How can cows and other herbivores survive on grass alone?
A cow is a ruminant, and a ruminant has four stomachs. The first stomach is called the Rumen, which is a large fermentation vat. The Rumen can hold 40 gallons of food! In this large fermentation vat (Rumen), cows have bacteria that break down grass. It is actually those bacteria, as they die in the Rumen that provide much of the nutrition and protein that the cow needs.
Have you ever heard of a cow chewing it's cud? Well, when a cow rests after grazing they will sit in the shade and chew their cud. They actually regurgitate Rumen contents and re-chew their food. This allows the cow to break down the grass into smaller pieces to make it easier to digest.
So what are the other three stomachs?
One is the Reticulum, which is a small stomach attached to the Rumen. Another is the Omasum which has multiple folds like the pages of a book. These folds absorb nutrients. Last is the Abomasum which is much like our stomach and secretes gastric acid and digestive enzymes.
So what are some other ruminants? Sheep, goats and buffalo are all ruminants.
Horses are not ruminants. They have one stomach much like ours (humans) but they also have a very large cecum (like a huge Appendix) and a very large colon. The Cecum acts kind of like the cow's Rumen and holds 8 gallons, while the horse's colon can hold approximately 20 gallons of digesting food and liquid.
Thanks Daddy Bird!