We started last week talking about how cows have 4 parts to their stomach, and then we talked about the first part, the rumen. The other three parts of the stomach are still important, but a lot less exciting.
So the cow takes a bite of grass, chews it, swallows it, and it goes down her esophagus into the rumen, where it gets fermented by bacteria, fungi, and protozoa. Then she ruminates, or gets a bolus of pre-chewed food back into her mouth for more chewing, and swallows it again. This better-chewed food goes back to the rumen, but not for very long.
Some of the food the cow has eaten can sort of move back and forth between the rumen and the reticulum. (This pair of stomach parts is often called the reticulorumen.) The wall of the reticulum is very muscular, and its job is to help grind the food and help break it down for the microbes. The photo below shows what the inside of the reticulum looks like. Even though this picture is from a sheep, it looks the same in a cow.
Once rumination and the reticulum have broken the food down and the rumen microbes have had time to to ferment the carbohydrates from the food, it is mostly liquid. This liquid passes from the reticulorumen to the omasum.
The inside of the omasum has a lot of folds. These folds give it extra surface area, where the tissue can absorb water and fermentation products from the food. This is where the cow first starts to get some energy from the food she ate – when the fermentation products get absorbed. The photo below is the inside of an omasum from a sheep (the cow’s looks the same).
Then the food goes to the abomasum, which is more like our stomach. Since most of what the cow actually ate has been digested by the microbes by this point, the cow’s abomasum doesn’t have to do that. It does, however, get to digest the microbes that did the hard work in the first place. (Remember, we talked about this before? The cow does some work for the microbes, and then microbes do some work for the cow.) The enzymes in the abomasum start to break down the microbes so the cow has a protein source. Most of the proteins (and more water) are absorbed in the rest of the intestines.
Are we all digested up? Clear as mud? This may be a bit more than you bargained for, but it is important to understand how a cow can digest the grass she eats. This is why we can raise grass-fed cattle, but not grass-fed pigs. (Pigs have a digestive tract that is very similar to people’s, and they don’t have the fermentation power that it takes to digest grass.)
Next we’ll talk a little more specifically about what beef cattle are fed around here in southern Indiana.