Editor's Note: The following article, authored by Bill Weiss, The Ohio State University, is the second in an eXtension six-part series on emerging nutrition topics.

"Dairy cows must follow the law of conservation of mass. What this means is that everything that goes into a cow must eventually come out. Nutrients consumed by a cow are secreted in milk, excreted in manure, or retained in the body (either in the cow or in her developing fetus). The nutrients secreted in milk are shipped off the farm several times each week, and most of the nutrients in the cow or her calf eventually leave the farm as a bull calf or a culled dairy cow. The excreted nutrients, although not as valuable as the nutrients in milk have substantial value as fertilizer. The most important nutrients when we consider both the cow and the crops are nitrogen (in nutrition, we call this crude protein), potassium, and phosphorus..."