The Basics of Equine
Digestive System Limitations
Photo & Diagram: C. Williams
The large colon, small
colon, and rectum make up the large intestine. The large colon is 10 to 12
feet long, and holds 14 to 16 gallons. It consists of four parts: right
ventral colon, sternal flexure to left ventral colon, pelvic flexure to left
dorsal colon, and diaphragmatic flexure to the right dorsal colon. The
sternal and diaphragmatic flexures are a common place for impaction. The
small colon leads to the rectum. It is 10 feet long and holds only 5 gallons
Horses require six main classes of nutrients to survive; they include water, fats, carbohydrates, protein, vitamins, and minerals.
isn’t one of the six nutrients because the horse cannot physically consume
energy, however, it is a requirement for sustaining life. The most dense
source of energy is fat (almost three times more than carbohydrates or
proteins); however, carbohydrates in the forms of fermentable fiber or
starch are the most common source. Horses exercising, growing, pregnant in
late gestation or early lactation need increased energy in their diet.
Signs of energy deficiency
include weight loss, decreased physical activity, milk production, and
growth rate. However, feeding a diet too high in energy can cause obesity
increasing the risk of colic, laminitis, and contribute to increased sweat
loss and exercise intolerance.
can be added to a feed to increase the energy density of the diet. Fat has 9
Mcal/kg of energy, which is three-times that of any grain or carbohydrate
source. Fat is normally found at 2 to 6% in most premixed feeds; however,
some higher fat feeds will contain 10 to 12% fat. See Fat Supplements
section for more.
are the main energy source used in most feeds. The main building block of
carbohydrates is glucose. Soluble carbohydrates such as starches and sugars
are readily broken down to glucose in the small intestine and absorbed.
Insoluble carbohydrates such as fiber (cellulose) bypass enzymatic digestion
and must be fermented by microbes in the large intestine to release their
energy sources, the volatile fatty acids. Soluble carbohydrates are found in
nearly every feed source; corn has the highest amount, then barley and oats.
Forages normally have only 6 to 8% starch but under certain conditions can
have up to 30%. Sudden ingestion of large amounts of starch or high sugar
feeds can cause colic or laminitis.
is used in muscle development during growth or exercise. The main building
blocks of protein are amino acids. Soybean meal and alfalfa are good sources
of protein that can be easily added to the diet. Second and third cutting
alfalfa can be 25 to 30% protein and can greatly impact the total dietary
protein. Most adult horses only require 8 to 10% protein in the ration;
however, higher protein is important for lactating mares and young growing
Signs of protein deficiency
include a rough or coarse hair coat, weight loss, and reduced growth, milk
production, and performance. Excess protein can result in increased water
intake and urination, and increased sweat losses during exercise, which in
turn lead to dehydration and electrolyte imbalances.
are fat-soluble (vitamin A, D, E, and K), or water-soluble (vitamin C, and
B-complex). Horses at maintenance usually have more than adequate amounts of
vitamins in their diet if they are receiving fresh green forage and/or
premixed rations. Some cases where a horse would need a vitamin supplement
include when feeding a high-grain diet, or low-quality hay, if a horse is
under stress (traveling, showing, racing, etc.), prolonged strenuous
activity, or not eating well (sick, after surgery, etc.).
Most of the vitamins are found in green,
leafy forages. Vitamin D is obtained from sunlight, so only horses that are
stalled for 24 hours a day need a supplement with vitamin D. Vitamin E is
found in fresh green forages, however, the amount decreases with plant
maturity and is destroyed during long term storage. Horses that are under
heavy exercise or under increased levels of stress also may benefit from
vitamin E supplementation. Vitamin K and B-complex are produced by the gut
microbes. Vitamin C is found in fresh vegetables and fruits, and produced
naturally by the liver. None of these are usually required in a horse’s
diet. Severely stressed horses, however, may benefit from B-complex and
vitamin C supplements during the period of stress.
Forages are classified as
legumes or grasses. The nutrients in the forage vary greatly with maturity
of the grasses, fertilization, management, and environmental conditions. In
order to determine the nutrient content in forage it is best to take samples
and get them analyzed by a forage testing lab (contact your local County
Extension Office for testing information or see the fact sheet, FS714,
Analysis of Feeds and Forages for Horses).
Legumes are usually higher in protein, calcium, and energy than grasses. They have more leaves than grasses and require optimal growth conditions (warm weather and good soil) to produce the best nutrients. Some legumes include clover and alfalfa. Some commonly used grasses include orchard grass, timothy, bluegrass, and fescue.
Appearance can be a good
indicator of the amount of nutrients in the hay, however, color should not
be used as sole indicator. Moldy or dusty hay should not be fed to horses.
For more information see Table 1.
Oats are the most popular grain for horses. Oats have a lower digestible energy value and higher fiber content than most other grains. They are also more palatable and digestible for horses than other grains; however, they can be expensive.
Sorghum (Milo) is a small
hard kernel that needs to be processed (steam flaked, crushed, etc.) for
efficient digestion and utilization by the horse. It is not palatable when
used as a grain on its own, however, it can be used in grain mixes. Like
corn, sorghum is high in digestible energy and low in fiber.
Barley also has hard hulls that should be processed to allow easier digestibility. It has moderate fiber and energy content, and can be a nutritious and palatable feed for horses.
Soybean meal is the most common protein supplement, which averages
around 44% crude protein. The protein in soybean meal is usually a
high-quality protein with the proper ratio of dietary essential amino acids.
Cottonseed meal (48% crude
protein) and peanut meal (53% crude protein) are not as common for horses as
Brewer’s grains (the mash removed from the malt when making beer) are a byproduct of the brewing industry. It is nutritious and palatable with about 25% crude protein and is also high in fat (13%) and B vitamins.
Rice bran is a newer fat
supplement on the market. It is distributed by some commercial feed dealers.
It consists of about 20% crude fat, giving it an energy content of 2.9 Mcal/kg.
References and Supplemental Reading
Lewis, L.D. 1995. Feeding and Care of the Horse (2nd edition). Williams & Wilkins, Philadelphia, PA.
National Research Council. 1989. Nutrient Requirements of Horses. National Academy Press, Washington, DC.
Ralston S.L. 1993. Analysis of Feeds and Forages of Horses. Rutgers Cooperative Extension. FS714.