What happens to the grass a cow eats?

A cow swallows grass all in one gulp. Then the grass goes into a large pouch inside her body. This is known as the first stomach, or rumen. This stomach is used for tripe. Tripe has a rippled surface because there are folds in the rumen. But when the grass reaches the stomach, these folds open out so that the first stomach now is smooth and can accommodate all the grass the cow eats. When the first stomach is full, the cow rests. Microorganisms in the first stomach break down the fiber in the grass. They also produce fatty acids, a component of fat. Next the food goes to the cow's second stomach, where it is made into little balls called cuds. Cuds go back to the first stomach.

Now a strange thing happens. The muscles of the rumen send the cuds back to the cow's mouth. The cow chews the cuds very well. Then she re-swallows them. This time, instead of going to the rumen, the cuds go to the third stomach. Here the water is squeezed out of them. Then the cuds go to the fourth stomach and are digested.

But wait! The cuds are no longer purely grass! Now they contain some microorganisms from the rumen, too. When the grass was in the cow's rumen, it provided a food source for the microorganisms. These microorganisms are where the cow gets most of her energy.

If a cow were to be fed on grain, strange things would happen. When the rumen was stuffed with grain, another type of bacteria would start growing there. These bacteria could kill the cow!