Citation
Thompson, F., A’mar, T., Berkenbusch, K., & Selvaraj, S. (2023). Trends in nitrogen loss from Canterbury dairy farms. Report prepared by Dragonfly Data Science for Overseer Limited. 32 p. Retrieved from https://assets.ctfassets.net/bo1h2c9cbxaf/16kKeBpg01Vob91kLtP1qC/2c0fdf968b99da9c91c96573aacc1464/2023-11_Trends_in_nitrogen_loss_from_Canterbury_Dairy_Farms.pdf
Summary
Agricultural activities, particularly dairy farming, are putting increasing pressure on New Zealand’s natural environments. Some of the impacts are associated with the loss of excess nutrients and the deterioration of natural habitats, such as freshwater ecosystems. Efforts to reduce environmental impacts from dairy farming include regulatory and farm management practices that are aimed at estimating and limiting nutrient losses to the environment.
One of the tools widely used to estimate nutrient flows in agricultural farming systems in New Zealand is the model OverseerFM. The model can be used to generate predictive nutrient budget analyses and to derive estimates of potential nutrient losses (e.g., nitrogen loss), documenting the inputs and outputs to a given scale (e.g., block or farm). Owing to its widespread use, the underlying database of OverseerFM contains a comprehensive dataset of New Zealand farming systems.
The current study focused on the OverseerFM dataset from Canterbury to characterise dairy farming enterprises in this region, and to analyse estimates of nitrogen loss from Canterbury dairy farms over time. Following an initial data exploration, the study dataset was restricted to the period for the farming years 2016–17 to 2021–22. Data from 1269 farms were included in the study, representing approximately 302,000 hectares of land used for dairy production. In the most recent reporting year, 2021–22, the analysis estimated a mean of 46.2 kilogrammes of nitrogen loss per hectare from these farms, representing a 27.5% decrease from 63.8 kg per hectare in 2016–17.
Comparison with data from other sources showed that the study dataset represented a comprehensive characterisation of the Canterbury dairy industry. There was a close match between independently‑reported numbers of dairy farms, total dairy production, and productive land and corresponding records in OverseerFM.
A statistical model was fit to the OverseerFM nutrient budget analysis data to assess annual trends in estimated nitrogen loss. Based on this modelling, nitrogen loss from Canterbury dairy farms was decreasing over the study period at an annual rate of 5.7% (95% credible interval (c.i.): 1.1% to 10.6%) per year, or 29.8% (95% c.i.: 6.6% to 48.9%) over six years. This decrease was estimated after the size of the Canterbury dairy industry was factored out by the model and, therefore, did not relate to any changes in dairy farm area over this period. The modelled decrease in nitrogen loss indicates changes in farming practice that were reported in OverseerFM nutrient budget analyses. Further analysis of OverseerFM data could be used to investigate this aspect.
The model was also designed to determine any variation between six geographical sub‑ regions (that were defined as part of the current study). This part of the analysis showed that the Ashburton sub‑region had the greatest reduction in nitrogen loss, whereas the Waimakariri sub‑region had the weakest evidence of a reduction. The South Canterbury sub‑region had lower nitrogen loss than the other regions, after taking into account farm size.