Dairy calves can be weaned from milk at 1 month of age by feeding a palatable, nutritious starter. Here's more about the early weaning program's "secret" - getting the calves on starter at an early age.
Van Horn, Harold H.; Rust, J. W.; Foreman, C. F.; and Jacobson, N. L.
"Wean Dairy Calves Early,"
Iowa Farm Science: Vol. 16
, Article 6.
Available at: https://lib.dr.iastate.edu/farmscience/vol16/iss10/6