Horses are herbivores, designed from their teeth to their digestive tract to eat plants. Equine teeth are flat and broad and form a crushing surface to grind plant material. About 65% of digestive capacity takes place in the horse’s cecum and large colon, the lower gut of the horse. Beneficial bacterial populations that help to break down fibrous material colonize these portions of the gut. Horses need to consume a minimum of 1% of their body weight in long stem forage per day in order to maintain proper gut function. When horses do not receive enough forage of the correct size, problems with digestibility and nutrient absorption can occur. Horses in light to moderate work need a total of 2-2.5% of their body weight in total feed per day. Ideally, most of that is fed in the form of hay. Recent studies have shown that horses fed high grain, low forage diets colic more frequently, develop gastric ulcers and exhibit behavioral problems such as cribbing. Eliminating the grain and increasing the hay in these horses alleviated behavioral problems in many of these horses. Forage is best consumed as pasture, but when that is not possible due to lack of acreage or seasonal conditions, then good quality hay should be fed.

There are many types of hay that can be safely fed to horses. Geographic conditions usually dictate what types of hay are grown in an area. Some common types of hay include: timothy, alfalfa, orchard grass, coastal Bermuda grass, pangola, brome, clover, and fescue. No matter what type of hay you feed, it is important to get good quality, clean hay. Clean hay means no dust or mold. Hay contains upwards of 19mg/m3 (milligrams of respirable irritants per cubic meter of air). It takes just 0.43mg/m3 for disease to start. Adding dust and mold to hay can cause significant permanent lung damage such as heaves in horses that are continuously fed bad hay.

There are many factors that go into determining the quality of hay. Most horse owners are looking for a green, clean and fresh smelling bale that appears free of weeds, pests (such as blister beetles), and foreign objects. These are all good characteristics to look for, but knowing the hay maturity at harvest has the greatest impact on quality. Hay should be harvested during the vegetative stage or just thereafter. Plants in the vegetative stage do not have visible seedheads (grasses) or flowers (legumes). Plants at this stage have higher protein concentrations. As plants mature, they lose protein and the non-digestible carbohydrate lignin content increases. Lignin decreases the overall digestibility of the hay. Immature hay has a higher neutral detergent fiber content (a measure of cell wall content). A higher NDF makes a hay more readily consumed.

Hay should be analyzed to properly determine its quality. It is important to get a representative sample to be analyzed. Core samples 3/8 inches in diameter and 12-18 inches long should be taken from several (10-20) bales of hay, mixed together, and then a pooled sample if sent in for analysis. There are several methods to accomplish this. Chemical analysis is the most complete method. It can determine the protein, structural and non-structural carbohydrate content, as well as, the trace mineral and vitamin content. The downside to chemical analysis it that it is expensive and has a slow turnaround time. Near-infrared Reflectance Spectroscopy (NIRS) can determine the protein and carbohydrate content, but cannot give the trace mineral content. It is, however, a less expensive and faster way to analyze hay if only a crude analysis is necessary. Contact your local agricultural extension office for exact instructions for submission in your area. The analysis report will include data such as crude protein, calcium, phosphorus, phosphate, acid detergent fiber (lignin and cellulose content), neutral detergent fiber, relative feeding value (how readily the hay will be consumed), and dry matter. Your local extension officer should be able to help you understand your report and offer suggestions for supplementation.

Horse owners are always concerned that they are feeding their horse the right kind of hay. While horses can thrive on just about any type of good quality hay, there are certain types of hay that are more appropriate for different types of horses. Different hays have different nutritional values and different horses have different nutritional requirements. The magic is to match up the two correctly. Horses with lower energy requirements such as healthy retired horses or horses in light work can do very well with late maturity hays. This type of hay has a lower nutritional content and more can be fed without causing the horse to gain weight. Broodmares, weanlings, and horses in heavy work will do better eating early maturity alfalfa and grass hay. Horses with gastric ulcers can benefit from alfalfa in their diet due to the high calcium content that acts as an antacid. The debate continues on whether alfalfa hay is good or bad for horses with laminitis. Keeping the non-structural carbohydrate content low is probably the most important aspect of feeding the laminitic horse. Not all horses can be maintained on a hay only diet and will require some supplementation with grain. All horses should have access to salt and mineral blocks.

Problems with hay

Feeding hay is very safe and the preferred diet for almost all hoses. There are a few problems that can arise and are discussed below.

Blister beetles

  • Blister beetles are members of the genus Epicauta. The beetle if found in the semi-arid regions of the Western United States. They produce cantharidin, which is toxic to people and animals. The adults feed on alfalfa and are often caught up in the hay during the baling process. Dead beetles are just as toxic as living ones. Cantharidin causes irritation of the urinary tract, gastrointestinal tract, and occasionally if fatal. Signs of blister beetle poisoning include blisters in the mouth and on the tongue, diarrhea, bloody diarrhea, blood in the urine, difficulty with urination, and colic. If your horse exhibits any of these signs after eating alfalfa hay, call your veterinarian immediately and save any uneaten hay for inspection.

Fescue Hay

  • Tall fescue hay is often infected by an endophyte fungus called Acremonium coenophialum. Broodmares consuming endophyte-infected tall fescue during late gestation may experience prolonged gestation (as long as 13 to 14 months), dystocia or foaling difficulty, thickened placenta (“red bag” foal) or agalactia (a decrease or absence of milk production), and reduced breeding efficiency following parturition. Foals may be born weak or dead. Endophyte-infected tall fescue apparently causes few adverse effects in non-pregnant horses. It is recommended that pregnant mares be removed from endophyte infected fescue pastures 60-90 days before their foaling date.

Coastal Bermudagrass

  • Coastal bermudagrass is a very fine, soft hay that does not require a lot of chewing on the part of the horse in order to swallow. As a result, long strands of hay are swallowed and can mesh together into a ball that can become lodged in the part of the small intestine (the ileum) that enters into the cecum. These “hay balls” will cause an impaction that blocks the small intestine. Horses will develop pain and exhibit signs of colic. This should be treated as an emergency and your veterinarian should examine and treat your horse. Some cases can be managed medically with analgesics and fluid therapy, but many require surgery to remove the impaction.

Clover slobbers

  • Clover (red, white, and alsike) and occasionally alfalfa, can become infected with the fungus Rhizoctonia leguminicola. This fungus produces the mycotoxin slaframine or slobber factor. Horses who ingest infected clover will have hypersalivation. Although the hypersalivation is harmless to your horse, it can be extremely profuse and annoying to deal with. The toxin persists in hay, although its activity decreases after about 10 months of storage. No treatment in necessary. Remove the feed source of the clover and your horse will stop drooling.

Moldy sweet clover

  • Moldy sweet clover hay can contain high levels of dicoumarol, an anticoagulant, produced by the action of molds on coumerol, a natural component of sweet clover. Dicoumarol ties up Vitamin K and causes the blood not to clot. The toxin is stable in hay for up to 4 years. Animals show signs associated with hemorrhage, such as reluctance to move due to blood in joints, anemia, increased heart rate, and death. Mildly affected animals often respond to treatment with vitamin K and blood transfusions. Veterinary care is critical.


  • Sudangrass/sorghum hybrids (Johnson grass) produce prussic acid only when on pasture. Prussic acid is related to cyanide and inhibits oxygen utilization in the body. Animals will exhibit signs of labored breathing, staggering, collapse and death. Death is usually sudden and can occur in as little as 20 minutes after consumption. Mildly affected animals can be treated with sodium nitrate to bind and inactivate the cyanide. Horses will preferentially avoid Johnson grass in a field as long as other forage is available. The toxin declines greatly in hay during the cutting and curing stage and is seldom a problem in well cured hay.


  • Many types of hay are sprayed with preservatives in order to aid in curing and storage. The most common preservative is propionic acid. Propionic acid has been found to be safe when fed to horses. One study did find when given a choice, horses preferred preservative free hay compared to preserved hay, but would consume all of the preserved hay if it was the only choice.

Hay should make up the bulk of your horse’s diet and that requires a lot of hay. Many horse owners wonder how much hay they should plan to buy. A rough estimate for a horse with no access to pasture is 600-700 lbs per month per horse. This amount is adjusted up or down based on the size of your horse (mini vs draft) and the amount of pasture available. Hay will store well and retain its nutritional value for up to 2 years if kept in a clean, dry area. So if you find good, clean, nutritionally sound hay, feel free to stock up. It will keep and it will keep your horse well fed and healthy.

Registered 2012 by Equestrian Collections
Author: Sallie S. Hyman, VMD, DACVIM, CVA

Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.

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