Excerpt from Revised Understanding Equine Nutrition: Energy and Carbs
by: Karen Briggs
June 04 2008, Article # 12004
If forages provide the "maintenance" energy horses need for the workings of everyday life--grazing, sleeping, wandering from pasture to pasture, maintaining internal temperature--then cereal grains are the turbo-charged portion of the diet. Their main function is to provide higher concentrations of energy, in the form of carbohydrates and starches, so that the horse can do the work we ask of him.
The amount of energy your horse needs rises in direct proportion to how fast, how long, and how hard you expect him to perform. At the lowest end of the spectrum are horses that are idle, or perhaps work only a few times a week at a very slow pace. Most pleasure horses and school horses fall into this category. At the opposite end are racehorses, which probably work harder than any other category of equine athlete (particularly because they're often asked for peak performance while they're still physically immature). Somewhere in between might be your equine athlete--whether he's a Western pleasure horse, a Grand Prix jumper, a polo pony, or one of a four-in-hand driving team. His energy requirements will more than likely not be completely met by hay or pasture alone.
Work isn't the only thing that can raise a horse's energy requirements above the maintenance level. Environmental conditions, his physical fitness, and his degree of fatigue all play roles. Even when all of these factors are identical, individuals can vary in their energy needs. We all know of high-strung horses that are "hard keepers" and their metabolic opposites, the easy-going types that maintain weight easily, even in hard work. Both breed type and temperament play roles here.
Pregnancy also places increased energy demands on the mare, especially in the latter half of gestation, when the fetus is developing most rapidly. Lactation and growth are two other situations in which energy needs are higher than usual.
Even a horse's size can have something to do with energy requirements. Studies have indicated that the energy requirement of horses at rest is proportional to the horse's bodyweight--so in theory, the energy requirement of a 500-pound pony is about half that of a 1,000-pound horse.
Unlocking the Energy
Carbohydrates and starches, contained in grains, are the most convenient ways to provide extra energy to your horse. A carbohydrate molecule is composed of simple sugars (also called monosaccharides) such as fructose, glucose, galactose, mannose, arabinose, and xylose. There are also disaccharides, which are two sugars bonded together. Lactose, made up of one glucose and one galactose molecule, is an important one for foals of nursing age.
Many glucose molecules, attached together by "alpha bonds," form the polysaccharides called starch (present in plants), and glycogen (present in animals). These two are sometimes called soluble or non-fiber carbohydrates, and both are readily used by the horse, providing much of his dietary energy. But other forms of carbohydrates contribute a substantial amount of "juice" as well. As we saw in the previous chapter, glucose molecules that are attached together by "beta bonds" instead of alpha bonds, form the polysaccharide cellulose (insoluble fiber). Likewise, hemicellulose is constructed of many molecules of the monosaccharide xylose, connected by beta bonds. So while we consider fiber and carbohydrates to be two entirely different things, they are really very closely related.
Monosaccharides are the only form of carbohydrate that can be absorbed from the intestinal tract, so the alpha or beta bonds of polysaccharides must be broken down in the gut before the horse can begin to use (or store) the simple sugars. The digestive enzyme amylase is responsible for this important job. All animals secrete amylase, primarily from the pancreas, into the small intestine. Amylase takes care of the first step of carbohydrate digestion, breaking the polysaccharide molecules down into a disaccharide (a two-sugar molecule) called maltose. After that, the disaccharide enzyme maltase takes over and further breaks down maltose into its monosaccharide components. Two other digestive enzymes, lactase and sucrase, also might be called into play if lactose (milk sugar) or sucrose (table sugar) is present in the gut. (Lactase is usually present only in young, nursing horses and later becomes scarce enough that adult horses have difficulty digesting milk products and usually end up with diarrhea.) Because these enzymes emanate from the interior intestinal wall, any damage to that area (as from enteritis, for example) results in impaired carbohydrate utilization. Large amounts of carbohydrates can remain in the gut, again causing diarrhea.
The simple sugars that pass through the intestinal wall are almost immediately available for energy use. Often, however, the energy isn't needed right at that moment, so the body busily begins re-assembling the sugars in the form of glycogen so they can be stored. Storage depots in the liver and muscles (and to a lesser extent, the kidneys) give the horse a substantial energy warehouse, and if these storage areas become full, any extra monosaccharides are then converted to and stored as fat. Both glycogen and fat can be drawn on for energy whenever they're needed (read more on fats). The hormone insulin acts as a glucose regulator in the bloodstream, determining how much sugar remains there and how much gets stored.