The Importance of Grain in Developing the Rumen
The rumen of the calf at 4 weeks of age is more developed than the 12-week-old calf that received no grain.
Continued grain feeding through 6 or 8 weeks leads to further papillae growth and development. Since grain intake is strongly related to rumen development and since we normally cannot see the inside of calves' rumens, grain intake can be used as an indirect estimator of rumen development and a criterion for weaning. If calves are healthy and eat 1.5 to 2 pounds of grain per day for three consecutive days, they can be weaned any time after 4 weeks of age.