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Effect of timing and formulation of dicyandiamide (DCD) application on nitrate leaching and pasture production in a Bay of Plenty pastoral soil

Abstract Urine excreted from grazing animals has been identified as the main source of nitrate leaching from grazed pastures. The nitrification inhibitor dicyandiamide (DCD) has the potential to be an important tool for reducing nitrate leaching in grazed pasture systems. A mowing trial near Rotorua examined the effects of different DCD formulations, and the timing of DCD application on plant growth and leaching of urine‐N. Treatments were urine‐only or urine+DCD (either granular or liquid DCD) applied at a rate of 18 kg DCD ha‐1. The effect of timing of DCD application was tested by applying the DCD to plots in May (autumn) or July (winter) immediately after urine was applied. The application rate of the urine was equivalent to 598 kg N ha‐1 in the form of artificial urine. There was no significant effect of DCD formulation (granular versus liquid) on any N leaching or yield parameter measured in either the autumn or winter application. On average, the autumn application of DCD significantly (P < 0.001) increased pasture dry matter (DM) from urine plots (34% increase or 21 DM ha‐1). Extrapolation of treatments with or without DCD to grazed pasture, with 5% of the area affected by urine at this grazing, would equate to an annual pasture response of approximately 7%. The increased pasture growth resulted in a greater apparent recovery of urine‐N in herbage from the urine+DCD treatments compared with the urine‐only treatment over the first 98 days of the study of 64% or 103 kg N ha‐1. In contrast to autumn application, winter application of DCD did not significantly influence pasture DM yield from urine plots. Total nitrate leaching losses for the autumn‐applied urine+DCD treatments were 17% less than the urine‐only treatments, but this was not significant. Interestingly, over the short‐term (30–55 days), the autumn urine+DCD treatments showed reduced nitrate leaching losses of up to 50% (P < 0.05), but after day 84 greater amounts of nitrate were leached from the urine+DCD plots giving cumulative losses at day 159 of 217 and 263 kg nitrate‐N ha‐1 for the urine+DCD and urine‐only treatments, respectively. It was apparent that the DCD had become ineffective in inhibiting nitrification, thus allowing the conversion of ammonium to nitrate and its subsequent leaching from the root zone. In contrast to autumn, the application of DCD in winter led to a reduction in nitrate leaching from urine‐N of 62% compared to the urine‐only treatment (average of 32 versus 86 kg N ha‐1 respectively; P < 0.001). Overall, our study showed that when applied at a rate of 18 kg ha‐1 the form of DCD (granular versus liquid) used was inconsequential. However, to gain the greatest benefit from DCD use, more than one application is needed during autumn/winter to maximise the potential for reducing nitrate leaching during this key period of high drainage and N losses.