Phosphorus utilization and environmental and economic implications of reducing phosphorus pollution from Ontario dairy cows.

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.