Q. A friend recently recommended that I switch my horse to a different commercial feed because his current one contains corn. She said horses cannot digest corn so to look for a product without it. Is this true?
A. Corn has been fed to horses for decades in a number of forms, often either cracked or steam flaked. Traditionally it’s been a popular feed ingredient because of its easy availability, low cost, and high calorie content (about 1.76 Mcals per pound). Steam-flaked corn was often combined with oats and barley, then coated with molasses to make a feed known as COB, which some owners still feed. Modern corn processing includes extrusion, and it’s sometimes ground so it can be mixed with other ingredients to form pellets.
Today, however, corn is a less common horse feed ingredient for a number of reasons, one being that a lot of corn is diverted away from livestock feed in favor of use for ethanol production. Another is that, overall, horse feeds today typically have lower starch contents than their predecessors. This is because research has shed light on the negative implications of feeding horses too much starch, especially those used in disciplines where high-starch diets are not necessary.
High-starch diets result in an increase in blood glucose levels and, subsequently, insulin. This can have negative health consequences for horses with insulin resistance and other conditions that require a diet low in non-structural carbohydrates (NSC), such as polysaccharide storage myopathy (PSSM). It is also possible that feeding high-starch diets could increase some horses’ risks of developing insulin resistance and equine metabolic syndrome later in life.
Another concern with feeding a high-starch diet is that if the starch is not digested and absorbed before leaving the small intestine (pre-cecal digestion) it will enter the hindgut, where it could disrupt microbial fermentation. This has negative implications for forage utilization, and can increase a horse’s risk of developing colic, acidosis, and potentially laminitis.
These concerns apply to all starch sources traditionally fed to horses. However, because corn contains more starch (about 65%) than oats (about 40%), barley (about 55%), and wheat (about 60%), it’s not seen on horse feed ingredient lists as commonly as it once was.
Additionally, in its natural state, corn starch is not particularly digestible in the equine small intestine—this is likely what your friend was getting at. While total tract digestibility of starches from grains regardless of processing is almost 100%, pre-cecal digestibility is most important. Corn’s chemical structure is also more complex than that found in other grains, and it is part of a complex with protein which makes it harder to digest pre-cecally. As a result, only about 30% of corn starch is digested in the small intestine when whole or crushed corn is fed. Grinding before feeding can increase this figure to 60 to 80%, however this is still lower than the 97 to 99% of ground oat starch being digested pre-cecally.
Mechanically processing corn breaks apart the macro structure of its starch granules and, in some cases, destroys the starch granule structure all together. This results in a larger surface area and greater exposure of the granules to digestive enzymes. Rolling and crushing corn does not achieve much more than the horse chewing the grain and, therefore, does not result in much improvement in starch digestibility.
Rate of passage through the digestive tract also impacts digestibility, with smaller particles moving more quickly than larger ones. The faster the rate of passage, the less time there is for digestive enzymes to work on feed particles and the lower the digestibility. So, there must be a balance between improving starch digestibility by grinding grain and reducing potential digestibility by increasing transit time.
Steam flaking and extrusion can increase corn’s digestibility. The heating involved with these processing methods destroys the grain’s macrostructure as well as the starch granule structure. The water involved in these processes disrupts the starch molecule’s crystal structure, resulting in gelatinization of the starch. Gelatinized starch is more soluble, and its susceptibility to enzyme attack is also increased, which makes it more digestible. The benefit of the larger particle size associated with steam flaked and extruded corn is that it does not increase rate of passage the way grinding does.
Keep Meals Small
One last note: Aside from feed processing, meal size also impacts retention time, with larger meals being emptied from the stomach and particles moving through the small intestine more quickly than smaller meals. Thus, high-starch ingredients should be fed in numerous small meals throughout the day rather than as infrequent large meals.
Some research suggests that starch from ground corn should be kept to no more than 3.5 to 4 grams of starch per kilogram of body weight—so, 1,750 to 2,000 grams (about 3.85 to 4.4 pounds) for a 500-kilogram (1,100-pound) horse—per single meal. The National Research Council recommends keeping starch from any source to no more than 0.2 to 0.4% body weight per day; this amounts to 1,000 to 2,000 grams (about 2.2 to 4.4 pounds) per single meal for a 500-kilogram horse. If you’re using commercial products, you should be able to obtain a feed’s starch content from the manufacturer so you can determine the correct meal size for your horse.
The Bottom Line
Corn’s digestibility, in its native form, is lower than other grains. However, processing with heat and steam and/or grinding can significantly improve its digestibility. As such, most feed manufacturers utilize these processing techniques to improve corn digestibility in their products.
As an owner you can also help improve starch digestibility in your horse’s ration through careful feeding management and a feeding plan that allows for several small meals throughout the day to keep starch intake at or below advised levels.
With these measures in place, corn can be a valuable source of calories for horses in heavy work that need readily available carbohydrates in their diets.