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Fructans—the sugars produced by cool-season grasses—are indigestible in the horse’s stomach and small intestine, so these nonstructural carbohydrates (NSC) end up in the hindgut, where they can disrupt the rate of fiber fermentation. This disruption can skew the pH level, unbalance microbial populations, and put horses at an increased risk for conditions like colic, laminitis, and hindgut acidosis.

Some horses are rarely bothered by this phenomenon, happily munching pasture forage throughout the day with no ill effects. Other equines quickly develop problems, some with the potential to become quite serious, if they are allowed free access to grazing.

Kathleen Crandell, Ph.D., senior nutritionist at Kentucky Equine Research, explained that while very sensitive horses might not be able to graze fresh pasture at all, many horses can graze for at least a few hours each day as long as owners watch the clock.

“NSC levels are lowest from midnight until about 10:00 in the morning,” she said. “The level increases as the day goes on, to where a horse grazing in the afternoon can get two to four times as much energy from the grass as when it grazes in the early morning.”

Mowing the pasture grass doesn’t eliminate the problem, Crandell said, because the fructans are stored in the lower part of the stem just above the ground level.

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