A cow’s digestive system is quite different from that of humans. Cows eat grass, hay and other plant material that contain hard-to-digest cellulose. To cope with this they have a large stomach with four compartments, with the largest being the rumen. The rumen contains millions of tiny organisms that live and die inside the cow, and help to break down plant matter and release nutrients that the cow can digest. Without these organisms the cow would starve.
Organisms Outnumber People
The rumen of a single cow contains more organisms than there are people on Earth. About half of the living organisms inside the rumen are bacteria, but they do more than half of the digestive work. There are probably thousands of different types of rumen bacteria, but only about 20 have been studied in detail. The other major organisms are protozoa and fungi. They all coexist and cross-feed.
Bacteria carry out the majority of the digestion of starch, protein, sugars and fiber for the cow. Different bacteria have different functions and some break down certain carbohydrates and proteins that are then used by others. Some help clean the rumen of the waste products of others so that the rumen does not become toxic. This using of products from others is called “cross-feeding.” Bacteria are constantly being produced as others die and are carried through the digestive tract. One type of rumen bacteria only takes 11 minutes to double its population.
Protozoa Population Boom
Protozoa are unicellular organisms that are much larger than rumen bacteria. Most of them have cilia (hair-like structures) that they beat to move about and to transport food into their mouth. One type convert sugars to starch while another type consume starch and cellulose. Some of them eat the rumen bacteria, while other bacteria attach to the surface of the protozoa and live there. Rumen protozoa can double their population in 15-24 hours.
Fungi Make Hay Out of Grass
Fungi account for about 5-10 percent of organisms in the rumen. They can digest cellulose and break open the starch fibers in the grass and plants, which make it easier for the bacteria to digest it. They are not as well understood as the other rumen organisms. They can be absent in the rumen of cows whose diet is poor in fiber, and have been found in high numbers in the rumens of cows fed on poorly digestible subtropical forages.
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