Pasture grass is a major source of sugar in the horse’s diet, but carbohydrate content varies with soil type, climate, hours of sunlight, species, season, and time of day. Access to pasture should be carefully restricted for horses with insulin resistance.
Species lower in sugars that allow for longer grazing times include timothy, brome orchardgrass, bermudagrass, and teff. Avoid ryegrasses, meadow fescue, and cereal hay, such as wheat and oats. Alfalfa is a good source of both protein and less-digestible fiber and can meet or exceed horses’ calcium and protein requirements.
Cool season grasses accumulate more starches, sugars, and fructans and are higher in total energy content. Although alfalfa hay is higher in total energy content than grass hays, most of the energy is from protein and fiber.
Avoid grazing any pasture grasses that are growing rapidly or going through a dynamic phase – like after a summer rain, or entering dormancy in late fall. Horses should be limited to 2 percent of their body weight in feed.
Hay should be fed before animals are released onto pasture – one good scenario is to feed hay all night before turning animals out early in the morning, when sugar levels are lower. Given a choice, horses will consume lush pasture grasses before hay every time. Soaking hay in cold water for 30 minutes lowers the sugar content.
Keep horses off weedy pastures, since weeds can have a greater concentration of iron than forage grasses, potentially leading to elevated insulin and can be higher in sugar. In spring, when grass is lush and growing rapidly, horses will want to consume more. Muzzles can also be used to limit intake.
Frost can greatly increase fructans in the plants within a few hours. And, hay can run the risk of higher sugar content if it is cut in bright sun.