Mineral nutrition: are animals nutritionally wise?
Utah State University Extension
A number of companies in the U.S. sell free-choice or cafeteria-style mineral supplements. Their rational is that ruminants will select the minerals they need and in the quantities they require for maintenance, pregnancy and growth. Is this a good idea? Many animal nutritionists will tell you ruminants are incapable of consuming minerals in correct amounts to prevent or correct mineral deficiencies and a number of studies support this claim. Unfortunately, researchers who designed and conducted these studies made questionable assumptions about diet selection. Research at USU has demonstrated that ruminants must learn about foods and the consequences of eating those foods before they can make correct choices. Diet selection is not innate nor is it driven by instinct.
Burritt, Elizabeth A., "Mineral nutrition: are animals nutritionally wise?" (2012). Wildland Resources Faculty Publications. Paper 1538.