When does a beef calf have functional rumen?

Beef calves have a functional rumen about 4-5 months of age. Before this time, the calf is strictly a nonruminant and obtains its nutrients from milk. As the calf grows, its digestive system begins to change, and it gradually starts to eat solid foods. The rumen, which is a large fermentation vat in the calf's stomach, begins to develop and grow, and by the time the calf is about 4-5 months old, it is fully functional.