The kind of digestive system an animal has determines what it can eat. Pigs and humans have mono-gastric systems or one-part stomachs. These stomachs have limited digestive capabilities. For example, humans cannot consume rough grass or alfalfa hay even though they contain good energy and protein. Domestic livestock and other animals such as goats, elephants, deer, elk, and mice have ruminant digestive systems. The digestive systems of ruminant animals have four part stomachs! (rumen, reticulum, omasum, abomasum). Food that is swallowed first goes into the rumen. The rumen of cattle and sheep contain bacteria that help break down the coarse fibers in range grass and hay. The reticulum of animals can convert rough forage into useful food, this food is chewed twice. The reticulum also catches longer pieces of hay. This food, called cud, is then regurgitated and chewed again before being swallowed into the omasum. The food travels into the abomasum, which contains acid and works like mono-gastric systems. Throughout this process the grass, hay and microbes produce proteins. For this reason, the digestive systems of livestock have an important role in human nutrition. Livestock can consume every source on rangeland and people can consume those livestock.

A fistulated steer was used to demonstrate how four part stomachs function. A fistula is an opening in the side of the steer made to examine how its stomach is functioning. The opening is usually kept plugged.