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 repe atedly shown that urinary N is a much more important source of pollution th an faecal N, specially under grazing conditions. A model was developed to p redict the amount and form of N excreted under different production systems . Analysis of N pollution was based on data collected from Holstein/Friesia n cows fed 30 different diet types consisting of 10 grass silages and 6 con centrates. While there was a strong correlation between N intake and N outp ut 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 a bove 500 g N/d. On the other hand, outputs of faecal and milk N increased b y less than 20% per unit increase in N intake. Model predictions also agree d 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 significa nt effects on the amount and form of N excreted. It is concluded that N pol lution may be ameliorated by using grass grown with moderate fertiliser app lication, and maize-based energy supplements, formulated to provide low deg radable protein and with N intakes of less than 400 g/d for average yieldin g cows.