Excessive gas in horses comes from fermentation in the hindgut, where billions of microbes break down fiber and carbohydrates, producing gas as a natural byproduct. The problem starts when something shifts the balance: too much fermentable feed, disrupted gut bacteria, slowed motility, or not enough water. Gas that builds up faster than the horse can pass it leads to bloating, discomfort, and the condition most horse owners know as gas colic.
How the Hindgut Produces Gas
Horses are hindgut fermenters. Unlike cattle, which ferment food in a multi-chambered stomach, horses rely on the cecum and large colon to break down fibrous plant material. Bacteria in this region digest fiber and produce short-chain fatty acids (the horse’s primary energy source), B vitamins, vitamin K, and gas. A certain amount of gas production is completely normal and continuous. Problems arise when fermentation speeds up, the types of bacteria shift, or the gas gets trapped.
High-Sugar and High-Starch Feeds
The single biggest dietary trigger for excessive gas is too much soluble carbohydrate reaching the hindgut. When a horse eats grain, sweet feed, or lush pasture grass, the small intestine can only absorb a limited amount of starch. Anything beyond roughly 4 grams per kilogram of body weight in a single meal overwhelms small intestine digestion and spills into the hindgut, where bacteria ferment it rapidly and produce a surge of gas and acid.
Cool-season pasture grasses are a common culprit because their sugar content (called non-structural carbohydrates) fluctuates throughout the year and peaks in spring and fall. Horses turned out on these pastures during high-sugar periods can take in far more fermentable carbohydrate than their gut is prepared for. Other high-risk feeds include beet pulp, lush clover, and large grain meals.
Research on feeding frequency supports keeping starch intake below 1.1 grams per kilogram of body weight per meal. For a 500-kilogram horse, that means no more than about 550 grams of starch in a single feeding. Dividing the daily grain ration into smaller, more frequent meals mimics natural grazing behavior, slows passage rate through the gut, and reduces the risk of a fermentation spike.
Gut Bacteria Imbalances
The composition of a horse’s hindgut microbiome shifts depending on diet. Horses on forage-only diets have bacterial populations dominated by fiber-fermenting species. When concentrate feeds high in starch enter the picture, starch-digesting bacteria like Lactobacillus and Streptococcus species multiply. These bacteria ferment soluble carbohydrates quickly, producing large quantities of lactic acid and gas as byproducts. The lactic acid drops the pH of the hindgut, which can damage the gut lining and kill off the beneficial fiber-fermenting bacteria, creating a vicious cycle of worsening imbalance.
Abrupt feed changes are especially dangerous because the microbial population hasn’t had time to adapt. Switching from hay to a grain-heavy diet overnight, or suddenly turning a stall-kept horse out onto rich spring pasture, forces the hindgut to process substrates it isn’t equipped for. Gradual transitions over 7 to 14 days give the bacterial community time to adjust.
Slowed Gut Motility
Even normal levels of gas become a problem when the gut isn’t moving it along. Several factors slow intestinal motility in horses:
- Dehydration. Insufficient water intake makes gut contents drier and harder to move. This is especially common in winter when horses drink less, or during travel and competition.
- Coarse or poor-quality forage. Overly stemmy hay, straw, and poorly chewed feed (often from dental problems) can pack into the cecum or large colon, creating impactions that trap gas behind them.
- Limited movement. Stall confinement reduces the natural gut stimulation that comes from walking and grazing. Horses that stand in a stall for long periods are at higher risk for both gas accumulation and impaction.
- Parasite burden. Heavy roundworm infections can physically obstruct the intestine. Deworming a horse with a large parasite load can actually worsen the problem temporarily, as paralyzed worms accumulate in the intestinal lumen and release substances that may inhibit muscular activity in the gut wall.
- Sand ingestion. Horses fed on sandy ground or on sparse pasture may ingest enough sand to accumulate in the large colon, weighing it down and disrupting normal motility.
When Gas Leads to Displacement
The horse’s large colon is loosely attached inside the abdomen, which makes it uniquely vulnerable to shifting position. When gas builds up, the colon essentially becomes a balloon. Because it isn’t firmly adhered to the body wall, a gas-filled colon can float upward and out of its normal position. This is called a displacement, and it ranges from mild (the colon shifts but corrects itself) to serious (the colon twists on itself, cutting off blood supply).
Colonic twists, known as volvulus, are presumed to be caused by disproportionate gas accumulation. They’re seen more frequently in broodmares around foaling, horses on grass diets, and horses eating highly fermentable feeds. Many simple displacements resolve on their own, but a full volvulus is a surgical emergency.
Recognizing Gas Colic
Gas colic is one of the most common forms of colic in horses. In one veterinary study, spasmodic colic (which includes gas-related pain) accounted for 75% of all colic cases presented. The signs are the same general colic behaviors: pawing with a front foot, looking back at the flank, kicking at the belly, lying down and rolling, stretching out as if to urinate, sweating, and reduced manure output. You may also notice visible abdominal bloating.
Heart rate in gas colic is typically only mildly elevated, in the range of 40 to 60 beats per minute (normal resting rate is 28 to 44). Listening to the abdomen with a stethoscope can reveal changes. Increased gurgling or fluid sounds may point to active fermentation, while silence suggests the gut has slowed or stopped moving. Most gas colic cases respond to pain relief alone, and the episode passes as the horse moves the gas through. Severe or prolonged cases, especially with a heart rate above 60 or signs of worsening distress, need veterinary evaluation to rule out displacement or obstruction.
Preventing Gas Buildup
Most excessive gas problems come back to management. The strategies that reduce risk are straightforward but require consistency:
- Feed small, frequent meals. Splitting grain into three or more feedings keeps starch loads per meal low and better mimics continuous grazing. Keeping starch below 1.1 grams per kilogram of body weight per meal is a practical threshold.
- Prioritize forage. Hay or pasture should make up the bulk of the diet. Fiber is fermented more slowly and steadily than starch, producing less gas per unit of time.
- Transition feeds gradually. Any change in hay type, grain amount, or pasture access should happen over at least a week. This applies to seasonal pasture changes too, particularly the spring flush.
- Ensure water access. Horses drink 5 to 10 gallons per day under normal conditions and more in hot weather or heavy work. Heated water buckets in winter can encourage adequate intake.
- Maintain dental health. Horses with sharp points, missing teeth, or other dental issues don’t chew feed thoroughly, leading to larger particles that are harder to ferment and more likely to cause impaction.
- Encourage movement. Turnout is better than stall rest for gut motility. Even hand-walking a stall-kept horse helps keep things moving.
- Manage pasture timing. Non-structural carbohydrate levels in cool-season grasses are highest in the afternoon and during cool, sunny weather. Turning horses out in the early morning, when sugar levels are lowest, can reduce the carbohydrate load.
Alfalfa-heavy diets are worth a specific mention. Alfalfa raises pH in the large colon and increases mineral concentrations, which has been linked to the formation of enteroliths (intestinal stones) in some geographic areas. These stones can partially obstruct the colon and trap gas. If you’re in a region where enteroliths are common, particularly the southwestern United States, limiting alfalfa and mixing in grass hay is a standard recommendation.