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Phosphorus Utilization and Environmental and Economic Implications of Reducing Phosphorus Pollution from Ontario Dairy Cows¹

E. Kebreab^*,,2, N. E. Odongo, B. W. McBride, M. D. Hanigan and J. France

^* National Centre for Livestock and Environment, Department of Animal Science, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
Centre for Nutrition Modelling, Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

² Corresponding author: kebreabe{at}cc.umanitoba.ca

A major source of environmental pollution has been overfeeding P to dairy cows, caused by the "safety margins" added to diets in order not to compromise the health and production of animals. An extant whole-animal model was evaluated using an experiment conducted in Ontario to assess its applicability for predicting P excretion. The objective of the study was to use the model to estimate P excretion levels and the economic and environmental implications of implementing mitigating options by following recommendations from studies that have reported sufficient levels of P inclusion in the diet. Mean square prediction error and concordance coefficient analysis showed that the overall predictions were close to the mean and that there was only a slight underprediction of fecal P output by the model. The majority of the error was random, with only 8.9% coming from error caused by deviation from the regression line, and the model did not show a systematic trend of over-or underprediction. The model was then used to predict P excretion in Ontario by using diets commonly fed to dairy cows on Ontario farms. It is estimated that Ontario dairy farms produce 7 kt of P annually at current levels of P inclusion in the diet. Reducing P levels from the current 0.41% P of dry matter to 0.35% is estimated to save producers CAN $20/cow per year and the environment 1.3 kt/yr without impairing cow health or productivity. Additionally, the reductions might be from inorganic P sources added to the feed, which are more polluting than organic sources because of their water-soluble nature and liability to leaching and runoff.

Key Words: phosphorus • pollution • economics • dairy cow