The effectiveness of a granular formulation of dicyandiamide (DCD) in limiting nitrate leaching from a grazed dairy pasture

Abstract

The effectiveness of a granular formulation of dicyandiamide (DCD) in limiting nitrate leaching from a grazed dairy pasture in southern New Zealand is reported. Treatments were an untreated Control managed as standard farm practice, and+DCD with two or three applications of DCD per annum at a rate of 10 kg active ingredient (a.i.) ha‐1 per application. Each treatment had hydrologically‐isolated plots 12 m wide × 15m long with separate mole‐pipe drainage systems from which drainage waters were collected and analysed for nitrate and ammonium over a 4‐year period. Pasture production, grass N uptake, grass nitrate‐N concentrations and DCD losses in drainage were also measured over this period. The application of DCD showed a clear and consistent trend in reducing concentrations of nitrate‐N in autumn and early winter drainage. On an annual basis, the application of DCD reduced the amounts of N lost in drainage by between 21 and 56%, depending on the year of study. Calculated mean annual losses of nitrate‐N in drainage over the 4‐year period were 12.9 kgNha‐1 from the control and 6.8 kg N ha‐1 from the DCD treatments (P < 0.05), with the greatest losses in the May‐July period. The application of DCD had no significant effect on annual or seasonal pasture production across all measurement years, with pasture yields in the DCD‐treated plots being less than 1% greater than those observed in the control plots. The application of DCD had little consistent effect on the botanical composition of the sward. Cost‐benefit analysis suggests that the small pasture responses observed at this site would not cover the costs of applying DCD unless there were additional benefits such as a carbon credit for reduced nitrous oxide emissions. The application of DCD had the additional benefit of lowering grass nitrate‐N concentrations on 22 of the 31 measurement dates. Between 2 and 16% of the DCD applied annually to the +DCD treatment was lost in drainage, representing c. 7% of applied DCD over the 4 years of measurement. Based on measured soil temperatures at this site and the observed monthly pattern of N loss in drainage, it is suggested that the scheduling of two autumn applications of DCD (e.g., at the March and May grazings) is the most effective strategy for minimising N losses in drainage at this site.