Spray irrigation of farm dairy effluent (FDE) to artificially drained land in accordance with deferred irrigation criteria causes minimal direct drainage of partially treated FDE at the time of irrigation. The influence of deferred irrigation of FDE on the subsequent nutrient enrichment of winter–spring drainage from mole and pipe systems is unknown. Research was conducted in the Manawatu region, New Zealand, to investigate the influence of deferred irrigation of FDE on the quality of water in artificial drainage. The experimental site was established on a Pallic soil (Tokomaru silt loam) at the No. 4 dairy farm at Massey University, Palmerston North. There were 6 plots (each 40 m by 40 m), each with an isolated mole and pipe drainage network. Four of the plots received fertiliser according to the farm’s fertiliser program (non-effluent plots), while the other 2 plots received applications of FDE according to the deferred irrigation scheduling criteria (effluent plots). All of the plots were subject to the farm’s standard grazing management.

The average concentrations of N and P in the 2003 winter drainage (average 236 mm) from both the non-effluent and FDE irrigated plots were well above the threshold concentrations that stimulate aquatic weed growth in fresh water bodies. Annual nutrient losses of 31.4 kg N ha/year and 0.65 kg P ha/year in drainage were recorded for non-effluent plots. Deferred irrigation of FDE in the summer period did not increase the loss of N in winter–spring drainage (N loss from effluent plots was 31.1 kg N ha/year) but did cause a significant increase (P < 0.001) in total P in drainage (an additional 1.03 kg P/ha, c. 160% of losses from non-effluent plots, a loss of 3.3% of applied P). Furthermore, an irrigation of FDE to near-saturated soil in mid September resulted in the direct drainage of partially treated effluent, and hence, N and P concentrations in drainage were 6–10-fold greater than those that would normally be expected from drainage events induced by winter–spring rainfall. This illustrates the importance of scheduling FDE irrigation in accordance with deferred irrigation principles.