Does variable irrigation decrease nutrient leaching losses from grazed dairy farming?

The irrigation of grazed dairy pastures can be highly profitable, but can lead to the enhanced leaching of nitrogen (N) and phosphorus (P) and impairment of freshwater quality. A six-year study was conducted to determine whether the use of variable rate irrigation (VRI), compared to uniform rate irrigation (URI), could decrease N and P leaching losses from a 143-ha area under intensively grazed dairy cattle that had been partly hydrologically isolated by the installation of artificial drainage pipes. Median concentrations in drainage of NH4-N, NO3-N and total N were enriched after passing through the irrigated area under both URI and VRI. However, median concentrations of N species and filterable reactive P and total P in drainage downstream of the irrigated area were much less under three years of VRI than under three years of URI. After accounting for potential differences in flow, annual load estimates of N and P species at the downstream site under VRI were about 80–85% less than that lost under URI. Wider adoption of VRI technology could therefore decrease farm leaching losses and nutrient concentrations in receiving waterbodies compared to areas with widespread URI or flood irrigation.