Modelling potential human exposure to the nitrification inhibitor dicyandiamide through the environment-food pathway

Abstract

Dicyandiamide (DCD) is a nitrification inhibitor that effectively reduces nitrogen losses from fertiliser applied to pastures and has the potential to reduce greenhouse gas emissions. Detection of DCD in dairy products in New Zealand led to safety concerns of food products from animals grazing pastures treated with DCD at high application rates (10 kg/ha). This study explores potential risks associated with DCD's application to agricultural grasslands wherein DCD is applied with fertiliser-N at recommended rates. Probabilistic risk assessment method was employed to calculate human Estimated Daily Intake (EDI) of DCD from milk following two application methods (co-application of DCD with granular N-fertiliser and liquid N-fertiliser). The exposure assessment model consists of five separate modules covering application/soil conditions, field conditions, animal feeding, milk production and human exposure/risk to characterise risk in humans. The predicted mean DCD concentration in milk was 0.005 mg DCD/kg milk for the granular method and 0.05 mg DCD/kg milk for the liquid method, respectively. Furthermore, EDI of DCD for both the granular and liquid methods (1 × 10−5 mg and 1 × 10−4 mg DCD/kg body weight/ day, respectively) were well below the European Food Safety Authority set Tolerable Daily Intake limit of 1 mg DCD/kg body weight/ day. Back-calculation revealed that under the conditions of co-application of DCD with granular and liquid nitrogen fertilisers at maximum permissible rates, for EDI to exceed TDI of 1 mg DCD/kg body weight/ day, DCD application rate would need to exceed DCD fertiliser limit (2.25–4.5% by mass of urea nitrogen) set in Commission Regulation (EC) No 1107/2008, and the bio-transfer factor would need to be over 100%. From the sensitivity analysis, for both application types it was found that phytoaccumulation factors, milk intake and the bio-transfer factors are critical factors influencing the EDI. From the present study, the Margin Of Safety values for both application types exceed 1; indicating DCD does not present a significant risk to human health.