Overview: Horse Digestive System

Overview of the Horse Digestive System

Each part of the horse digestive system serves a different purpose and has a unique function. The horse is a nonruminant herbivore or a plant consumer. Rabbit, swine, zebra, donkey, tapir, elephant & rhinoceros are also nonruminant herbivores. Animals consume food based on gut design, with one exception – the giant panada. The length of the gut and number of compartments are two important factors that determine the gut design of horses.

Based on function, the gut is broadly divided into foregut and hindgut and is designed for the horse to consume small amounts of grass continuously. Horses are continuous grazers. The foregut consists of mouth, esophagus, stomach, and small intestine. The hindgut consists of cecum, large colon, small colon, and rectum. The connection of the foregut & hindgut is the ileo-cecal junction; specifically, the merger of the ileum & cecum.

FOREGUT

Mouth

• Used to grasp & chew food as well as to produce salvia to lubricate and buffer food mass
• Buffering effect of salvia prepares food entering the stomach acid environment
• Chewing is a critical component of the initial digestive process, thus the importance of dietary long-stem fiber (pasture, hay) that requires significant chewing

Esophagus

• 4 ft. long
• Transport ‘tubing” for feed mass from mouth to stomach; esophagus allows food mass to move in only one direction, thus difficult for horse to vomit

Stomach

• 2 – 4 gal. capacity
• Considered small for the body size
• 75% of stomach content passes through within 30 minutes
• Acid environment & enzyme prepare food for the more specific digestion in small intestine

Small Intestine

• 60 – 70 ft. long, 2½ – 4 inches diameter, 10 – 12 gal. capacity
• Composed of 3 segments: duodenum, jejunum, ileum and is the major site of sugar, starch, protein & fat digestion
• Liver, pancreas & lining of small intestine contribute enzymes for the digestive process but there is no gall bladder
• Duodenum, the small intestine’s first segment, is where bile (fat emulsifier) is directly secreted
• Jejunum, the middle segment of the small intestine, is the primary site of nutrient absorption
• Ileum, the last segment of small intestine, has a large population of bacteria that will digest fiber (fermentation) and other feed components that had escape enzymatic digestion

Ileo-Cecal Junction

• The connection of the foregut & hindgut or the merger of the ileum & cecum

HINDGUT

Cecum

• 3 ft. long, 7 gal. capacity
• A site of fermentation or volatile fatty acid (VFA) production
• Food enters and exits from same opening; interesting point relative to how long food mass is retained in the cecum and the reason for cone shape

Large Intestine

• 24 ft. long, 21 gal. capacity
• More than 60% of gut capacity, retains feed for 48 – 64 hours
• Consist of large colon, small colon, rectum
• Hindgut bacterial population brakes down fiber & other feed components that escape enzymatic digestion
• Bacteria produces VFAs which are the primary energy source for horses

Large Colon

• 12 – 14 ft. long, 3 – 4 inches diameter, 15 gal. capacity
• Consists of right ventral colon, sternal flexure to left ventral colon, pelvic flexure to left dorsal colon, diaphragmatic flexure to right dorsal colon (followed by transverse colon & small colon)
• Cecum & proximal colon (right ventral colon to left ventral colon) are sites of VFA (acetic, propionic, butyric) production

Small Colon

• 10 ft. long, 6 gal. capacity
• The small colon absorbs water
• Contains the descending colon (10 ft. long, 3 – 4 inches diameter, 5 gal. capacity) which is the primary site of water absorption

Rectum

• 1 ft. long
• Site that forms & holds the fecal mass prior to evacuation

The type & volume of food influences the rate in which food mass passes through the gut. The frequency of feeding and the environment with other horses are also factors that influence this rate of passage. Gut health is influenced by a healthy, stable, consistent population of the microbiome.