The rumen comprises about 25 percent of the young calf's stomach, increasing to 80 percent in the mature cow. Formed by a temporary partial closure of various tissues in the rumen and reticulum, the esophageal groove allows colostrum and milk to go directly to the true stomach for fast and efficient digestion and absorption. Groove formation is initiated by a variety of stimuli, including suckling. Whether fed by a nipple feeder or open bucket, milk causes the esophageal groove to function until the calf is about twelve weeks old. However, some of the nonmilk proteins in milk replacers may not stimulate full closure of the esophageal groove.
Incomplete closure results in less efficient digestion and absorption.
The calf's digestive system gradually matures in the first two months of life, depending largely on the animal's diet. Rumen development depends primarily on how long milk is fed, how soon dry feed is given, and the amount of milk and dry feed consumed. Under normal conditions, the calf's rumen begins to expand and develop at two weeks of age. A microbial population becomes established as the calf consumes increasing amounts of dry feed. After weaning, the calf's digestive system rapidly develops to that of a full ruminant.