Sheep are classified as ruminants, a category of mammals that includes cattle, goats, deer, and antelopes. This classification is based on their digestive system, which features a specialized four-compartment stomach. This anatomy allows sheep to efficiently break down and extract nutrients from fibrous plant materials like grass and hay, enabling them to thrive on vegetation indigestible to many other animals.
What Makes an Animal a Ruminant
A ruminant is defined by two characteristics: a multi-chambered stomach and the behavior known as rumination. The specialized stomach allows for foregut fermentation, where microbes break down plant matter before it reaches the true stomach. This architecture gives ruminants an advantage in utilizing high-fiber forage.
The second trait, rumination, is the act of “chewing the cud.” This involves the animal regurgitating partially digested food, or cud, back into its mouth for thorough re-chewing. Re-chewing significantly reduces the particle size of the forage, enhancing the efficiency of microbial action and subsequent digestion. Ruminants often perform this process while resting for several hours each day.
The Four-Chambered Stomach and Digestion
The sheep’s stomach is separated into four compartments: the rumen, reticulum, omasum, and abomasum. Digestion begins when the sheep quickly consumes forage, which is briefly chewed before being swallowed into the rumen. The rumen is the largest chamber, acting as a massive fermentation vat that houses billions of symbiotic microorganisms.
These microbes break down complex carbohydrates, like cellulose, which the sheep’s own enzymes cannot digest. This microbial fermentation yields volatile fatty acids (VFAs), the sheep’s primary energy source, along with microbial protein and B vitamins.
The reticulum, often called the “honeycomb stomach,” works closely with the rumen to sort ingested material. It traps larger particles and stimulates the regurgitation of the cud for re-chewing.
Once the cud is re-chewed and swallowed, it bypasses the rumen and reticulum, moving directly into the omasum. The omasum, with its numerous folds, primarily functions to absorb excess water and residual VFAs from the partially digested food.
The final chamber is the abomasum, considered the “true stomach” because its function is similar to that of non-ruminant animals. Strong acids and digestive enzymes are secreted here to further break down microbial protein and other nutrients before the material passes into the intestines for final absorption.
Why Diet is Critical for Ruminant Health
The sheep’s digestive mechanism relies on maintaining a balanced population of microbes within the rumen. The diet must be managed carefully because the microbial community is highly sensitive to changes in the acidity (pH) of the rumen environment. A proper diet includes significant high-fiber forage, such as grass or hay, which encourages rumination and saliva production. Saliva acts as a natural buffer, helping to keep the rumen pH in the ideal range of 6.0 to 7.0.
Problems arise when the sheep consumes too much highly fermentable carbohydrate, typically low-fiber grain. A sudden influx of starch causes a rapid increase in the production of volatile fatty acids and lactic acid. This rapid acid production can drop the rumen pH below the healthy threshold, a condition known as ruminal acidosis. Acidosis can be severe, causing the rumen to stop functioning, depressing appetite, and leading to systemic metabolic issues. The health of the ruminal microbes dictates the health and productivity of the sheep.