Nitrogen inhibiting fertilisers, known as DCD, are unlikely to be used this spring following the discovery of traces of it in export milk products.

Earlier this year, both Ravensdown and Ballance Agri-Nutrients withdrew Dicyandiamide-based nitrification inhibitors from the market.

'DCDs are considered safe and there is no evidence to suggest otherwise, however, there is no internationally-agreed acceptable level and so the default is the level of detection,” says Dr William Rolleston, Federated Farmers spokesperson on food safety.
However, humans have not been tested for effects of DCD – other than studies of external exposure of workers applying the product, a spokesperson for the Ministry for Primary Industries has told Coast & Country.
The toxicity studies on DCD are mainly animal studies. 'Human exposure could only be to transient and very small levels of DCD – which makes it extremely unlikely for concentrations reaching the human gut bacteria to be enough to have any effect at all. DCD is also unlikely to affect any human bacteria because of its selective reaction on only one enzyme system in soil bacteria.”
The spokesperson says domestic maximum residue limits could be set in New Zealand within a few months.
'International limits take much longer because they have to be agreed at meetings that take place only once per year. It is our guess that the international process could take up to four to five years to complete.”
Risk assessments are undertaken to determine both safe levels of application of the product to pasture, and maximum residue levels that will be allowed in foods.

Safety margin
These assessments take into account the findings from animal studies, designed to establish the highest amount or dose of the chemical that does not cause any toxic effects. The spokesperson says this determines the No Observed Adverse Effect Level for the chemical. A safety margin is then applied to the NOAEL to establish a tolerable daily intake (TDI) in humans – this is at least 100 times lower than the NOAEL.
Then, analysis occurs to work out what application rates to pasture would be required to achieve the desired effect, what residues in different foods would be expected from this use, and whether a withholding period would be effective in limiting human exposure to chemical residues. Finally, a dietary intake exposure assessment checks that, at the levels used legitimately, the overall human dietary intake will be below the tolerable daily intake (TDI), and thus pose no health risk.

Synthetic
DCD is described by Warwick Cato of Ballance Agri-Nutrients as a synthetic chemical, not a biocide that does not inhibit bacteria. 'It works to slow down bacteria in the soil, not kill them. Its specificity is to pause the nitrification process for a period of time allowing plants to take up ammonia and nitrate. DCD does not have longevity. It is 67 per cent nitrogen and 46 per cent urea and can be used as a nitrogen fertiliser,” says Warwick. DCD breaks down to carbon dioxide, water and ammonia, and lasts in the order of 10 days, depending on rainfall and temperature.

Meat
MPI carried out a risk assessment on the likelihood of residues in meat and concluded that there was no scientific value in carrying out tests. 'This is because the exposed animals (those that were grazing on particular dairy farms during the short window of application) were highly unlikely to be culled immediately after grazing. DCD breaks down very quickly in live animals – likely to be less than two days. Dairy cows are not typically culled in spring, but months later after having been dried off.
'Similarly, if any bobby calves had been exposed through drinking milk containing traces of DCD, any residues would have depleted to extremely low levels by the time they were culled because there is a withholding period of four days. Finally, meat is generally not held for long periods at either processors or retailers, further reducing any value of testing.”
MPI has confirmed that DCD has not been used on dairy goat farms in New Zealand.
The spokesperson says a working group has been set up to look at future use of DCD and will continue to investigate this issue.
The Ministry says no other products achieve the three DCD benefits of slightly increased pasture growth (3-5 per cent), reduced nitrate leaching (up to 20-30 per cent) and reduced nitrous oxide (GHG) emissions (up to 50 per cent) particularly for nitrogen deposited through animal urine under grazing situations.
This is the main source of nitrogen cycling in grazed pasture systems.