Here we can compare the rumen papillae development of 6-week-old calves fed three different diets. The calf fed grain from day 3 shows a great deal more papillae development and a much thicker, darker, and more vascularized rumen wall. In the calf fed good quality hay from 3 days of age, the papillae are not developed at all, and the rumen wall is quite thin. 