For most mature horses, grass is an excellent dietary staple. Not only is a well-maintained pasture abundantly nutritious, it’s great for a romp.

Can horses really be kept with grass as their only food?

The answer is a qualified "yes." In many instances, a well maintained pasture is the perfect way to feed an adult horse. After all, horses didn’t evolve with grain and baled hay on the menu. Horses browsed for their meals, and the nutritional content of the diet changed with the season, environmental conditions, and the range they traversed.

However, several things need to be taken into consideration when determining if grass can be the sole dietary component for your own horse.

Amount of Grass Available
From the standpoint of calories only, there has to be enough grass available for your horse to get the energy he needs to thrive. Grass is about 80% water. Compare that to dried hay which contains 10% to 13% water. The horse has to eat about eight times as much grass to get the same nutrition. As a rule of thumb, with irrigated, well-maintained, high-quality pastures, you need to allot at least one acre per horse for mature horses; one and a half to two acres for yearlings to 3 year olds; and two acres for each mare and foal pair. If the pasture begins to look short and thin, it is being overgrazed. Fencing off the available pasture into at least two lots can solve this problem. Rotate the horses between lots every three to seven days.

Nutritional Concerns
Many people worry that their horses will not get all the nutrients they need if they don’t feed something extra. However, grass is actually the best source of vitamins you can imagine, fresh and in their natural state. With the possible exception of vitamin E, no supplemental vitamins should be needed. However, horses that are working regularly with pasture as their main diet may need additional vitamin E to promote optimum health.

Pastures in good condition are also an excellent protein source, although you should double check with your vet or agricultural extension office for such high requirement groups as weanlings and nursing mares, who may need a boost, depending on the forage.

Minerals are another story. Horses on pasture should always have plain salt available to them. Other minerals that may be needed will depend on your soil, growing conditions, the growth stage of the grasses, and also the types of grass in the field. If you are relying on pasture for a significant portion of the horse’s nutrition, it’s always advisable to have the pasture plants analyzed. This is generally done at both early and mature-growth stages. Your state university or agricultural extension agent can provide you with detailed information on how to properly sample the pasture and assist in analyzing the results.

Too Much of a Good Thing
The suitability of good grass as the mainstay of the equine diet is validated by the high-sheen coats of most pasture-fed horses. But that slick, glossy hair can go hand-in-hand with something that’s not so desirable—fat. Early growths of grass, as well as regrowths in the fall, are generally high in simple carbohydrates and calories, and low in nonfermentable fiber. In other words, the horse gets a lot out of what he eats. In fact, a horse can get very fat, very quickly, on good grass.

The Main Course

Watch your horse’s weight. A horse can get fat very quickly on good grass. Put out a salt block. Have pasture plants analyzed by your agricultural extension agent. Be wary of new varieties designed as high sugar or "fattening" feedsfor livestock. Avoid endophyte infested fescue if you’re breeding mares or raising youngsters. Prevent overgrazing by cross-fencing, rotating, and limiting pasture access. Do not turn out insulin-resistant horses on grass, as laminitis can develop from even short-term grazing.

Overindulging on rapidly growing grasses can also easily lead to digestive upset, with bloating and gas, and even diarrhea and colic. The worst case scenario is the effect nutrient-rich grasses can have on horses that are prone to insulin resistance. Such horses can easily become laminitic on low-fiber, high-sugar, and high-starch grasses. Even short-term grazing by insulin-resistant horses can result in laminitis. It’s like a diabetic who eats chocolate and throws his chemistry out of whack. These horses really need to be kept off pasture. Blood testing is the best way to identify insulin-resistant (IR) horses. But there are also clinical signs that should cause you to be suspicious. They include:

• Horses with a history of easy weight gain under conditions where other horses do not become overweight.
• A cresty neck, or lumps of fat, even when the horse’s body weight overall is not high.
• History of pasture-related laminitis.

Are There Safer Varieties?
When it comes to laminitis risk, the growth stage of the grass is more important than the type of grass. Pasture-associated laminitis typically occurs in the spring, although fall regrowths can also be risky. Mature stands of grass in June, July, and August are generally safer. However, weather also comes into play here. Pastures stressed by weather conditions can also be high in sugar. For a horse with uncontrolled insulin resistance, pasture access is Russian roulette and is not recommended.

Laminitis aside, you may want to restrict high sugar/starch/fructan pasture plants if you have mature adult horses that are not breeding or getting much exercise simply because they are likely to get fat on unrestricted grazing. See the table on page 69 for some pros and cons on common pasture species.

If you are planning to establish or reseed your pastures, make your agricultural extension office your first stop. Avoid seeding with strains of grass or clovers developed specifically to promote rapid weight gain in cows, sheep, or other farm animals, or grasses cultivated to be included in silage. These will be high in sugar and/or starch, or fructans. Also be suspicious of grasses billed as highly traffic-resistant, drought-resistant, or cold-resistant for the same reason.