Mitigating nitrous oxide and manure-derived methane emissions by removing cows in response to wet soil conditions

In pasture-based grazing systems, urine deposition is the major source of the greenhouse gas nitrous oxide (N2O). Livestock treading damage and high soil water contents increase the risk of N2O emissions. Duration controlled grazing (DCG) practices that are implemented in response to soil water conditions above a threshold may therefore provide an effective means of reducing greenhouse gas (GHG) emissions from dairy farms. The objective of this study was to evaluate the potential decrease in GHG emissions from dairy farms when implementing DCG when soil water content is above a specific threshold (akin to ‘wet’ days). We used the DairyNZ Whole Farm Model and APSIM model to assess the cost-benefit of implementing DCG to reduce total N2O and manure-derived CH4 emissions from dairy farms. We modelled scenarios on poorly drained or imperfectly drained soils in four regions of New Zealand including Waikato, Manawatu, Canterbury and Southland, where the grazing time on wet days was 0, 13, 17 or 21h per day. Emissions were estimated using a refined version of New Zealand's current national greenhouse gas inventory methodology. Our analysis suggested that reducing the grazing time from 21h to 0, 13 or 17h per day when soils were wet could reduce annual N2O and manure-derived CH4 emissions by up to, respectively, 12, 9 or 5% on farms with poorly drained soils. The 13h per day grazing duration was the least costly, particularly if there were >150 ‘wet’ days per year. In contrast, for dairy farms on imperfectly-drained soils, DCG increased emissions, suggesting this management approach for reducing GHG emissions is not suitable for these soils.