By Carole Knight
Bulloch County CEA
It’s a match made in grazing heaven – the ruminant animal and the forage producer. No digestive system is better suited for a diet of grasses and legumes. The ruminant animal is uniquely designed to digest fibrous, high roughage feedstuffs through fermentation. An understanding of how the ruminant digestive system works can be beneficial to producers striving to feed livestock on a forage based nutritional program. There are livestock nutritionists out there that have spent their lives studying the ruminant digestion system; this is only a quick overview of this amazing process.
The actual definition of a ruminant is “an even-toed ungulate mammal that chews the cud regurgitated from its rumen.” Ruminant animals include cattle, sheep, goats, deer, moose and even giraffes. Cud is a food bolus that is regurgitated, re-chewed, and re-swallowed. (Imagine the force needed to regurgitate cud up the neck of a giraffe. Wow!) These animals eat copious amounts of forage at a time and then relax and ruminate. Ruminant animals are further classified by their foraging behavior: grazers, browsers, or intermediate grazers. Grazers, such as cattle, consume mostly grasses while browsers such as moose and mule deer stay in the woods and eat highly nutritious twigs and shrubs. Intermediates, such as sheep, goats, and white tail deer, have nutritional requirements midway between grazers and browsers. Of this group, sheep are more of a grazer, while goats and deer are browsers.
The ruminant digestive system is made up of the mouth, tongue, salivary glands, esophagus, four compartment stomach, pancreas, gall bladder, small intestine, and large intestine. The process all begins when the animal uses its mouth and tongue, by either grazing or consuming harvested forages. These animals eat rapidly, swallowing much of what they consume without chewing it sufficiently. Ruminants only have lower front teeth, but don’t have top front teeth, instead having a hard dental pad. They do however have powerful back molars and these teeth crush and grind plant material during initial chewing and rumination. Consumed forage and feed mixes with saliva, which helps to buffer rumen pH, to form a bolus. That bolus then moves through the esophagus towards the stomach. The esophagus functions bi-directionally, allowing ruminants to regurgitate the bolus or “cud” for further chewing.
Contrary to popular belief ruminants do not have 4 stomachs but rather have 1 very large stomach with 4 compartments: the rumen, reticulum, omasum, and abomasum. Once the cud is adequately chewed, then swallowed again, it passes into the reticulum. Then the solid portion slowly moves into the rumen for fermentation, while most of the liquid portion rapidly moves from the reticulorumen into the omasum and then abomasum. The solid portion left behind in the rumen typically remains for up to 48 hours and forms a dense mat in the rumen, where microbes can use the fibrous feedstuffs to make precursors for energy.
The reticulum sits underneath and toward the front of the rumen. Contents flow freely between the reticulum and rumen. The main function of the reticulum is to collect smaller digesta particles and move them into the omasum, while the larger particles remain in the rumen for further digestion. The rumen is where the real magic happens. The rumen acts as a fermentation vat by hosting microbial fermentation. About 50 to 65 percent of starch and soluble sugar consumed is digested in the rumen. It is home to a population of “rumen bugs” that include bacteria, protozoa, and fungi. These microbes ferment and break down tough plant cell walls and produce volatile fatty acids (VFAs) from these carbohydrates. The animal later uses these VFAs for energy. These microbes also synthesize protein from nonprotein nitrogen, and synthesize B vitamins and vitamin K.
The omasum is spherical and connected to the reticulum by a short tunnel. Water absorption occurs in the omasum. The abomasum is the “true stomach” of a ruminant. It is the compartment that is most similar to a stomach in a nonruminant. The abomasum produces hydrochloric acid and digestive enzymes that help prepare proteins for absorption in the intestines.
The small and large intestines follow the abomasum as further sites of nutrient absorption. The small intestine measures about 20 times the length of the animal. Digesta entering the small intestine mix with secretions from the pancreas, liver, and gall bladder. Active nutrient absorption occurs throughout the small intestine, including rumen bypass protein absorption. The intestinal wall contains numerous “finger-like” projections called villi that increase intestinal surface area to aid in nutrient absorption. Muscular contractions aid in mixing digesta and moving it to the next section.
The large intestine absorbs water from material passing through it and then excretes the remaining undigested material as feces from the rectum. The cecum is a large blind pouch at the beginning of the large intestine. The cecum serves little function in a ruminant, unlike its role in horses.
To effectively feed livestock, like cattle, sheep and goats, it is necessary to understand, at least to some degree, the complexities of the ruminant digestive system. The ability of these amazing animals to digest material that is mostly indigestible to other food animal species provides an important place for ruminant livestock in the food production industry.