Most Horses Tolerate Carbohydrates Well

The slogan “hay, oats and water” harkens back to a simpler time when horses performed free of drugs and anabolic steroids. It also is a reminder that traditionally cereal grains have been staple feeds for horses. For generations, work horses, racehorses and show horses have been fueled by rations composed mainly of forage and grains. Cereal grains are high in nonstructural carbohydrates (NSC), including starch and water-soluble sugars that can be enzymatically digested and absorbed as glucose in the small intestine.

Recently, the use of cereals as a major component of horse feed has been questioned by a small but vocal group of zealots who have used the Internet to effectively voice their opinions. Their concern is based on the somewhat flawed premise that feeding even moderate quantities of NSC will lead to metabolic disturbances and disease in clinically normal horses. Nonetheless, this has led many feed manufacturers to drastically alter their horse feed formulation to cater to the “low-carb” craze.

Of course, there are legitimate concerns about feeding excessive NSC to horses. Feeding too much starch in a single meal can overwhelm the small intestine’s digestive capacity, resulting in large quantities of starch escaping to the large intestine, where it is rapidly fermented to volatile fatty acids and lactic acid. Changes in the pH of the hindgut due to alterations in the microbial populations and acid profiles may result in hindgut acidosis. Horses suffering from hindgut acidosis may develop anorexia, colic or display stereotypical behaviors such as wood chewing and stall weaving.

Furthermore, long-term exposure to pH levels of less than 5.8 will begin to have deleterious effects on the epithelial lining of the colonic and cecal walls, which may affect absorptive capacity.

With good feeding management, carbohydrates in grains are well tolerated by most horses. There is, however, a small but important population of horses that do not tolerate carbohydrates well. Knowing how much carbohydrate should be included in a particular horse’s daily ration is key to successful feeding.

Performance horses depend on NSC as a major source of dietary energy. Digestion of NSC results in increases in blood glucose. Under the influence of insulin, blood glucose is taken up by the liver, muscle and adipose tissue, and stored as liver glycogen, muscle glycogen or fat. These substrates are later used as fuels for muscle contraction during exercise.

Kentucky Equine Research (KER) has evaluated the rations of hundreds of sport horses and racehorses competing successfully at the highest level. The vast majority of these horses consume feeds that contain 30-40% NSC, which supplies 35-50% of the concentrate’s digestible energy (DE) content. This is lower than the 44-65% NSC found in straight cereal grain diets since modern performance feeds also derive a significant quantity of DE from fat and fermentable fiber. A typical high-performance ration (forage plus concentrate) contains 18-22% NSC, which provides 28-32% of the ration’s total DE.

At the other end of the equine spectrum are horses that cannot tolerate even moderate amounts of NSC in their rations because of metabolic disorders. The most prevalent of these disorders is equine metabolic syndrome (EMS), which results in insulin resistance (IR) and an increased risk of laminitis.

Horses and ponies with EMS tend to be obese with cresty necks. These animals often have had prior bouts of laminitis and are easy keepers. Management strategies to reduce the incidence and severity of EMS include exercise, weight loss and a ration that contains no more than 10% NSC.

In between the extremes of the elite equine athlete and the obese, laminitic pony lies the majority of horses in the population. These horses are clinically normal, and while many may be older and sedentary, they have not displayed any signs of EMS.

Will a carbohydrate-rich ration make these horses insulin resistant and thus more susceptible to EMS as suggested by the low-carb zealots? Are high-fat, low-carbohydrate diets more suitable?

Testing

To answer these questions, KER recently conducted a study with aged Thoroughbred geldings 21.5 ± 3.32 years old with body condition scores of 5.0-6.0 to determine whether an oat-based ration (20% NSC, with 31% DE from NSC) or a high-fat ration (12.7% fat, with 30% DE from fat) would affect glucose tolerance as compared to an all-grass hay diet (9.4% NSC, with 17% DE from NSC).

Glucose tolerance was measured using an intravenous glucose tolerance test. In this test, a solution of glucose is infused into the horse and blood samples are taken over a six-hour period. The amount of time it takes for blood glucose to return to normal indicates how well the horse’s liver, muscle and fat cells are able to take up and utilize glucose. Horses with impaired glucose tolerance take longer for blood glucose to return to baseline levels.

Each dietary period of the Latin square design study lasted 28 days. During the intravenous glucose tolerance test, it took significantly less time (126.6 ± 25.8 minutes) for blood glucose to return to baseline in the oat-fed horses compared to either the all-hay (198.4 ± 40 minutes) or the high-fat diet horses (216.7 ± 23.5 minutes).

These results suggest that feeding normal, non-obese horses a ration with a significant quantity of its calories coming from NSC is not detrimental to—and may even improve—glucose tolerance.

For obese horses, the level of energy intake is more important than the source of calories. Obesity and lack of exercise are the two predisposing factors for insulin resistance and EMS. The main goal of feeding an obese horse is to reduce its caloric intake below its caloric requirement so that it will burn body fat and lose weight.

Concentrates containing more fiber and less fat and NSC are not as calorically dense and can be used to deliver fewer calories without drastically reducing feed intake. Feeding hay and a low-intake balancer pellet is also a good alternative for obese horses.

As a management tool for obesity, increasing exercise is just as important as lowering caloric intake since exercise increases caloric expenditure and has been shown to improve insulin sensitivity, even in overweight horses.

In conclusion, horses with specific metabolic issues certainly can benefit from low-NSC (less than 10%) diets, but these types of “super low-carb” diets are not necessary for normal sedentary or exercised horses. Concentrates that contain 20-30% NSC and that produce a lower glycemic response than higher-NSC performance feeds may be useful for sedentary and lightly exercised horses and for horses that tend to become more excitable on higher glycemic feeds. Heavily exercised performance horses need NSC in their diets, and cereal grains and grain by-products remain important and cost-effective sources.

To paraphrase Mark Twain: The news of grain’s demise in horse feed has been greatly exaggerated.