The relative sizes of the four stomach compartments of the ruminant vary with age of the animal. In the newborn calf
or lamb, the first three compartments represent less than 30% of the total capacity of the stomach. As the ruminant
animal grows, forages and grains are added to the diet. At that stage, the rumen begins to develop. Normally, the
rumen of calves becomes functional in six to eight weeks. Food normally passes first into the rumen. The rumen
makes up 80% of the total stomach capacity in the mature animal.