A 10-year evaluation of management practices and nutrient losses from dairy farms in New Zealand – Trends and drivers

Good management practices (GMPs) on dairy farms have been shown to reduce contaminant losses and improve water quality. Few national long-term datasets exist globally on management practices on dairy farms over time and their effect on nutrient losses. Here, we examine 50 parameters across a 10-year period (from 2013 to 2022) thought to influence estimates of nitrogen (N) and phosphorus (P) losses (kg ha⁻¹ yr⁻¹) to water from dairy-farmed land in New Zealand. The number of farms in our database increased from 137 in 2013 to a total of 378 in 2022. The years from 2013 to 2017 were classed as ‘period 1’ and from 2018 to 2022 as ‘period 2’, which aligned with more intensive extension of GMPs. Nationally, there was a small increase in median N and P loss rates (38 – 40 kg N ha⁻¹ yr⁻¹ and 1.1 – 1.2 P kg ha⁻¹ yr⁻¹), fertiliser applied 140 – 141 kg N ha⁻¹ yr⁻¹ and total milk solids produced by 11 % between periods. However, between 1 – 42 % of farms exhibited decreasing N loss trends regionally, which were related to (in order of decreasing importance): N fertiliser applied, irrigation type, and forage establishment (cultivation) practice. Similarly, 1 – 25 % of farms with decreasing P trends regionally, trends were related to soil order, P fertiliser applied, and effluent storage method. We also found that these farms showed increased adoption of effluent and forage establishment method GMPs between periods, for example, the use of low-rate effluent application, direct drill, and minimum tillage, and increased effluent storage practice. These data suggest good management practices shown to decrease N and P losses from dairy-farmed land to water in New Zealand are being adopted; however, continued uptake on all farms will be required to achieve further improvement.