Horses eat hay, not straw. Hay is the foundation of a horse’s diet and provides the calories, protein, vitamins, and minerals they need. Straw is the leftover stalks from grain crops like barley or wheat, and it contains almost no nutrition. Horses will chew on straw if it’s available, but it can’t replace hay as a feed source.
Why Hay Is a Horse’s Primary Forage
Horses are hindgut fermenters, meaning bacteria, fungi, and protozoa in the cecum and colon break down plant fiber into volatile fatty acids the horse uses for energy. Hay contains cellulose and hemicellulose that these microbes can actually digest, along with sugars and protein. Straw is mostly lignin, a rigid structural fiber that passes through the gut largely undigested. It fills the stomach without feeding the horse.
A horse should eat roughly 2% of its body weight in forage per day. For a 1,000-pound horse, that’s about 20 pounds of hay daily. The absolute minimum is 1% of body weight, a threshold typically reserved for horses in heavy work that get a significant portion of their calories from grain. Dropping below that minimum risks digestive problems and behavioral issues tied to insufficient chewing time.
Grass Hay vs. Legume Hay
The two broad categories of hay are grass hay and legume hay, and they serve different purposes. Grass hays like timothy and orchard grass average around 10% protein and provide a moderate, steady energy supply suitable for most adult horses at maintenance. Legume hays like alfalfa pack nearly double the protein (close to 20%) and two to three times the calcium of grass hay. That makes alfalfa useful for growing foals, lactating mares, and horses in heavy work, but it can be too calorie-dense for easy keepers or lightly worked horses.
The calcium-to-phosphorus ratio also differs. Grass hay sits at roughly 2.3:1, while alfalfa runs closer to 5:1. Horses need more calcium than phosphorus, so both types fall within a safe range, but feeding alfalfa as the sole forage can push calcium intake high enough to warrant balancing with other feeds.
When Straw Has a Role
Straw isn’t horse feed in the traditional sense, but it does have a specific, practical use: helping overweight horses lose weight without taking away their ability to chew. Horses are designed to spend 12 to 16 hours a day eating, and simply cutting hay rations can lead to stress, ulcers, and wood chewing. Replacing part of the hay ration with straw lets a horse keep chewing while consuming fewer calories.
A study from Scotland tracked 40 ponies over four winter months. One group received a normal hay ration, while the other got a 50/50 mix of hay and unchopped barley straw. Every pony in the straw group lost weight, dropping an average of about 27 kilograms (roughly 5% of body weight). The hay-only group actually gained weight on average. No episodes of colic or laminitis occurred in either group during the study.
If straw is introduced, it needs to happen gradually over at least two weeks. Abrupt feed changes are a well-documented trigger for colic. The hindgut microbial population needs time to adjust to any new forage source, and straw’s high lignin content makes that transition especially important.
Risks of Feeding Straw Alone
A horse living on straw as its only roughage faces real nutritional deficits. Straw lacks meaningful protein, calcium, phosphorus, and vitamins. Without supplementation from a quality concentrate feed, horses on straw-only diets will lose muscle, develop mineral imbalances, and decline in condition.
There are digestive risks too. Research has linked straw as the sole roughage to an increased risk of gastric ulcers, possibly because straw’s low calcium and protein content reduces the stomach’s natural buffering capacity. Some individual horses are also more prone to impaction colic when consuming large amounts of indigestible fiber, since the dense, dry material can compact in the gut. Certain breeds may carry higher susceptibility to this, though the research hasn’t pinpointed which ones definitively.
How to Judge Hay and Straw Quality
Whether you’re buying hay or using straw as a partial forage supplement, quality matters. Both hay and straw need a dry matter content of at least 86% to store safely. Below that threshold, moisture creates conditions for mold growth. The most concerning mold types in horse forage are Aspergillus and Penicillium species, which have been detected at elevated levels in forage linked to respiratory problems and coughing in horses.
Good hay should smell fresh and slightly sweet, feel dry and pliable, and show a green to light-gold color depending on the type. Dusty, musty, or dark-colored hay is a warning sign. Straw should be golden, dry, and free of visible mold or dark patches. The maturity of the plant at harvest directly affects nutritional value: the older the plant when cut, the more its digestible fiber has converted to indigestible lignin, and the less energy it provides. This is exactly why straw, harvested after the grain is already removed, has almost nothing left nutritionally.
Straw Bedding and Accidental Eating
Many horse owners use straw as bedding rather than feed, which raises the question of whether horses will eat their beds. They will. Horses are natural grazers, and if they’re hungry or bored, they’ll pick through straw bedding and consume it. For most horses this is harmless in small amounts, but for horses prone to colic or those on carefully managed diets, it can be a problem. Switching to shavings or rubber matting eliminates the issue. If you want to keep straw bedding, monitor how much your horse is actually eating and factor it into their overall forage intake.