Understanding Digestion in the Horse: Comparative ApproachBy Kentucky Equine Research Staff · October 28, 2007
Have you ever considered the gastrointestinal tract of the mouse-chasing tabby that lurks around your stable? Ever thought about how market hogs derive energy from their diets of corn and soybeans? Does a three-ton elephant really have anything in common with your horse, digestive or otherwise?
In today's highly specialized world, animal nutritionists tend to consider themselves specialists in either ruminant or nonruminant nutrition. Ruminant nutritionists appreciate the significance fermentation plays in meeting the nutritional components of animals with multiple stomachs, but they tend to think of most nonruminants as “simple-gutted.” Nonruminant nutritionists, conversely, often dismiss the importance of microbial fermentation to the health and well-being of the animal. These are critical oversights, because fermentation plays a key role in the nutritional ecology of almost every species of animal including horses.
Therefore, a basic understanding of the function fermentation plays in a wide range of species is critical when considering its importance in the horse. An appreciation of the continuity of microbial fermentation across several species allows information gathered in one species to be used for the benefit of other species.
Digestion of Carbohydrates
In order to understand the significance of fermentation in animal digestion, a brief explanation of carbohydrate digestion is needed. The digestion of starch occurs primarily though the work of enzymes in the small intestine. In terms of equine feed management, sources of starch are usually cereal grains such as oats, barley, and corn. The final product of starch digestion is chiefly glucose.
Though most starch digestion occurs in the small intestine through enzymatic action, minimal fermentation of starch occurs in the stomach and portions of the large intestine (the cecum and the colon). The end products of starch fermentation in the large intestine are volatile fatty acids (VFAs) and lactic acid.
In contrast to starch, plant fiber is digested entirely by fermentation, which results in the production of VFAs. Fermentation of plant fiber occurs in the hindgut of the horse. Not all animals are anatomically similar to horses. Others possess distinctive digestive tracts that determine where fermentation takes place.
Four Basic Digestive Systems
Animals can be divided into four basic groups according to the structure of their gastrointestinal anatomy and its ability to ferment feedstuffs. First, animals can be classified as either pregastric fermenters or hindgut fermenters based on the primary location of microbial fermentation in relation to the stomach.
Pregastric fermenters are then subdivided into ruminants or nonruminants. Common ruminants are cattle, sheep, goats, deer, antelope, and camels. These animals have highly developed digestive tracts that use fermentation to degrade feedstuffs.
Large, multicompartment stomachs selectively sort and retain plant fiber for extended periods of time. Digesta then moves to the animal's “true stomach,” hence the adjective “pregastric.”
Nonruminants in this category include hamsters, voles, kangaroos, and hippopotamuses.
Hindgut fermenters are also split into two classifications according to whether they depend primarily on the cecum or colon for microbial digestion.
Cecal fermenters include rabbits, guinea pigs, chinchillas, and rats.
Large nonruminant herbivores such as horses, rhinoceroses, gorillas, and elephants depend more on the colon for microbial fermentation. Omnivores such as pigs and man have sacculated colons where a good deal of digestion takes place. Carnivores such as cats and dogs have little or no cecal capacity and an unsacculated colon.
Adaptations for Microbial Fermentation
In order for microbial fermentation to be useful, animals must have digestive systems that can retain digesta and microorganisms for a long period of time while simultaneously maintaining an environment suitable for fermentation of plant material. The degree in which a particular species is able to use fermentation will depend primarily on three factors: (1) the total volume available for fermentation in the digestive tract, (2) the retention time of ingested material, and (3) the makeup of the microbial population inhabiting the hindgut.
Volume available for fermentation The importance of microbial fermentation as a means of digestion in various species can be demonstrated by the proportion of the digestive tract devoted to fermentation. Ruminants typically allocate the largest proportion of their digestive tracts to fermentation. Nearly 75% of the bovine digestive tract, for instance, is suitable for supporting microbial fermentation. The vast majority of this fermentation capacity is in the reticulum and rumen, two compartments of the stomach. Nonruminant herbivores such as horses tend to dedicate a smaller proportion of their total digestive capacity to fermentation. Both ruminant and nonruminant grazers such as horses and cows usually have more developed digestive tracts than selective herbivores like rabbits and hamsters.
Omnivores vary greatly in their fermentation capacity. For example, pigs have a voluminous hindgut accounting for about 48% of their total digestive capacity, but humans devote only approximately 17% of their tracts to microbial fermentation. As mentioned previously, carnivores usually have unsacculated colons that represent a small proportion of total digestive capacity.
Retention time The extent to which plant material is fermented depends on how long it is in contact with the microbes. Longer retention results in more complete digestion, but there is a limit to the total amount of time the material can be subjected to fermentation before energy production becomes compromised. Herbivores such as horses depend to a large degree on volatile fatty acids (VFAs) as a source of dietary energy. These VFAs are by-products of microbial fermentation. If digesta is retained too long in the fermentive organs, VFAs will be degraded by certain anaerobic microorganisms, thus depriving horses of energy.
As ruminants become larger, mean retention time increases. Buffaloes, which at maturity have a body weight of about 2200 pounds (1000 kg), have retention times of between 90 and 100 hours. Retention times longer than this would make animals susceptible to the aforementioned degradation.
Animals larger than 2200 pounds must therefore employ a digestive system that is different than the ruminant to allow for rapid digesta transit, which in turn supports optimal microbial fermentation.
Elephants and rhinoceroses are hindgut fermenters with digesta transit times that are much faster than ruminants. These massive mammals have adopted the dietary strategy of ingesting large quantities of dry matter and passing it through the digestive system fairly quickly. Any loss of digestive efficiency is offset by increased intake. In general, the larger the hindgut fermenter, the more rapid digesta transit.
A notable exception to the relationship between body size and transit rate in nonruminants is the giant panda. These animals are actually carnivores that have evolved to survive on a diet of bamboo.
They have simple, short digestive tracts with little volume to accommodate microbial fermentation, yet they live in the wild as herbivores. Researchers determined the rate of passage and digestibility in giant pandas. The giant pandas were fed bamboo and gruel diets, while the elephants and horses were fed grass hay.
The horses and elephants in these studies ate 1.5% of their body weight per day in hay, while the giant pandas consumed 4.3% of their body weight. The pandas have adopted a dietary strategy of extremely high intake and short retention time. Although the panda's diet of bamboo is high in fiber, digestibility of fiber is quite low. Instead, pandas extract the cell contents from the bamboo, and the animals depend very little on microbial fermentation. Protein digestibility was 90% from the bamboo because most of the protein in bamboo is located within the cell contents rather than in the cell wall.
Horses and elephants illustrate the general trend in rate of passage and digestibility in large nonruminant herbivores as it relates to body size. Horses have a rate of passage equal to about 30 hours, and they digest about 50% of the dry matter in hay. Elephants, conversely, have a shorter retention time, about 24 hours, and lower dry matter digestibility. Fiber and protein digestibilities follow the same trend.
Microbial populations in different species Although animals vary greatly in their dependence on microbial fermentation, the populations of microbes that inhabit the organs utilized for fermentation and the environments within these organs are remarkably similar.
Despite the fact that pigs, dogs, and ponies vary tremendously in their dependence on microbial digestion, they all have hindgut environments conducive to fermentation. While VFA concentrations are high in the large intestine of each of these species, pigs and dogs have higher colonic VFA concentrations than ponies. This shows that species that are normally thought of as monogastrics have active sites of fermentation in their large intestines.
Pigs are quite capable of utilizing high-fiber diets, though this fact has been largely ignored as intensive swine management programs have developed. These days, pigs are often kept in confinement and fed high-starch diets that contain significant amounts of corn. In their classic animal husbandry text, Feeds and Feeding (Nineteenth Edition), Henry and Morrison state, “Pastures are so important to pork production that they often make all the difference between profit and loss. Few facts in swine feeding have been so clearly proven, both by scientific experiments and in the common experience of successful farmers, as the high value of pasture or forage crops for all classes of swine.”
In horses the microflora in the hindgut are susceptible to diets that veer severely from primarily forage. A close look at starch, which is abundant in cereal grains such as corn, barley, and oats, proves that it is a versatile energy source. However, problems arise when it is overfed. Studies at Kentucky Equine Research (KER) have shown that pH of the hindgut drops significantly in horses following a grain meal rich in starch, with the lowest point occurring between four and eight hours after feeding. Changes in hindgut pH make horses susceptible to colic and laminitis. For this reason and others, KER nutritionists generally recommend grain meals be small, generally not exceeding five pounds. For horses that must consume large quantities of grain in order to fuel exercise or maintain body weight, a hindgut buffer is appropriate because it steadies the pH, preventing sudden downward shifts that could harm microflora.
A constant source of debate among animal nutritionists is whether information from one species can be used to improve the way other species are fed. Are data gathered from ruminants relevant to horses? Are data gathered from horses transferable to pigs? In light of the similarities between microbial fermentation across many species, the answer would appear to be yes.
One example of this involves yeast culture. Early work with ruminants showed that yeast culture affected microbial fermentation in a number of beneficial ways. Initially, nonruminant nutritionists dismissed this information as unimportant for monogastric animals primarily because the rumen was deemed an inappropriate model for rabbits, pigs, or horses. Research with rabbits and horses, including some conducted at KER, showed that yeast culture affects the intestinal microbes in these species in much the same way that rumen microflora are affected. Transfer of information across species is important for the wellbeing of all animals. An open-minded approach to feeding often yields benefits for multiple species.
Microbial fermentation is important for most animals. The anatomical adaptations that each species has developed depend primarily on body size and natural diet. The total volume of the digestive tract devoted to fermentation and the amount of time digesta spends in these organs varies greatly from species to species. The types of microflora that inhabit the digestive tracts of various animals, however, are similar. Thus, it would seem logical that dietary manipulations that affect one species may also affect other species if the differences that exist between intestinal architecture are taken into account.