Submitted to: Extension Circular
Publication Type: Research Notes
Publication Acceptance Date: 10/15/2004
Publication Date: 5/15/2005
Citation: Powell, J.M and L.D. Satter. 2005. Dietary phosphorus levels for dairy cows. Best management practices for agricultural phosphorus. Four-Page fact sheet. SERA-17. Organization to Minimize Phosphorus Losses from Agriculture. http://www.sera17.ext.vt.edu/Documents/BMP_dietary_phosphorus.pdf Interpretive Summary: Guidelines for manure application to cropland are becoming increasingly based on a combination of manure phosphorus (P) content, soil test P level, crop P requirements and a field's risk to lose runoff P to surface water. Given the direct relationship between dietary P the amount of manure P excreted, the simple practice of adopting the dietary P recommendations of the National Research Council would greatly reduce manure P levels and would help farmers meet manure P-based management practices. This four-page research note provides information on the dietary P requirement of lactating dairy cows, and the impact of mineral and protein supplements on dietary P, manure P, the amount of land needed to effectively recycle manure P through crops, the nitrogen:P ratio of manure vs. crop requirements, and runoff P from fields after manure application. Many dairy producers already have reduced diet P levels and have saved approximately $15 per cow per year in cost of mineral P supplements. The reduced environmental risk associated with P runoff loss from manure-amended fields, and the potential to reduce eutrophication of fresh-water systems through reductions in dietary P and manure P levels would be potentially enormous. Any strategy aimed at improving P use on dairy farms, including dietary practices, must be done in partnership with the consultants, feed industry, veterinarians and others that assist farmers in making nutrient management decisions.
Technical Abstract: Manure application to cropland is becoming increasingly regulated based on a combination of manure phosphorus (P) content, soil test P level, crop P requirements and a field's risk to lose runoff P to surface water. Feeding P in excess of NRC-recommended levels does not increase milk production, milk composition or reproductive performance of dairy cows, it simply increases manure P excretion, the amount of land needed to effectively recycle manure P through crops, and the hazard of runoff P after manure land application. Holstein cows producing milk containing 3.5% fat and 3.0% true protein have a dietary requirement (dry matter basis) of 0.32, 0.35, 0.36 and 0.38% P for daily milk production levels of 25, 35, 45, and 55 kg., respectively. Many dairy farmers feed P in great excess of these NRC-recommended levels. When manure from dairy cows fed a high (0.49%) and low (0.31%) P diet were applied at equal amounts, difference in P runoff between plots amended with high P manure was 8 to 10 times greater than from plots amended with the low P manure. When manure was applied at equivalent rates of P (40 kg P per hectare), runoff concentrations and loads from plots amended with the high P manure were approximately four to five times those from the low P manure. The simple elimination of mineral P supplements and/or selection of protein supplement low in P may be sufficient to eliminate excessive P feeding, decrease land area needed for manure spreading and reduce the hazard of runoff P from manure-amended fields.