, sometimes to as much as 100% fiber, if the horse is not working, or if it maintains body condition easily.

How does the horse use fibrous feedstuffs?

Fermented in the cecum and colon by billions of bacteria, the fiber ends up as volatile fatty acids, which can then be absorbed by the horse and converted to energy. Quality and digestibility Depending on its origin, fiber can vary considerably in quality and digestibility. Fiber is comprised of three main substances: cellulose, hemicellulose, and lignin. Lignin, which gives plants their rigidity, cannot be digested by the bacteria in the horse's large intestine. The horse’s energy needs are therefore met by cellulose and, especially, hemicellulose. A fourth substance available in fiber, pectin, is drawing growing attention. Extremely digestible, pectin is found in large quantities in beet pulp, soybean hulls, and young forages. Much more than crude fiber (CF), a very imprecise value usually found on feed labels, the best indicators of a fiber’s digestibility are the percentage of NDF (neutral detergent fiber) and ADF (acid detergent fiber). The only way to determine the NDF, ADF, and CF values in what you are feeding your horse is through sampling and laboratory analysis. Principal fiber sources There are numerous fiber sources for the horse. The best known remain, of course, fresh grass and dry hay. It is important to understand that the digestibility of the fiber found in pasture and hay varies according to the climate, the time of year, and the degree of plant growth. Fresh grass is the most natural fiber source for horses, the latter having evolved to eat large quantities of it. Pasture grass that is 4 to 8 inches high generally offers very good digestibility for the horse. On the other hand, grass that is over 8 inches high is more mature and ligneous, with a corresponding drop in digestibility.

The nutritional content of alfalfa at different growth

This is why pasture maintenance is highly recommended, especially if it is the horse's only nutrition source. About hay, if harvested early in the season, while the plant is still young, the general fiber content will be lower than that of hay harvested later in the season, but its nutritional value will be higher. Hay cut when young will contain lower levels of lignin and, by the same token, display higher fiber digestibility than mature hay cut when it is in flower; at that point, the stem becomes hard and fibrous, which affects palatability and fiber digestibility, in addition to lowering its nutritional value.