Why do cows eat grass to express milk?

The composition of grass is mainly cellulose, as well as fructose, protein and other ingredients, generally speaking, there are more macromolecules;

Milk is mainly a suspension composed of protein and lipids.

Cattle can't digest completely after eating grass. It is digested with the help of some bacteria in the gastrointestinal tract. Under the action of a series of enzymes, it decomposes sugars such as cellulose and pectin into glucose and protein into amino acids. These small molecules are absorbed in the intestine. In vivo, these simple small molecules synthesize macromolecules through a series of reactions such as glycolysis and tricarboxylic acid cycle, but at this time, macromolecules are far from grass macromolecules, but constitute the ingredients of milk.

The whole process includes both physical and chemical reactions, but chemical reactions are the main ones. In the chemical reactions in the body, there is a macromolecule, namely enzyme, which plays a vital role. Their induced fit is to turn grass into milk step by step.