For years, there has been "theoretical" reasoning about the impact of various feedstuffs on rumen development in young calves.
But researchers at Pennsylvania State University have taken those theories a step further. They have actually extracted and photographed the rumens of calves at various ages to make comparisons on the impact of various early life calf diets.
The images are highly illustrative. As an example, the image above compares the rumen of a 4-week-old calf fed milk and grain (left), to that of a 12-week-old calf fed only milk (right). The rumen is larger and has more extensive papillae development.
More of these descriptive images – evaluating calves in a range of ages and fed various dietary ingredients – can be accessed here.