Interviewer: So what's the real reason why cows need four stomachs?

CowboyBenWade: Converting grass into hamburger meat is a very long process - grass is very unlike muscle fiber and to convert it, you need a lot of stuff going on. Grass is also a super low-energy dense food - so you have to eat a lot of it. That's why cows eat all day - that's really all they do.

Animals that eat meat to grow their muscles like cougars, panthers, dogs, people - that is a very easy process - meat in/meat out, but grass in/meat out - this is much harder.

Interviewer: I see what you are saying - converting something like grass into muscle requires a lot more than using meat directly as an energy source. Could you explain what the main functions of each of the four cow stomachs?

CowboyBenWade: Yes, the bovine digestive system consists of the following four parts. Keep in mind, this is also true for other animals that also eat grass, such as sheep, goats, deer, and, elk.

Rumen: This first stomach is really just a composting area for chewed grass. It bunches up into these nasty looking balls of cud. The cow likes to spit these back up and re-chew them in the cool of evening when it's sitting down. When cows are grazing or eating hay, they don't chew the grass enough the first time; they try and stuff as much in there as they can before someone else eats it, so the pieces are usually too large for the next step. So it needs to soak in this rumen pouch, get wet and soft, and then the cow can work on it some more. It's got to be pretty disgusting at this point, and I'm glad that I'm a higher creature and don't have to do this with my food.

The rumen still has bacteria, and it mixes all in there with the cud that the cow regurgitates. It becomes this big swirling mass and on a large cow this can hold about forty gallons. Bacterial help to break down the grass, but a lot of cows are fed grain, which is not their natural diet. When a cow is fed grain, the bacteria in this rumen will adapt to breaking down the protein and sugar in the corn as that's usually what they are fed to fatten them up. We don't know the biological effect this has on cows' bodies, but some people aren't taking any chances. Grass fed beef is a big thing these days, and it has less fat, obviously healthier for a person to eat. I think the grain/bacteria change that occurs causes the cows to put on fat. It's not just that they are eating more food.

The rumen is the main stomach on a cow, hence the name Ruminants. The rumen make it possible for an animal to consume grass and grain as its sole food source. The other stomachs, as I'll explain below, are secondary to the rumen. For demonstration purposes, vets sometimes cut a hole in the side of a cow and let you stick your hand into the rumen - it kind of feels like a big swirling washing machine full of mulch.

Reticulum: This is really part of the rumen (only separated by a thin wall), and its collects larger particles that need to be spit up and re-chewed. This what the cow barfs up from when it's sitting down in the cool of evening. Sometimes the there might be nuts and bolts in feed troughs, and if the cow swallows those, they get in here in the reticulum and cause all sort of havoc. This is called Hardware Disease - because it's basically means the cow ate metal hardware like nuts and bolts. If they are sharp enough, they can poke holes in things. To treat hardware disease, the farmer will toss a magnet down the cows throat, and that's supposed to aggregate the metal bits and prevent them from doing any damage. Another option is to put magnets in the bottom of the feet trough, so they don't swallow the metal in the first place. Just don't feed your cows bits of sharp tin, screws, broken mower blades, etc. and these wonderful beasts will be free of hardware disease. Magnets won't help if the cow swallows sharp pieces of tin - that's very common on farms.

Omasum: Smaller than the rumen and the reticulum, the omasum is a big strainer where chewed up grain and grass flows through a bunch of paper-thin folds of tissue. These folds absorb nutrients and water. Once the food gets to this point, there's no throwing it back up to re-chew. This area allows the cow to re-absorb water that it has expended in the digestion process. It take a lot of water to digest dry grass.

Abomasum: This is the same type of stomach that you, a non-ruminant, have. It's where normal digestion takes place, and this is the only area where glands secrete hydrochloric acid and other strong chemicals. These digestive accelerants break down what's left after the other stomachs finish their part. The abomasum is the smallest stomach.