What Goes In Must Come Out

Phosphorus Balance on Dairy Farms

Abstract

Phosphorus (P) stimulates growth of algae in freshwater lakes and streams. The loss of P to surface runoff from fields that contain excess P are typically greater than from fields managed to supply adequate but not excessive P for crop growth. As dairy operations have increased in size, manure application rates of P have often exceeded plant uptake of P, resulting in elevated soil test P. High levels of P in lactating cow diets exacerbates the problem, since P fed in excess of the cow's requirement is excreted in the feces in a largely soluble form. Removing excess P from dairy diets not only reduces P content of manure, but sharply reduces the amount of soluble P excreted, thus reducing risk of surface runoff. Most lactating dairy cow diets can have their P content reduced by 20%. This results in a 25-30% reduction in P content of manure, and a similar reduction in the amount of land required to accommodate the manure.