Effects of dairy farming intensification on nitrous oxide emissions

A dairy farm system trial was conducted between September 2003 and August 2005 to evaluate the effect of integration of maize silage forage on nitrous oxide (N2O) emissions. Potentially, the integration of low-protein forage (e.g. feeding cows with maize silage) to reduce dietary-nitrogen (N) concentration can mitigate environmental N emissions and increase N use efficiency. The dairy farm systems consisted of a maize supplementation system with a stocking rate of 3.8 cows ha−1 of grazed pasture with maize silage brought in and a control system with a stocking rate of 3.0 cows ha−1 of grazed pasture. Direct and indirect N2O emissions from all components of the farm systems were either measured using a closed chamber technique or calculated using the New Zealand IPCC inventory methodology. Annual average N2O emissions were slightly lower on the maize supplementation pasture than on the control pasture. Annual total N2O emissions from the “whole” farm systems (including direct and indict emissions from the grazed pastures, maize growing land, N fertilizer use and associated land application of farm effluent) were 7.71 and 8.00 kg N2O–N ha−1 of dairy farm on the control and maize supplement farm systems, respectively. The corresponding annual milk production was 13,437 and 17,925 kg ha−1. Therefore, the N2O emission per kg of milk production from the maize supplementation was 22% lower than that from the control system. This was due to the much greater efficiency of N use from low-protein maize silage than from pasture. The results suggest that the integration of low-protein forage can be an effective management practice to mitigate adverse environmental effects of increasing stocking rates in the New Zealand dairy farm systems, in terms of N2O emissions per unit of milk production.