What the heck does that mean? Well, basically it means that they have a rumen. So do sheep and goats.
And what the heck is a rumen? You’ve probably heard that cows have four stomachs. That’s sort of right, but not exactly right. They have four compartments, and the last one is the most like our stomach. The first compartment is called the rumen.
The rumen is far and away the most important part of the cow’s digestive tract. It is basically a giant fermentation vat and can hold up to 50 gallons of contents (mainly fluid, but also some solids and gas). By itself, it takes up about 50% of the cow’s abdomen – it almost completely occupies the entire left side of the cow’s abdomen.
Cows can’t actually digest the cellulose (starch) in grass that is their normal diet by themselves. They need help of thousands of different types of microbes that normally live in their rumen. There are lots of different kinds of bacteria inside the rumen. These bacteria digest sugar, starch, fiber, and protein in the cow’s feed. There are a few fungi in the rumen as well. Their job is to break open the starch fibers in the grass to make it easier for the bacteria to digest it. There are also protozoa, that digest some starches and fiber, but also dead and dying bacteria and fungus.
The rumen microbes multiply and grow inside the rumen. As one bacteria reaches the end of its lifespan, there are already new ones there to take its place. The dead microbes pass through the rest of the cow’s digestive tract and are used as a protein source for the cow.
So the microbes have a happy, warm, moist environment to live in, a constant source of food, and their job is to help the cow digest her food and then be the cow’s food.
The cow provides the happy living environment, is frequently eating to give the microbes something to do, and in return the microbes help the cow utilize the nutrients in the grass (cellulose and other starches and fiber) and then give her a protein source.
Protozoa come in two sizes. The videos below are from a sample of rumen fluid that was viewed under a microscope. The first video was filmed at 400 times normal magnification. In the first video you can see the smaller protozoa – there are a bunch of different types, but it’s hard to tell them apart by just looking at them. You can see how fast they zip around!
The second video shows the larger protozoa. These were filmed at 1000 times normal magnification. They move much more slowly than their smaller counterparts. You can see that there is a short row of cilia (sort of like whiskers) along the outside of the large protozoa. The cilia beat together and move the protozoa through the fluid in the rumen (sort of like those crews who work together to row a boat).
(Videos courtesy of Dr. Jonathan Townsend and Dr. Shawn Donkin, Purdue University.)
More details about the functions of the rumen microbes can be found in this publication from the USDA Agricultural Research Service.
Next time, we’ll talk “ruminating.” Unfortunately for cows, it’s not so simple as chew, swallow, and done. There’s a bit more that goes into eating for them!