Animal Science Department


Date of this Version



Published in J. Anim. Sci. 80(E. Suppl. 2):E143–E156. Copyright © 2002 American Society of Animal Science. Used by permission.


Much of the effort expended on nutrient management has focused on the post-excretion product. It is important to keep in mind that management of the diet can have important impacts on quantitative and qualitative aspects of the excreted nutrients. Surveys of nutritionists and extension specialists show that dairy producers are advised to feed 0.45 to 0.50% phosphorus (P) (DM basis) in their lactating cow diets. This is 20% in excess of NRC (2001) requirements. Feeding to requirement would reduce P excretion by 25 to 30% and would reduce solubility and potential for runoff of the P that is applied to fields. Nitrogen (N) excretion by dairy cows can also be decreased, but by a lesser amount. Balancing ruminally undegraded and degraded protein and use of protected methionine along with strategic selection of protein supplements that are relatively rich in lysine, may permit a 10 to 15% reduction in total N excretion, with most of the reduction occurring in urinary N. Urinary urea, following conversion to ammonia, is the N excretion product most vulnerable for loss to the environment. Feedlot cattle routinely consume P in excess of NRC (1996) predicted requirements, and recent research suggests the NRC estimates of the P requirements are high. Decreasing dietary P from the industry average (0.35% P) to the NRC predicted requirement (0.22 to 0.28%) decreased P input by 33 to 45% and excretion by 40 to 50% in nutrient balance studies. With grain-based feedlot diets, overfeeding P is inevitable. At minimum, supplemental P sources should be removed from diet formulations. More accurate formulation of feedlot diets for protein provides opportunity for reducing N excretion. By using the NRC model for metabolizable protein, and by employing phase-feeding, N inputs may be decreased by 10 to 20% from the feedlot industry average of 13.5% dietary CP. This translates into a 12 to 21% reduction in N excretion, and 15 to 33% reduction in ammonia volatilization in open-dirt feedlot pens. Diet formulation can have an important impact on the amount of N and P excreted in both dairy and beef. It is much easier to control potential pollutants by managing their release into the environment than to recover or confine them once they are released.