This article is part of our series of original articles on
emerging nutrition topics. Please check here to see other
articles in this series. 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. Based on the average milk yield in
the U.S., an average lactating Holstein cow eats about 48
lbs./day of dry matter. Survey data on the nut