Nutritional and Metabolic Concerns with Equine Forages

While many of us want to simplify nutrition for our horses, selecting the best forage for a horse isn't always simple, particularly if he is chronically laminitic and/or metabolically challenged. Diets for these horses require particular care; however, in order to optimize the diet, first you must understand the horse's need for forages.

The first "Equine Forage: Risks and Rewards Seminar and Summit," held Oct. 22-23 at Rutgers University, offered seminars for owners and veterinarians, open discussions, and hands-on labs. Three presenters focused on carbohydrates in forages, with an eye toward better managing hay and pasture to meet the needs of normal and problem horses.

Sarah Ralston, VMD, PhD, Dipl. ACVN, associate professor in the Department of Animal Science at Cook College, Rutgers University, began the conference by discussing the horse's natural diet, feeding patterns, and digestive physiology.

"We all know forage is important for horses," she began, "but until recently, we took it as a given. Many, especially here in the East, however, rely more on grain for nutrients and view hay as just something for horses to chew on. But if we could rely more on our hay and pastures, we could meet our horses' needs more cheaply, and they would be happier, healthier animals."

She briefly described the horse's evolution into an almost continuously grazing animal consuming a high-fiber, moderate-protein, and low-starch and -fat diet, noting that the average adult horse can meet his nutritional needs on good quality hay and pasture alone, with only water and salt needed as supplements . In other words, the average horse doesn't need a rich diet with lots of starch, sugar, and/or fat to meet his needs, and his rather unique digestive system keeps him from efficiently processing single meals larger than 0.4% of his body weight (four to five pounds for the average horse).

Ralston explained the role of hay and pasture in gastrointestinal health and function, noting that low forage/high concentrate rations are associated with a higher risk of the following conditions:

  • Colic/gastric ulcers (especially associated with extreme exercise and limited forage intakes);
  • Gastrointestinal acidosis--the fermentation of starches and grains generates more acids in the large intestine than the higher fiber forages;
  • Metabolic  acidemia (low blood pH): A meal of grain can cause significant decreases in blood pH, whereas meals of hay tend to cause a slight increase;
  • Laminitis; and
  • Rhabdomyolysis (tying-up).

"Horses just do best with free access to forages," she stated. "Contrary to popular belief, most horses do not need grain. If the hay is poor-quality or the horse is in strenuous training or has exceptional needs, you'll need to supplement higher energy concentrates, but on the whole, the more forage you can get into your horse, the better off he is."

The potential problems with forages, however, include:

  • Inadequate energy/protein to meet the horse's needs (i.e., with poor-quality or rained-on hay).  Hard working horses, broodmares in late gestation/early lactation and young foals often need more energy/protein than they can consume in more forages and should be supplemented with appropriately balanced concentrates.
  • Minerals that are inadequate or imbalanced (she cited one instance of finding grass at the top of a hill with a reverse calcium:phosphorus ratio compared to that at the bottom, because all the fertilizer ran down to the bottom of the hill);
  • Toxins or contaminants (from weeds, fungus, mold, and/or blister beetles); and 
  • High soluble sugar content (saccharides and fructans), though these are primarily a concern only for horses that are especially  prone to laminitis.

The soluble sugar content in grasses has always been considered to be low, she said, but many new grass varieties contain 22% sugar or more, which can be a problem in horses that are insulin resistant or that suffer chronic bouts of laminitis. Also, forage stressed by drought or cold will have more sugars than unstressed forage.

"What do soluble sugars do in horses and why are we worried about it?" Ralston asked the audience. She discussed the dynamics of glycemic response (rise in blood sugar following a meal) and insulin resistance, and noted that high-sugar/starch diets have been ireported  to worsen or cause clinical signs of, laminitis. Hyperinsulinemia (abnormally high blood insulin responses to high starch meals or standardized oral glucose tolerance tests) in weanlings has also been correlated with an increased risk of osteochondritis dissecans (OCD), a defect in the mineralization of growth plate cartilage. Insulin resistance, an abnormally high insulin response to a standardized glucose challenge, is also common in obese horses, perhaps contributing to their increased susceptibility to founder.

"Unfortunately, we don't know what's normal (in terms of insulin sensitivity) for animals at different times, or know what degree of individual variation to expect," she said. "But in general, sweet feeds have a high glycemic response." A similar response  might be expected  with higher-sugar forages. However, her research on feeding 22% soluble sugar hay to both weanling and adult horses resulted in glucose/insulin responses, that, while significantly higher than when 6% or 12% soluble sugar hay was fed, were not even close to the responses usually seen with a common meal of grain.

"It's not the insulin itself that's a problem, it's the effects on the body, which are more than just regulating blood sugar," she explained. "Insulin also affects growth and thyroid hormones and enhances the deposition of body fat. These actions not only can contribute to obesity but also abnormal glucose metabolism and possibility bone growth However, both the young and old horses appeared to adapt to the higher-sugar hay and after three months had glycemic responses that did not differ from those in matched horses that were fed the lower-sugar hay..

Several other factors  can affect a horse's insulin sensitivity, she added. These include:

  • Age (older horses might have decreased insulin sensitivity, or increased resistance due to pituitary or thyroid dysfunction.;
  • Stress/time of day (the "stress hormone" cortisol decreases insulin sensitivity, and is normally higher in the early morning than in the evenings);
  • Exercise (moderate exercise increases insulin sensitivity regardless of ration or age in humans, rats, and horses); and 
  • Possibly genetics.

If you have an older horse, Ralston warned that many senior feeds are very high in molasses so they'll be eaten "with relish," but a metabolically challenged horse definitely doesn't need that extra sugar. And if you don't have time to exercise your horse more, she suggested chasing him around the pasture for a bit or distributing his hay around the pasture to make him work for it.

In sum, Ralston offered the following recommendations to optimize insulin sensitivity:

  • Avoid feeding large meals of low-fiber/high-starch and -sugar concentrates;
  • Added fat (7-12% in concentrates) might be beneficial to replace sugar calories;
  • Don't fear high-sugar forages for young horses, as they are more of a problem in older, Cushingoid adults; and 
  • Reduce stress and maximize a horse's opportunity to exercise.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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