Nitrate (NO3-) leaching and nitrous oxide (N2O) emission from urine patches in grazed pastures are key sources of water and air pollution, respectively. Broadcast spraying of the nitrification inhibitor dicyandiamide (DCD) has been shown to reduce these losses, but it is expensive. As an alternative, it had been demonstrated that feeding DCD to cattle (after manual mixing with supplementary feeds) was a practical, effective and cheaper method to deliver high DCD rates within urine patches. This two-year study carried out on simulated urine patches in three application seasons (spring, summer, autumn) explored the efficacy of DCD feeding to cattle to reduce N losses from grazed pasture soil in a heavy-textured soil under temperate climatic conditions. In each application season, DCD fed to cows, then excreted with urine and applied at a rate of 30kgDCDha-1 (treatment U+DCD30-f) was as effective as powdered DCD mixed with normal urine and applied at the same rate (treatment U+DCD30) at reducing cumulative N2O-N emissions and the N2O-N emission factor (EF3, expressed as % of N applied). Increasing DCD loading within urine patches from 10 to 30kgDCDha-1 improved efficacy by significantly reducing the EF3 from 34% to 64%, which highlights that under local conditions, 10kgDCDha-1 (the recommended rate for commercial use in New Zealand) was not the optimum DCD rate to curb N2O emissions. The modelling of EF3 in this study also suggests that N mitigation should be given more attention when soil moisture is going to be high, which can be predicted with short-term weather forecasting. DCD feeding, for instance in autumn when cows are not lactating and the risk of N losses is high, could then be reduced by focusing mainly on those forecasted wet periods.
Keywords: Animal feeding; Dicyandiamide; Nitrate leaching; Nitrification inhibitor; Nitrous oxide; Urine patch.
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