The reason there are no international standards governing residues in food is that DCD has proved itself safe over 30 years of use, Professor Keith Cameron said.
It is not a pesticide.
Because there are no fully international standards governing residue levels, the sale of eco-n by Ravensdown Fertiliser Co-op and DCn by Balance Agri-nutrients has been suspended, after low levels of DCD were detected in whole milk powder.
It has been sprayed on dairy pastures to help reduce nitrate leaching and greenhouse gas emission levels.
DCD is covered by European Union regulations, which allow residue levels 100 times greater than that found in the Fonterra powder. This was measured in decimal points parts per million, Ravensdown chief executive Greg Campbell said.
Other countries do not have standard levels, though DCD is used in the United Kingdom and in the United States to reduce nitrogen losses in pasture, Cameron said. Organisation for Economic Co-operation and Development (OECD) studies had concluded it met all food safety requirements.
Cameron, professor of the centre for soil and environmental research at Lincoln University in Canterbury, and his colleague Professor Hong Di did the research work that resulted in Ravensdown developing eco-n.
Eco-n is applied twice a year, in autumn and late winter-early spring. The usage guideline is that the spray would be washed off pasture leaves by 10mm of rainfall, making it less likely it would be ingested by stock after that, Cameron said.
The main target areas are the urine patches, though the entire paddock would be sprayed.
The DCD compound remains in the soil for about three months.
It prevents nitrate leaching by slowing bacteria activity and retaining the nitrogen fertiliser as ammonium, rather than allowing it to convert to nitrate. Ammonium is held in the soil for longer and is better taken up by plants, whereas nitrates leach through the soils, potentially into waterways.
The follow-up spray in late winter-early spring ensures the DCD-ammonium is available to the plant roots for spring growth, Cameron said.
DCD is a winter-active compound because the impact of nitrate leaching is greatest at this time of year. This favours use in areas with cooler, drier winters and where soil type allows the working agent to be kept longer in the soil.
The warmer, wetter winters and free-draining pumice and ash soils in the North Island make it less suitable.
As a result most of the 500 or so farms using eco-n or DCn are in the South Island, mostly in Canterbury. This is a low rate among the 12,000 or so dairy farms in New Zealand.
Not everybody accepts that DCD achieves all the benefits that are claimed for it. After doing a study for Ravensdown in 2008, independent scientist Doug Edmeades doubted the claimed effects on pasture production and to a lesser extent the nitrate leaching and remains sceptical.
The industry and scientists have to go back to the drawing board, he says.
Cameron is satisfied that enough research has been done to show that the gains are feasible rather than just being potential.
These findings suggest a 20% to 40% reduction in nitrate leaching in the South Island and 10% to 30% in the North Island. Dry matter pasture production can increase by up to 21% in the South Island and 14% in the North Island, through the nitrogen being available longer for the plants than would otherwise be the case.
The North Island results were still good and helpful for farmers, Cameron said.