All members of the equine family have evolved to extract their necessary nutrition from grasses. Horses and donkeys especially are able to extract nutrients from coarse grasses and plants that might not support a larger horse. Their teeth are suited to grinding the coarse plant fiber, and the long, slow digestive system efficiently extracts nutrients and energy from the plants it eats.
Some horses are easy keepers and are able to live quite well on minimal pastures. Others will be starved for nutrition. A horse that is working very hard may not get enough nutrition. It really depends on the type of horse you have and what its job is as to whether or not it will thrive on a diet of grass, especially one that isn’t top-notch.
Grass and Nutrients
On the best quality pasture, horses should be able to get all the nutrients they need. This, after all, is what they naturally eat in the wild. Unfortunately, very few owners, through no fault of their own, have top quality pastures. Overgrazing, drought, freezing, poor management, poor soil, and snow cover all affect the quality of the grass and the horse’s ability to extract adequate nutrition. Some of these conditions can be compensated for with good pasture management and soil amendments.
Even with good care, a perfect growing season, and excellent soil, most horses will require supplementation with minerals, fodder, or concentrates for at least part of the year. Early spring, winter, and fall may slow grass growth and make the grass less than optimum for your horse. Be prepared to supplement with hay and a multi-vitamin/mineral supplement. Watch your horse’s condition and for signs of weight loss. Again, each horse is an individual, so what works for one might not be suitable for another.
Avoid overgrazing by limiting the number of horses on pasture and by rotating pastures, so the grass has time to recover after grazing. Horses will crop grass down to the soil level, which means in hot, dry conditions, grass may burn and dry out before it has a chance to grow. A lot of horses kept in a small area means the soil will become compacted, and that makes it very difficult for anything but the most aggressive weeds to grow.
Although a weedy pasture may look green, it doesn’t provide a lot of nutrition for your horse, and some may even be toxic if eaten in quantity.
In some areas, minerals or other nutrients might be depleted from the soil, leaving a deficit in the nutritional value to the horse. Selenium, in particular, is of concern to horse owners. This mineral is scarce in many areas of North America, and that means horses won’t be getting enough from grass or hay. A balanced supplement is ideal for replacing this essential mineral. Selenium is an important antioxidant and a deficiency can cause something called white muscle disease. If your horse is prone to tying-up, which is a painful stiffness of the muscles after work, a lack of selenium may be part of the problem.
Ponies, donkeys, and some mules may need to be restricted from eating too much fresh grass as they metabolize their food much more efficiently. Overeating can easily cause obesity and health problems like founder, which is a painful inflammation within the hooves. Even horses that do quite well on rich pasture may need a period of adjustment if they’ve been eating hay, say after a long winter. A quick change can cause colic and other health problems.
The simple answer is yes. A pasture can potentially be the sole source of nutrition for a horse. Given the variability of a horse’s own metabolism and needs, though, pasture alone may not be sufficient for your horse. This is why keeping a careful watch over your horse’s condition is essential.
The Basics of Equine Nutrition. Rutgers University
Plants Toxic to Horses. Pennsylvania State University, College of Agricultural Sciences
Selenium in the Equine Diet. American Association of Equine Practitioners
Laminitis: Prevention and Treatment. American Association of Equine Practitioners
Colic In Your Horse. University of Minnesota