Aren’t all the necessary minerals already in the feed?
In most cases, no. The world’s cropland has been under cultivation for many decades. With each crop cycle, plants remove trace minerals from the soil or the elements are lost through erosion. Since most fertilizer programs only replace N, P and K, crops become deficient when the soil has been depleted of the trace elements. Most feed ingredients are lacking the full range of minerals that were deficient in the soil. Of course, animals can complete their life cycle without the full range of minerals but they will not produce at their full potential or be healthy and adequately resistant to disease.
