The effectiveness of the nitrification inhibitor dicyandiamide (DCD) for mitigating nitrogen leaching losses from a winter grazed forage crop on a free draining soil in northern Southland.
Evidence suggests that the wintering of stock on forage crops is a significant contributor to N losses from livestock farming. Losses are likely to be exacerbated if crops are grown on shallow free-draining soils types and grazed by dairy cattle. A three-year trial (December 2008 - November 2011) was conducted in northern Southland on a soil classified as having severe vulnerability for nutrient leaching to groundwater. Porous ceramic cups were installed under a brassica crop which was grazed by dairy cows in June each year and the leachate collected regularly for N analysis. The treatments evaluated were with and without a single application of DCD applied at the time of crop grazing. Concentrations of nitrate-N in drainage water ranged from <1 mg/L initially to a peak of >40 mg/L in May 2011. Concentrations of dissolved organic N (DON) also increased from a low initial value (<4 mg/L) to peak at 48 mg/L in May 2011. Estimates of mean annual losses of total nitrogen (TN) in drainage to groundwater averaged 81 kg N/ha/yr and ranged from 8.5 kg N/ha in 2009 to 125 kg N/ha in 2011. The single post-grazing application of DCD did not significantly reduce nitrate leaching losses. Keywords: winter forage crop, cow urine, nitrate leaching, dicyandiamide, DCD.
