Straw can contribute to colic in horses, but it’s not an automatic death sentence for your horse’s gut. The risk depends on how much straw a horse eats, whether it has adequate water intake, and how the straw fits into the overall diet. Straw is low in digestibility and high in dry fiber, which makes it harder to move through the intestinal tract compared to quality hay or pasture grass. When large amounts build up, they can form a blockage known as an impaction.
Why Straw Can Cause Impaction
A horse’s large colon has natural bottlenecks where its diameter abruptly narrows. The two most common trouble spots are the pelvic flexure (where the colon loops back on itself) and the junction between the right dorsal colon and the transverse colon. Well-hydrated, finely chewed forage passes through these narrowing points without issue. But straw is tougher and drier than most hay, and its long, rigid fibers are harder for the gut to break down and push along.
The large colon’s main job, beyond fermentation, is absorbing water. In an average adult horse, roughly 90 to 135 liters of water move back and forth across the colon wall each day, keeping the food mass moist enough to flow. Straw’s high dry matter content and tough cell walls pull more water into the gut to compensate, but if the horse isn’t drinking enough, that food mass dries out. A dry, fibrous plug at one of those narrow transition points is exactly how an impaction forms.
Horses with poor dental health face additional risk. If worn or missing teeth prevent proper grinding, straw enters the gut in larger, coarser pieces that are even more likely to accumulate. Older horses and those overdue for dental work are especially vulnerable when straw makes up a significant portion of what they’re eating.
Straw Bedding vs. Straw as Feed
Most horses encounter straw as bedding, not as intentional feed. The problem is that many horses nibble their bedding out of boredom or hunger, especially when forage runs out between meals. A horse stabled on straw bedding that also has free-choice hay will usually eat only small amounts of straw. But a horse that finishes its hay ration and has nothing else to do may eat enough straw bedding overnight to create a problem.
Horses that transition from pasture to stall management are at particular risk. Research published in Frontiers in Veterinary Science found that when horses moved from pasture to stalls, their water consumption doubled, total fecal output dropped by about half, and fecal dry matter increased. Those changes in intestinal water balance, combined with decreased motility in parts of the large colon, are implicated as possible causes of colonic impactions. Add straw bedding to that scenario and the risk compounds.
How Much Straw Is Safe
Straw isn’t toxic to horses, and small amounts mixed into a proper forage diet rarely cause problems. The key is proportion. Current nutritional guidelines recommend horses receive at least 1.5 to 2% of their body weight per day in forage on a dry matter basis, and at least 70% of the total diet should be roughage from quality sources like hay, pasture, or chaff. Straw doesn’t provide enough nutrition to count as a meaningful forage source for horses the way it does for donkeys, which have evolved to thrive on low-energy, high-fiber diets. For donkeys, a mix of 70 to 75% barley straw with 25 to 30% moderate-quality grass hay is actually recommended.
For horses, straw works best as a minor supplement or as a tool to slow down fast eaters when mixed with hay. If you’re using straw intentionally in the diet, keep it to a small fraction and make sure the horse’s primary forage is good-quality hay or pasture. Chopped straw mixed into a hay ration is safer than long-stem straw because the shorter pieces are easier to chew and digest.
Signs of Straw-Related Impaction
Impaction colic from straw typically develops gradually rather than all at once. Early signs include intermittent, moderate to severe abdominal pain caused by the gut distending and contracting around the blockage. You might notice your horse looking at its flank, pawing, lying down and getting up repeatedly, or showing a temporary response to pain relief that wears off within hours. Gut sounds are often still present early on, which can be misleading.
As the impaction worsens over 6 to 12 hours, the signs change. Gut sounds diminish, pain becomes more persistent, and the horse stops responding to pain medication. The longer the blockage sits, the more the intestine behind it fills with fluid and gas, creating pressure that can compromise blood supply to the gut wall. Ileal impactions, which occur in the last section of the small intestine, are particularly associated with high-roughage, coarse forage and can escalate to a surgical emergency.
Reducing the Risk
The simplest way to prevent straw-related colic is to make sure your horse always has access to adequate quality forage so it doesn’t fill up on straw out of hunger. Allowing free-choice hay for stalled horses mimics natural grazing behavior and reduces the impulse to eat bedding. If your horse is a determined straw-eater, switching to shavings, paper, or rubber matting eliminates the temptation entirely.
Water access matters just as much as forage quality. Horses eating dry, fibrous feeds need significantly more water to keep gut contents moving. In cold weather, water intake drops because horses are reluctant to drink near-freezing water. Offering lukewarm water or using heated buckets during winter can make a measurable difference in consumption and colic risk.
When changing any part of the diet, including introducing or removing straw, do it gradually over 7 to 14 days. Mixing old and new feeds allows the microbial population in the hindgut to adapt. Sudden dietary shifts destabilize fermentation and fluid balance in the colon, both of which set the stage for impaction. If you’re feeding grain alongside forage, keep individual meals to 1.3 kilograms (about 3 pounds) or less to avoid flooding the large intestine with rapidly fermentable carbohydrates, which disrupts the fluid dynamics that keep things moving.