Cow has one stomach, with four compartments. They are the rumen, reticulum, omasum, and abomasum, the rumen being the largest compartment. Cow sometimes consume metal objects which are deposited in the reticulum, the smallest compartment, and this is where hardware disease occurs. The reticulum is known as the "Honeycomb." The omasum's main function is to absorb water and nutrients from the digestible feed. The omasum is known as the "Many Plies." The abomasum is like the human stomach; this is why it is known as the "true stomach".
Dairy farming and the milking of cow - once performed largely by hand, but now usually replaced by machine - exploits the cow's unique ruminant biology. Cows are ruminants, meaning that they have a digestive system that allows use of otherwise indigestible foods by repeatedly regurgitating and rechewing them as "cud." The cud is then reswallowed and further digested by specialised microorganisms in the rumen.
These microbes are primarily responsible for decomposing cellulose and other carbohydrates into volatile fatty acids that cattle use as their primary metabolic fuel. The microbes inside of the rumen are also able to synthesize amino acids from non-protein nitrogenous sources such as urea and ammonia. As these microbes reproduce in the rumen, older generations die and their carcasses continue on through the digestive tract.