Assessment of good farming practice implementation by dairy farms in New Zealand: nutrient loss reductions and timeframes for detecting improvement.

Good Farming Practices (GFP) decrease contaminant losses while minimising impacts on productivity and profitability. In this study we quantified GFP implementation in New Zealand in 2023 using the land area of dairy farms supplying milk to Fonterra Ltd (81% of the dairy sector) and estimated percentage reductions in nitrogen (N) and phosphorus (P) losses relative to 2015 baseline loss rates (kg ha−1 yr−1). On average, nationally 64% of dairy land area had implemented GFP in 2023. This reduced N and P losses by 20% and 14%, respectively. Full GFP implementation should reduce losses by 37% for N and 26% for P. Predicted timeframes to reliably detect water quality improvements from dairy land with 64% GFP implementation ranged from 5 to >50 years, with an average of 36 years for N and 41 years for P. With full GFP, average detection times decreased to 18 years for N and 30 years for P. If non-dairy land was included without GFP implementation response times for improvement from dairy actions may be longer. Our results suggest that GFP implementation on dairy land will lead to nutrient concentration reductions within 1–4 decades, on average, but may take longer without collective action across all land uses.