You can feed cows with dry corn

All about animals

The rumen is the first of a total of 4 stomachs of a cattle, it is a powerful fermentation chamber. It lies on the left side of the cow and fills almost the entire left abdominal cavity with its volume of 150 to 180 liters. It is divided into an upper and lower rumen sac and can hold a total of 50 to 60 kilograms of feed.
The rumen has no glands such as the stomach or intestines - the ruminant uses other living beings to break down the plant matter, which is difficult to digest. Many billions of bacteria and protozoa (around 7 kg in total) populate the rumen and produce acid during their activity.

Initially, the cow just swallows the grass. Then she lies down comfortably to rest and begins to chew. Plant parts that have already been attacked by bacteria are fed back into the mouth by a reflex and then chewed thoroughly. The strong molars grind the food into a fine pulp. Every bite is saliva vigorously. A cow produces 200 liters of saliva a day. The saliva not only has the task of making the food slide well, it is also strongly alkaline (pH value 8.3) and increases the acids produced by the bacteria in the rumen to a neutral pH value of approx. 6.5 .
IMPORTANT: Structured feed such as hay or grass, which the cow has to chew properly, is vital for her, because without ruminating there is no saliva production and the rumen would "acidify" to the point of stopping.

Breakdown of cellulose (crude fiber) in the rumen
A large part of the diet of a cattle consists of cellulose, which is largely indigestible for us as humans (straw even consists of up to 40% cellulose). No digestive ferment from vertebrates is able to break down cellulose and thus make it digestible. Only bacteria produce such a ferment - cellulase. However, it only manages to break down 50 to 60% of the cellulose present in the rumen. The end product of the breakdown of cellulose is acetic acid. A considerable part of this is converted into milk fat in the metabolism.

Breakdown of starch and sugar (carbohydrates) in the rumen
Another group of bacteria break down the carbohydrates, starch and sugar contained in feed. These become propionic acid and butyric acid and thus provide the cow with energy. Approx. 95-100% of the carbohydrates in the feed are digested in the rumen.

Build-up of protein in the rumen
A large part of the protein in the cow's diet is broken down into ammonia by the bacteria. Together with the carbohydrates, this serves as food for the bacteria. They build up the amino acids that are essential for the cow in the form of bacterial mass. The cow is therefore independent of essential (= indispensable) amino acids. Approx. 70% of the protein in the feed is digested in the rumen.