Evaluating the effects of dicyandiamide (DCD) on nitrogen cycling and dry matter production in a 3-year trial on a dairy pasture in South Otago, New Zealand

The nitrification inhibitor dicyandiamide (DCD) has been reported to significantly reduce nitrate () leaching and nitrous oxide (N2O) emissions under urine patches in New Zealand trial conditions. However, results on the efficacy of DCD for increasing pasture production have been inconclusive. As part of a national series of nitrous oxide mitigation research (NOMR) trials, a 3-year study was conducted at a dairy farm in South Otago to examine the effect of DCD on pasture production and N leaching under mowing and/or (simulated) grazing conditions and on N2O losses from dung and urine patches. Over the three trial years, DCD significantly inhibited nitrification in urine-amended soil, reducing peak concentrations in the soil by >50% and the N2O emission factor from urine by 33%–72%. Nitrate concentrations in drainage under simulated grazing plots with or without DCD applications were low and consequently no effect of DCD was detected. The N2O emissions and N leaching results suggested that high rates of denitrification occurred in the imperfectly draining soil, particularly in years 2 and 3 of the study. DCD significantly increased spring pasture dry matter (DM) yields from urine patches by 9%–21% in all years under mowing, but no significant annual effect on DM response was detected under either mowing or grazing. The observed increases in DM response in the urine patches under mowing were not detectable under the variable pasture responses under true grazing conditions.