Ruminant digestion

 

Often, one of the first things a person is told about cattle is that they have four stomachs. That is both yes and no. There are four chambers: rumen, reticulum, omasum, and abomasum; but the reticulum can be considered as a pouch on the side of the rumen as they are not well separated.

Ruminant is the term applied to species having this type of digestive system. It is a rather complex process so I will not go into detail of how the products of digestion are converted into other products that can be used by the body. The group includes all species (both wild and domestic) of deer (including elk, moose, caribou, etc.), antelope, sheep, goats, cattle (including bison, musk ox, buffalo), and the giraffe. These animals are designed to be able to consume a lot of food in a short time and then go to a more secluded or sheltered area. The benefits of this include reduced exposure to predation, avoiding excessive heat, and preventing excessive loss of body heat in cold weather.

In order to gather their food quickly, they do not chew and then swallow each bite of food as it is taken. They usually take several bites, chew just enough to slightly crush the food and coat it with saliva. That clump of food (called a bolus) is swallowed, and they continue the same procedure until the end of that feeding period. Each bolus of food goes to the rumen which is an anaerobic (without oxygen) fermentation vat. Here, microbes (bacteria and protozoa) digest some components of the food. Fermentation in the rumen is continuous as it always contains some partially digested food so that recently eaten food is inoculated with the microbes needed for fermentation.

After the animal has finished feeding and retired to an appropriate area, rumination occurs. This is often called “chewing the cud”. The process involves a bolus of the recently eaten food in the rumen being regurgitated by the reticulum back up into the mouth. Now it is more thoroughly chewed in a grinding motion of the cheek teeth to greatly reduce the particle size of the food. It is then swallowed to the rumen again and another bolus is immediately brought up for chewing. This continues until the recently eaten food is well chewed.

There are very many kinds of microbes in the rumen in order to digest the wide variety of components of plants. Soluble sugars and other cell contents are rapidly fermented in the rumen. The microbes also digest complex carbohydrates, cellulose, and hemicelluloses. Some of the compounds produced by the microbes are utilized by other microbes to produce yet other compounds. End products are carbon dioxide, methane, and volatile fatty acids. The gases are excreted, and the fatty acids are absorbed through the rumen lining and oxidized for further use by the body. Microbial synthesis of vitamins also occurs here which add to the vitamins that are released during food digestion.

Fine particles in the rumen then enter the omasum, but large particles are screened to prevent them from entering it yet. Here there is absorption of soluble nutrients and excess water. The contents of the omasum then pass into the abomasum which produces mucus, pepsin, and hydrochloric acid. Here, there is acid digestion of some plant materials that could not be digested by the rumen microbes. Also digested are the microbes which then provide an additional source of energy and protein.

The contents then enter the small intestine for enzymatic digestion and absorption. Then it passes to the caecum and large intestine for additional fiber fermentation, bacterial vitamin synthesis and absorption of water and soluble nutrients.

While the very young are on a diet of milk, fermentation digestion would be detrimental for them. Fortunately, there is a groove that extends from the esophagus to the abomasum that bypasses the rumen. The rumens of the young acquire microbes when the mother licks and grooms them and they swallow some of her saliva that still contains some rumen fluids and tiny particles.

Ruminants are often divided into three general, but not totally distinct, categories based on food preferences and digestion capabilities. There is a gradient of species through these categories as some are only slightly closer to (or farther from) the verbal description for each category.

Grazers or roughage feeders such as cattle, bison, and domestic sheep eat mainly grasses which are high in fiber. They have a large rumen as it takes a long time for fermentation to break down that material. They also have very long intestines; my book on this type of digestion says 25 to 30 times their body length.

Browsers or concentrate selectors such as moose, white-tailed deer, and mule deer prefer easily digested foods such as forbs, leaves and new growth shoots (stems) of deciduous trees and shrubs. If an abundant supply of preferred foods is not available, they will eat some less desirable foods as spending an inordinately long time searching for food can be counter-productive and even detrimental. Their rumens are smaller than the above as fermentation can more rapidly digest their preferred foods. Their intestines are reported to be only 12 to 15 times their body length.

Mixed or intermediate feeders such as elk, caribou, and domestic goat eat grasses, forbs and browse. Their rumen and intestines are (as one would expect) intermediate between the other two.

Although fermentation is involved in the making of alcoholic beverages by brewers, rumen fermentation produces products that are quite unpleasant to both smell and taste. That is why great care is taken not to contaminate meat with rumen fluids and contents. If it should occur, one must cut all affected meat away from the meat to be eaten as there is no way to remove or hide the taste or the odor.