Linking the uptake of best management practices on dairy farms to catchment water quality improvement over a 20-year period

Intensive land use, such as dairying, can impair water quality. Although many guidelines exist on how to mitigate the loss of dairy-associated contaminants from land to water through best management practices (BMPs), few datasets exist on the success of implementation on-farm.

Five dairy-dominated catchments (from 598 to 2480 ha) in New Zealand were studied from 2001 to 2020. The first period, from 2001 to 2010, involved comprehensive “extension” advice to farmers consisting of workshops, stream water quality and flow monitoring, farm practice surveys, and identified solutions to address site-specific contaminant losses. In the second period (2011−2020), termed “post-extension”, only water quality monitoring and farm practice surveys were continued.

Of the water quality contaminants (including dissolved reactive phosphorus (DRP), total phosphorus (TP), ammoniacal-nitrogen, nitrate-nitrite-nitrogen [NNN], suspended sediment and E. coli), 83 % of water quality trend directions were either improving (n = 16) or showed no change (n = 9) during the extension period.

Over the 20-year dataset, which included the post-extension period, 20 out of 30 contaminant-catchment combinations (67 %) were improving, but nine were degrading, dominated by NNN (n = 4), DRP (n = 2) and E. coli (n = 2). Abrupt decreases in contaminant concentrations, were correlated with on-farm practice changes mainly associated with transition from direct discharge of farm dairy shed effluent to waterways to land application, and the capture of effluent from off-paddock facilities (like stand off or feed pads). Best management practices reduced phosphorus (P) forms, E. coli and sediment concentrations. Increase in NNN concentrations was caused by transitioning from flood to spray irrigation and a commensurate increase in cow numbers and NNN leaching.

These data indicate that extension advice and on-farm practice change have helped to improve overall water quality over time. Nevertheless, recent regulatory threshold values for some contaminant concentrations are not being met, meaning that more actions are required, over and above the BMPs implemented.