A plan to grow paddocks of genetically modified grass created to cut the greenhouse gases produced by cows is in the pipeline at Crown-owned company AgResearch.

The proposal has already provoked criticism from anti-GM groups, who say outdoor trials could jeopardise New Zealand's reputation as a GM-free dairy exporter.

If successful, the grass could take a slice off New Zealand's methane and nitrous oxide emissions, which make up about half the country's emissions under internationally agreed standards.

AgResearch's applied biotechnologies manager, Jim Suttie, said an application would not be made to the Environmental Risk Management Authority until next year.

Enough grass would be needed in the test paddocks to feed "tens" of animals, but the location and size of the trial had not been decided.

When it was, iwi and others in the community would be consulted before the application was made.

Suttie said it was too early to say how much of a difference the grass could make to greenhouse gas emissions.

A trial mixing ordinary cut grass with lipids, or fat, found animals fed the mixture gained more weight from eating the same amount of food - reducing the number of animals needed to produce the same amount of milk or meat and potentially cutting emissions from each beast.

The goal of the trial would be to grow grass with double the lipid content of the best previously created.

GM-free campaigner Claire Bleakley said the trials would divert money from work being done using non-GM legumes and grasses to cut greenhouse gases and increase productivity in animals.

She said the Government should keep funding that research to give farmers the option of staying GM-free.

In May, Federated Farmers used the verbal image of New Zealand cows "grazing outdoors on GM-free grass" to rebut criticism by a British dairy company trying to persuade shoppers to boycott Fonterra's Anchor butter.

But exporters believe animal emissions from making milk and meat will also become an issue for shoppers.

Suttie said that although high-lipid grass had not been tested, it should cut emissions by growing each beast to productive size more quickly, and helping it digest food more efficiently.

Methane and nitrous oxide are both waste products produced by grazing animals.

If Erma approval was granted, Suttie said, the first test would be to graze animals on the GM grass and see if they would eat it. Researchers would then measure the emissions and growth of the animals.

This year, a GM vegetable trial by Crown company Plant and Food Research was axed after campaigners discovered plants supposed to have been destroyed had been left to flower.

Asked about concerns that the grass would escape, Dr Suttie said "quite a bit of thought" was needed to design proper controls.

"Of course it is going to be an issue and it is something that AgResearch takes very, very seriously."