Considering that grasses are the most natural food sources for a horse, it surprises many people to learn that there is a proper technique for turning a horse out to graze in lush pasture and that problems can occur if you don't do it correctly. If your animal has been eating hay or browsing on sparse pasture all winter, suddenly turning it out onto a very lush pasture can cause colic and laminitis (also called "founder," a painful inflammation of the hooves). Prolonged exposure to too much high carbohydrate grasses can cause metabolic syndrome—a cluster of clinical problems that often include obesity, along with the potentially life-threatening illnesses of diabetes and cardiovascular disease.
The key is a gradual transition. If your horse or pony is grazing in the same pasture from the time the ground thaws in early spring, it will be introduced to the new growth of grass in a slow, natural transition. Many people keep herds of ponies this way without a problem.
Many pony breeds evolved in areas of very sparse pastures, such as mountainous and arid regions, where grass didn't grow thickly. In such environments, ponies spent much of their grazing time searching and navigating rugged terrain, and the constant travel provided considerable exercise to balance the eating. But the thickly grassed pastures common in today's environment make grazing too easy for most ponies, causing them to fill up quickly and remain relatively sedentary, as compared to their wild ancestors. For this reason, you must be very careful to introduce ponies very slowly to grass and watch them to make sure they aren't eating too much, too quickly. Allow ponies just a few minutes of grazing when you first introduce to pasture, gradually building up the time spent there. The full transition can take several weeks until you are satisfied that the ponies can spend all day out in pasture.
It's also possible your ponies will never be able to spend full days in lush pasture. Ponies can gain weight very easily, and you may need to restrict the grazing time to prevent them from gaining weight too quickly to the point of obesity. Founder is very common in ponies, and obesity can lead to problems such as metabolic syndrome and heart disease. To prevent these problems, allow your pony only short periods for lush grazing, with the bulk of their time spent in a much sparser area. Your pony will no doubt try to convince you that it's starving, but you’re the best monitor of its actual condition. Most ponies can thrive on sparse rations and don’t need any supplements or concentrates.
Laminitis and obesity problems can occur in horses, too. Horses put out on lush pasture can suddenly develop problems, such as colic, as well as longer-term problems, like metabolic syndrome and founder. Introduce your horse gradually, starting with a half-hour and adding about 10 minutes every day, until you can leave your horse in the pasture for the entire day. It’s a good idea to feed hay to your horse before turning it out, to prevent it from gorging on lush grass.
Keep in mind that having a completely empty stomach isn’t good either, as this can possibly lead to EGUS (equine gastric ulcer syndrome), which leads to stomach ulcers. A horse’s digestive system is made to digest small amounts of food over long periods of time. So, organize your horse’s feeding routine so that it's neither feast nor famine.
The Bottom Line
It’s tempting to turn your horse out in the spring and watch it enjoy that beautiful green grass. However, doing so may endanger its health, or even foster potentially fatal diseases such as diabetes and heart disease. Err on the side of caution when introducing your horse to pasture. Go slowly and make sure that your horse or pony doesn’t gorge on too much of a good thing.
The Role of Nutrition in Horse Colic and Laminitis. United States Department of Agriculture.
Johnson, Philip J et al. Diabetes, insulin resistance, and metabolic syndrome in horses. Journal of diabetes science and technology vol. 6,3 534-40. 1 May. 2012, doi:10.1177/193229681200600307
Overview of Equine Metabolic Syndrome. Merck Veterinary Manual.
Gastric Ulcers in Horses. Merck Veterinary Manual.