Nitrogen pollution by dairy cows and its mitigation by dietary manipulation

One of the major contributions to atmospheric pollution comes from nitrogen (N) derived from cattle and especially dairy cows. Although most estimates of ammonia volitilization are based on total N excretion, it has been repeatedly shown that urinary N is a much more important source of pollution than faecal N, specially under grazing conditions. A model was developed to predict the amount and form of N excreted under different production systems. Analysis of N pollution was based on data collected from Holstein/Friesian cows fed 30 different diet types consisting of 10 grass silages and 6 concentrates. While there was a strong correlation between N intake and N output in general, urinary N was exponentially correlated with N intake and the model predicted about 80% loss of N in urine for levels of N consumption above 500 g N/d. On the other hand, outputs of faecal and milk N increased by less than 20% per unit increase in N intake. Model predictions also agreed well with published data and provided reasonable estimates of the form in which N was excreted. Concentrate composition with respect to energy type and its degradation and protein degradability and silage type had significant effects on the amount and form of N excreted. It is concluded that N pollution may be ameliorated by using grass grown with moderate fertiliser application, and maize-based energy supplements, formulated to provide low degradable protein and with N intakes of less than 400 g/d for average yielding cows. This revised version was published online in August 2006 with corrections to the Cover Date.