Get horse supplement “just right”

Horses may need grain supplement, but feed carefully

Filling out my 1040 made me think that all the line-by-line addition and subtraction on the form is similar to figuring what to feed horses. Ultimately, the ability of a forage to provide nutrients dictates the amount and extent of concentrate you need to give a horse. So in IRS parlance you would “add line forage nutrients to line concentrate nutrients” to get horse requirements. In both cases, knowing what the answer is supposed to be is important.

Usually, forage is the major component of any given horse’s diet. Forages provide fiber, energy, protein, minerals and vitamins, which go a long way to cover the animal’s nutrient requirements. All sorts of good things come along with good forage. Forages are needed to maintain normal microbial function in the horse’s hindgut and to support good fermentation. The end products of such fermentation include volatile fatty acids (which are a principal source of energy for the horse), B vitamins, and, to a limited degree, some microbial protein. However when energy needs are high (say when a horse is rapidly growing, nursing or working hard), grain supplementation becomes important.

Grains have high digestible energy, modest fat, protein and low mineral content. In some horse diets, grains will be the major source of digestible energy. However, if grain is overfed, significant amounts of starch can enter the lower gut and affect fermentation. At that point, bad things can happen. You may see increased lactate production causing lower hindgut pH. When that happens, the proportions of volatile fatty acids will change. You may also see problems such as colic, laminitis, variable growth rate in young horses and hormonal changes.

Many horse owners have limited space. This, at times, results in stabled horses with no access to pasture. That’s a situation that pushes the practice of feeding dry hay twice a day—or providing ad lib hay along with a concentrate. Horses have a relatively high passage rate with a high rate of cecum-colon (“hindgut”) fermentation, so the rapid consumption of highly fermentable carbohydrates such as grain-rich rations is potentially more troublesome that it is for steers. I can feed more pounds of rolled corn to a 1,200-pound steer or 1,200-pound milk cow than I can to a 1,200-pound horse.

The horse has a digestive tract that is designed to ingest large amounts of feed continuously over time. Its stomach is just 10 percent of its digestive tract volume, which allows short retention time. Inside a horse, the digestive strategy is basically “get what is soluble and get rid of the rest.”

This has some advantages—it allows a horse to subsist on a lower quality diet than cattle. In a horse’s digestive tract there is partial chemical-enzymatic digestion and limited bacterial fermentation. That brings some disadvantages, especially in a nutrient-dense environment such as feeding grain supplements. It is easy to overfeed them.

The small intestine of the horse is about 30 percent of its digestive tract volume and is the primary site for digestion.
The fiber fraction of the diet cannot be digested by the horse’s digestive enzymes—it is fermented by bacteria in the hindgut, the largest part of the digestive tract.

In a mature, medium-sized horse, the hindgut might be 18 to 19 gallons of volume. The hindgut also maintains an environment that is suitable for bacterial fermentation of fiber to volatile fatty acids, a significant source of energy for the animal. The horse’s hindgut is similar in its physiological function to the rumen of a cow, but a faster rate of passage through the horse’s hindgut results in shorter time for the undigested feed to be fermented. It’s less efficient than a cow’s rumen. Horses can’t ferment plant fibers as efficiently as cattle because of the reduced numbers and activity of cellulolytic bacteria.

And that’s where it gets a little technical. When we consider forage compared to concentrate feed for horses, we have to consider hindgut health. Horses fed concentrate will have lower hindgut pH from four to six hours after feeding compared to horses just fed hay. But there are times such as gestation, lactation and hard physical work when a horse will require more nutrients than can be attained through forage.  

Grain type and processing method affect the amount of energy available to the horse. Dry processing methods such as crimping, rolling, or cracking increases surface area of the starch granules exposed to digestive enzymes. Wet methods such as steam flaking or steam rolling, extrusion or pelleting result in gelatinization of starch, which substantially increases digestibility.

I usually try to explain it with a grocery store example: Take a bag of popcorn off the shelf and throw it in a bucket of water, the kernels fall to the bottom. But if you pop it first and then put it in the water, it melts.

In horses, most dietary starch, especially from processed cereal grains, is hydrolyzed by the pancreatic and intestinal enzymes to glucose which gets absorbed in the small intestine (popped popcorn melting). The undigested starch fraction then enters the cecum-colon and the bacteria have a chance at it. The grain source will have an effect on digestibility. In the small intestine, raw oat starch is about 90 percent digestible. Corn starch is closer to 70 percent digestible. Dry grinding will improve starch digestibility, but a wet process will improve digestibility even more.

As more starch is fed, the small-intestine digestibility of starch declines. That means even more starch in the hindgut, which can lead to starch overload.
The negative effects of overfeeding concentrate include starch escaping the small intestine and enter the hindgut. Once the starch gets to the hindgut, bacteria that favor starch (Streptococcus and Lactobacillus) start to use it. The reason that Lactobacillus are called lactobacillus is because they are really good at making lactic acid. The resulting spike of lactic acid production will irritate the gut lining, cause a drop in pH and reduce numbers of fiber-degrading (good) bacteria.

These fiber-degrading bacteria are known to favor a pH close to neutral (7.0). It is worth noting that different cereal grains have been shown to vary in lactate production in the horse’s hindgut. Wheat more than corn; corn more than barley; barley more than oats. Additionally a pronounced and quick increase in the starch content of a horse diet can lead to acidosis, which will make the animal more prone to hoof lesions. Under extreme conditions the majority of fiber-digesting bacteria might die off, which has been shown to increase endotoxin and amine release. Symptoms may include: colic, laminitis, post-feeding acidemia, osteochondrosis, excessive gas and altered bowel motility.

At what point does increased grain feeding reduce fiber digestibility? Somewhere between 0.25 to 0.5 percent of body weight as starch.

Dr. Jim White is ruminant nutritionist for MFA Incorporated.