Comparing nitrogen management on dairy farms: Canterbury case studies

Abstract

Five Canterbury dairy farmers participate in the Forages for Reduced Nitrate Leaching programme (FRNL) to co-develop options for less environmental impact. Farm practices were adapted and new mitigation options were implemented. To assess farm environmental performance, the Overseer model was used to estimate nitrogen (N) leaching, N surplus and N conversion efficiency (NCE) for each farm and each year. When discussing the results with farmers, it appeared that these indicators for environmental performance are limited when comparing farm management strategies. The Overseer estimates include N fixation, which is influenced by model assumptions, and N leaching, estimates that strongly depend on soil type and climate entered into the model. To enable better comparisons between farms and years, a simplified N surplus and NCE were calculated using farmer recorded N inputs and N outputs, i.e. fertiliser, imported supplement, production and exported supplements. Effects of improved management and new mitigation options are presented. Four of the farms improved their N surplus and NCE and three reduced their Overseer-estimated N leaching over 3 years (2014, 2015 and 2016).