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An important fertiliser aid has been culled from farmers' chemical shopping lists after it turned up in milk products destined for human consumption.
The suppliers, Ballance Agri-Nutrients and Ravensdown, have voluntarily suspended sales and use of the granules and spray, called dicyandiamide (DCD), because of fears that importers of New Zealand milk products might object to its presence in foods.
Dairy giant Fonterra's testing of 100 samples from products made last September identified "very-low levels" of DCD residues in 10 samples of whole milk powder, skim milk powder and buttermilk powder made with milk from the North and South Islands.
A Fonterra spokesman said that because there were no food-safety concerns associated with the residues, the batches from which the affected samples were drawn were dispatched as normal. Most of the products would have gone to export markets.
The Ministry for Primary Industries said DCD was not found in products made in November; nor was it detected in 2010 in tests of 48 random raw milk supplies.
"Testing completed so far has not shown any DCD residues to be present in protein products, butter, cheese or anhydrous milk fat."
The ministry said that in 2003, the year before the chemical was released for commercial use, Landcare Research was engaged by Ravensdown to assess the potential environmental and human health affects of spraying DCD onto pasture. No impediments to use were found.
"Research shows no food safety risk or human or animal health concerns with DCD use," the ministry said.
The compound's other uses are in industries including electronics, pharmaceuticals and food packaging.
A deputy director-general of the ministry, Carol Barnao, said that even very low levels of DCD residues in milk could be a trade issue.
"... there is no internationally set standard for DCD residues in food ...
"Because no standard exists, the detectable presence of DCD residues in milk could be unacceptable to consumers and our international markets, even in the small amounts found in recent testing."
In some countries there was a zero tolerance for residues when no standard existed.
The ministry is investigating the process to have a maximum allowable residue level set, if necessary, but this could take more than five years. It is also in a working group with the fertiliser and dairy industries to investigate future use of DCD.
DCD is water soluble and is said to wash rapidly into soil with rain, putting it out of reach of consumption by stock.
The Local Government Association, representing water quality regulators, expressed concern at the suspension of DCD, a product intended to help minimise the environmental footprint of farms by improving water quality and reducing greenhouse gas liabilities.
"The withdrawal of nitrogen inhibitors from fertiliser is a major concern for both New Zealand's agriculture-based economy and for our environment," said Wellington Regional Council chairwoman Fran Wilde.
Dairy NZ said it supported the suspension but hoped a solution could be found to allow use of the nutrient-loss tool to resume.
The product
• Dicyandiamide (DCD), brand names Eco-n and DCn
• Fertiliser aid sprayed or spread on to pasture at around 500 dairy farms
• Reduces nitrogen loss, which improves water quality, reduces greenhouse gas emission and promotes grass growth
• Used commercially since 2004
Is it safe in food?
• No international standard for maximum residue in food
• State food agency says there's no food-safety risk to humans