Whenever two items substitute for one another in doing the same job, it will often pay to use a little more of one and a little less of the other as costs for the two vary. Take broiler rations, for example.
Balloun, Stanley and Heady, Earl O.
"Least-Cost Rations for Broilers,"
Iowa Farm Science: Vol. 11
, Article 3.
Available at: https://lib.dr.iastate.edu/farmscience/vol11/iss3/3