Why a proposed change to glyphosate (or Roundup) residue levels is so controversial

The Spinoff

25-05-2025

MPI has proposed an increase to the amount of glyphosate allowed to be left on New Zealand wheat, barley and oats. What does this mean, and how could it impact human health and New Zealand's exports?
What is glyphosate?
Glyphosate is a herbicide, often sold under the brand name 'Roundup' and used to kill weeds. It can be sprayed onto plant leaves, where it will be absorbed into the leaves. It moves into the plant's roots, where it prevents the plant from absorbing nutrients from the soil. It's used extensively, both at a small scale in people's backyards, and in the agriculture sector.
Sounds great, might be good for the weeds in the back of my garden! Is there a catch?
Yes, unfortunately. There are some human health risks associated with glyphosate. Some government bodies, including New Zealand's ESR and the US EPA, have said that it is not carcinogenic, but the International Agency for Research on Cancer has said it is a 'probable' carcinogen.
Proving that something is a carcinogen, especially after years of exposure, is really hard. Roundup's owner, Bayer, has had to pay billions of dollars in lawsuits from people alleging their cancer was caused by the herbicide. Glyphosate-containing Roundup formulations have been banned in some US states and household use is banned i n some countries. There are possible links between glyphosate and damage to people's livers and kidneys, and it might impact people's gut biome too.
Because glyphosate is widely used in agriculture, it gets into food, usually at low levels. Agricultural workers who use it regularly are particularly exposed. '[Glyphosate] is widespread in food and in human blood worldwide. As a consequence, it is difficult to establish the full range of impacts on human health because there are almost no unexposed people to compare with those who are exposed,' said John D. Potter, a professor at Massey's Centre for Public Health Research.
'The precautionary principle would suggest that it is up to glyphosate's advocates to prove its safety and, in the absence of such proof, to reduce or eliminate exposure,' said Potter.
'We agree with most regulatory bodies around the world – including in the European Union, United States, Australia, and Canada – that glyphosate is unlikely to cause cancer and is safe to use if the rules are followed,' says New Zealand's Environmental Protection Authority.
Because glyphosate is widely used in agriculture, it gets into food, usually at low levels. Agricultural workers who use it regularly are particularly exposed. '[Glyphosate] is widespread in food and in human blood worldwide. As a consequence, it is difficult to establish the full range of impacts on human health because there are almost no unexposed people to compare with those who are exposed,' said John D. Potter, a professor at Massey's Centre for Public Health Research.
That's a bit worrying but seems inconclusive. How is it used in New Zealand?
Glyphosate is used in New Zealand's agriculture sector, including around grains like wheat, barley and oats. Products with glyphosate have to be approved by New Zealand's Environmental Protection Authority before they can be used. It recommends that glyphosate is used with safety gear like gloves and goggles, is not used when rain is forecast and is kept away from water systems and stormwater drains, so that it doesn't affect plants outside of the target area.
In the food system, there is testing to make sure that people aren't being exposed to large amounts of glyphosate through what they eat. Glyphosate residue levels are set at a default level of 0.1 milligrams per kilogram for oats, wheat and barley.
What is the government proposing to change?
The Ministry for Primary Industries released a consultation document in March for public feedback, proposing a review changing the Maximum Residue Level (MRL) for glyphosate and other agricultural chemicals.
It proposed that the glyphosate residue level be lifted to 10mg per kg, a hundred-fold increase.
How does that compare to other places?
It's double the 5mg MRL allowed in Australia, which New Zealand imports wheat and other grains from. The EU currently has a 20mg MRL for glyphosate in barley and oats, and 10mg for wheat.
What impacts could this change have on New Zealand's international trade?
Ian Shaw, a toxicologist from the University of Canterbury told Newsroom that health-wise, this amount would have little risk – but it might impact New Zealand's trade.
While this change would apply to grain, we've already seen how other products have been impacted by glyphosate testing. In 2021, honey shipments to Japan were rejected; Japan's limits were 0.01 parts per million (a default setting), while New Zealand's is 0.1 and the EU's is 0.5. Japan warned New Zealand that if more than 5% of honey imports exceeded their glyphosate limits, they would ban all honey from New Zealand, prompting panic in New Zealand honey manufacturing. The Ministry for Primary Industries then required all honey shipments to Japan to undergo more stringent testing. It wasn't a health risk, but it was a trading risk.
'A five-year-old child who was consuming honey with the default maximum residue level in New Zealand would need to eat roughly 230kg of honey every day for the rest of their life to reach the World Health Organisation acceptable daily intake for glyphosate,' MPI food risk assessment manager Andrew Pearson told RNZ.
Boring Oat Milk founder Morgan Maw is concerned that similar reputational damage could happen to New Zealand's grain exports if the limit changes. 'Perception is reality – and we risk losing the high ground,' she told BusinessDesk. The wider impact would likely be smaller, as unlike honey, grains are not a major export product for New Zealand.
Could there be other issues with changing the glyphosate level?
In some places, Bayer, the biotechnology company that sells glyphosate under the brand name 'Roundup' also sells 'Roundup Ready' crop seeds, which have been genetically engineered to be resistant to the herbicide. These currently aren't available in New Zealand due to restrictions around genetically engineered crops. Some, including professor Ian Shaw and Green MP Steve Abel, have wondered if the change to GMO regulations is linked to the changing the limits for glyphosate.
Harraways Oats, New Zealand's only oat mill, has submitted against the proposal, as has Organics Aotearoa New Zealand. Because it's a 'maximum' level, and glyphosate residue is usually much lower than 10 milligrams a kg, Federated Farmers spokespeople have said that the raised level will create the impression that there's more glyphosate residue in New Zealand crops than there really are. 'But from a Federated Farmers point of view, we struggle to see why it needed to be increased to 100 times what's been, from 0.1% to 10%, when we're well underneath that,' said Andrew Darling, a grain farmer and Federated Farmers representative in south Canterbury, to RNZ.
'What is clear is that as 'acceptable' levels rise, spraying will result in increased exposure for those who apply this herbicide for a living and for those who live downwind from sprayed crops,' said Potter. According to RNZ, 3,100 people submitted about the change on the MPI website.