*Dr. Joe D. Pagan is president and founder of Kentucky Equine Research Inc., which, through consultation and research, aims to bridge the gap that may exist between basic research and horse production.
OVER the past 30 years, there has been a dramatic shift in the way performance horse feeds are formulated, and the greatest change has occurred in the sources of dietary energy used.
In North America, dietary energy for horses is usually expressed in terms of digestible energy (DE), which refers to the amount of energy in the diet that is absorbed by the horse. DE requirements are calculated based on the horse's maintenance DE requirement plus the additional energy expended during exercise.
Basically, DE can be provided by four different dietary energy sources: non-structural carbohydrates (NSC), fat, protein and fiber.
The quantity of each of these energy sources in equine feed can be expressed either as a concentration or as its relative contribution to the total DE content. For instance, a feed may contain 35% NSC, which provides 42% of the feed's DE content.
When horses were first domesticated thousands of years ago, their rations were supplemented with high-NSC cereal grains and byproducts to provide the additional dietary energy required for work. These feeding practices continued well into the 20th century.
A typical "sweet feed" for horses in the 1970s consisted mainly of cereal grains mixed with molasses. These feeds derived 60-65% of their DE content from NSC.
In the 1980s, several research groups began studying the effect of adding fat to horse feed. On the surface, the idea of feeding fat to horses seemed illogical since horses evolved eating very low-fat diets and they do not possess gall bladders to assist in fat digestion. Surprisingly, early research showed that horses digested fat well and were able to utilize fat as an energy source for exercise.
A number of feed manufacturers began offering horses feeds containing up to 10% fat. In most of these mixes, fat was added as a replacement for grain, which lowered the contribution of DE from NSC to less than 50% and increased the DE contribution from fat from 10% to 20%. Since fat contains 2.5-3.0 times as much DE as grain, the total energy densities of these fat-supplemented feeds increased by around 15%. Therefore, they required higher levels of fortification to prevent a dilution of nutrient:calorie ratios.
At around the same time, other alternative energy sources were introduced into horse feeds. In particular, the use of sugar beet pulp became popular as an ingredient for performance horse feeds. Beet pulp had previously been used as a fiber source in complete horse feeds, but its unique fiber characteristics had not yet been fully appreciated.
Beet pulp is quite high in total dietary fiber, containing around 38% neutral detergent fiber and 30% soluble fiber. Digestibility trials at Kentucky Equine Research (KER) determined that these fiber fractions are highly digestible, resulting in beet pulp having a DE content nearly as high as oats, with 83% of its DE coming from fiber fermentation.
Combining 10-20% beet pulp and 6-8% fat with cereal grains resulted in an energy density that was similar to a traditional sweet feed but with less than 45% of the DE from NSC. These types of mixes continue to be popular today as high-performance horse feeds.
In the 1990s, Dr. Stephanie Valberg at the University of Minnesota began studying recurrent exertional rhabdomyolysis (RER), or "tying-up," a muscle disorder that occurs commonly in nervous fillies of Arabian, Standardbred and Thoroughbred breeding. These individuals often develop RER when they are excited or stressed and/or when a period of stall rest precedes exercise.
Preliminary genetic research and breeding trials point to this condition as an inherited trait in Thoroughbred horses.
RER is caused by an abnormality in the way muscle contraction is regulated in the horse. Muscle biopsies from horses with RER have revealed an increased sensitivity to contraction when exposed to various chemicals compared to normal horses. The altered contraction and relaxation of muscle suggests that abnormal intracellular calcium regulation is the cause of RER.
Research conducted at the University of Minnesota, in conjunction with KER, evaluated how dietary energy source affect horses suffering from RER. This research demonstrated that replacing the grain in the diet with a low-NSC (less than 10% DE)/high-fat feed (30% DE) significantly decreased the amount of muscle damage in RER horses.
In a feeding trial, Thoroughbred fillies with RER were exercised on a treadmill for five days a week while they consumed hay and a variety of energy supplements. When the horses consumed a high daily caloric intake of a high-NSC ration, they showed symptoms of tying-up, including high post-exercise serum creatine kinase (CK) levels. In contrast, if extra calories were provided from a low-NSC/high-fat feed, no increase in post-exercise serum CK activity occurred.
The reason for this improvement appeared to center on a change in temperament. When the fillies were fed the low-NSC/high-fat feed, they had a calmer demeanor, and their heart rates were lower before and during exercise. Subsequent research at KER showed that the glycemic response of a feed affected the behavior of certain horses.
Since behavior and tractability are key components to successful competition in a wide range of equestrian disciplines, feed manufacturers began formulating performance feeds with lower and lower NSC contents. To maintain a constant energy density, high levels of fat and fermentable fiber have been substituted for NSC so that many modern performance feeds derive less than 20% of DE from NSC.
Today, low-NSC/high-fat feeds that were developed for horses suffering from RER have morphed into widespread use for healthy performance horses. These feeds work well for a wide range of horses, but they aren't appropriate for every horse. Horses depend on muscle glycogen as a primary substrate for energy generation. The amount of muscle glycogen used is related to exercise intensity and duration.
KER recently conducted an exercise trial where horses were fed rations providing high, medium or low levels of NSC. The horses completed a series of exercise tests that depleted around 30% of their muscle glycogen stores. Muscle glycogen repletion was then measured over a three-day period. When the horses were fed high- or medium-NSC feeds, they completely repleted their muscle glycogen stores, but when fed the low-NSC feed (15% DE), muscle glycogen repletion was incomplete.
Choosing the correct formulation for a specific performance horse will depend on the discipline and individual temperament of the horse.
In disciplines such as dressage, where a calm and consistent temperament is essential, lower-NSC/higher-fat feeds are preferable.
On the other hand, more-intense activities such as Thoroughbred and Standardbred racing require higher levels of NSC to replenish muscle glycogen.