Deer are ruminants. Ruminants have specialized four-part digestive systems that allow them to thrive on their natural diets of high fibre such as leaves, old grass, and woody browse. The first part of their digestive system is the rumin.

When ruminants feed, they barely chew their food. They mix it with large quantities of saliva and then swallow it into the rumin. There the food is churned and mixed with microorganisms that start a fermentation process. The fermented food generates methane and carbon dioxide gasses, which the animal then burps up.

The rumin then separates the feed into layers of solid and liquid material. The solids clump together to form a cud. The cud is then brought back up to the mouth and chewed slowly (chewing the cud). During that process fiber is further broken down by microorganisms.

The microorganisms present in the deer’s rumin cannot digest a diet of high carbohydrate such as chicken feed, which contains about 80% carbohydrates in the form of corn and cereal grains. If fed chicken feed, this can cause a rapid change in stomach chemistry and disrupt the microorganisms present. The same condition applies to allowing deer to feed on new, sweet spring grass. The rumin will not be able to work to digest the food or expel the gasses formed. It will then shut down.

Other serious outcomes are the release of toxins which are absorbed into the deer’s system and lactic acidosis which causes stomach ulcers and secondary infections.

The microorganisms in the deer’s rumin change during the year. This is why you may see deer feeding on apples and other fruits as the rumin prepares for winter. The microorganisms change further during the winter, allowing the rumin to change a high fibre diet of woody browse into protein.

It is not uncommon for deer to starve to death with a rumin full of food.