When calves are born, they are essentially monogastric, because the rumen does not function at birth.
Of the four stomach compartments – the rumen, reticulum, omasum and abomasum – the rumen and reticulum make up 30 percent of the stomach. But they are undeveloped, sterile and nonfunctional. The abomasum, or true stomach, represents 60 percent and the omasum the other 10 percent. This means nutrition for the newborn calf requires feeding essentially a single-stomached, non-ruminant that is unable to digest fiber.
Within the first day after birth, large concentrations of bacteria start to populate the rumen. Feeding dry feeds, such as calf starter, stimulates the number and species of bacteria in the rumen. Starting to feed calf starter at two to three days of age stimulates the growth of anaerobic bacteria, which are involved in methane production, breaking down protein and digesting fiber. Calf starter also stimulates the development of rumen papillae, or the finger-like projections that absorb nutrients.
The developing rumen and rumen bacteria need water to be able to ferment dry feeds, which is why offering fresh, clean, free-choice water to all calves is so important. It is important to remember that milk and milk replacer bypass the rumen and reticulum and goes straight to the abomasum, so supplemental water is needed for complete rumen development.
Figure 1 shows the progression of rumen development and shift in stomach compartment sizes from birth to approximately 84 days of life. At this point, well-cared-for calves should have fully functioning rumens.
Source: “Calf Nutrition” by Hugh Chester-Jones, PhD, University of Minnesota Dairy Extension