Restricted autumn grazing to reduce nitrous oxide emissions from dairy pastures in Southland, New Zealand

Animal excreta deposited on pasture during grazing represent the single largest source of N2O emissions in New Zealand. These emissions are highest when pastures are grazed during the wet autumn/winter season. The strategic use of a feed pad on dairy farms could restrict the amount of excreta N returned to pasture during this time of year, and thus reduce N2O emissions and other environmental losses. The effect of restricting autumn grazing to 3 h per day on N2O emissions and NO3 leaching losses was measured in a 3-year field study. Nitrous oxide emissions were measured weekly between April and September using a soil cover methodology. Nitrate leaching losses were measured from the NO3 concentration of drainage water that was collected from the hydrologically isolated and artificially drained field plots. Restricted autumn grazing reduced both N2O emissions and NO3 leaching losses from grazed pasture by about 40%. The effect of this grazing regime on total on-farm N2O emissions was estimated using the field measurements and the New Zealand IPCC inventory methodology. These calculations indicated that restricted autumn grazing could reduce direct and indirect on-farm N2O emissions by 7–11%, and could thus be an effective tool for reducing N2O emissions, while also reducing NO3 leaching losses, and preventing soil and sward damage. The study further highlighted that the currently used IPCC inventory methodology cannot easily account for reductions in national N2O emission following adoption of N2O mitigation strategies. It also reinforced the need for assessing the impact of mitigation strategies at a whole farm level.