If you're interested in amounts, sources and importance of various ingredients of supplements, here are some answers to questions about protein, vitamins, minerals, energy, unidentified factors and additives.
Summers, Charles E.
"What's In a Good Beef Cattle Supplement?,"
Iowa Farm Science: Vol. 17
, Article 2.
Available at: https://lib.dr.iastate.edu/farmscience/vol17/iss3/2