Greenhouse Gases: Reductions Being Achieved On Manawatu Dairy Farms

In 2018, the Manawatu Wanganui Regional Council (Horizons) began examining natural resource management in the region in preparation for a plan review. This includes the information provided by dairy farmers as part of their land-use consent applications. In this paper the authors describe the reductions in greenhouse gas emissions (GHG) emissions being achieved by dairy farmers in the Tararua District as a co-benefit from reducing nitrogen losses to water. The sample of 126 dairy farms came from a relatively high rainfall area (1000-2000mm/yr) and mixed soil types (mostly brown and allophanic soils). In 2012-13, the annual losses of nitrogen to water averaged 40 kgN/ha (ranging from 24-60 kgN/ha). The annual GHG emissions averaged 11.2 t/ha (ranging from 10-15 t/ha). There was a very poor relationship between individual farm nitrogen losses to water and their GHG emissions (R2 <0.1). To model the effect of management practices that reduce nitrogen losses to water, the farms were placed into five groups using cluster analysis. Five clusters of farms were modelled in Overseer®, to represent all the dairy farms in the catchment. Management mitigations were introduced sequentially to each cluster farm and the nitrogen losses to water calculated over an expected 20 year timeframe. When the changes in GHG emissions were compared with the expected reductions in nitrogen losses, a possible co-benefit became apparent. Across the representative dairy farms in the catchment, after introducing the management mitigations for improving water quality, the GHG percentage reductions were estimated to be around 64% of the percentage reductions in nitrogen losses to water. From these results, it appears likely that dairy farmers in the Tararua District will achieve significant reductions in GHG emissions from the adoption of management practices designed to reduce nitrate losses to water.