Italian ryegrass is a common weed in field crops
John Cussans
Weeds resistant to the herbicide glyphosate have been found in the UK for the first time. The species in question, called Italian ryegrass, is widespread in the UK, but samples impervious to the chemical have only been found on one farm in Kent.
Glyphosate has helped farmers adopt more environmentally friendly farming methods known as regenerative agriculture, says John Cussans, a weed management expert at consultancy ADAS. His team confirmed that the plants were resistant in tests in a greenhouse.
“It could affect our ability to adapt our agricultural system,” says Cussans. “Herbicide resistance to glyphosate is a very significant practical problem on a farm.”
In the UK, farmers use glyphosate mainly to clear all the plants in a field before planting seeds. This allows them to avoid plowing, which damages soil health, increases erosion and reduces carbon storage.
Minimizing soil disturbance is one of the key planks of regenerative agriculture, along with crop rotation and ground cover maintenance.
“Glyphosate is relatively environmentally friendly compared to other agricultural chemicals,” says Helen Metcalfe at Rothamsted Research in Harpenden, UK. “It shows very little bioaccumulation and low toxicity. And it supports regenerative practices like minimum tillage, which is really great for soil health. It can actually have environmental benefits compared to some of the alternatives.”
Weeds are a massive problem for farmers, says Paul Neve at the University of Copenhagen in Denmark. “We lose more crop yield to weeds than we do to pests and pathogens.”
Around the world, at least 56 weed species, including Italian ryegrass (Lolium multiflorum) have developed glyphosate resistance, and this has happened independently in hundreds of different locations. Such weeds are a major problem for many farmers in the United States and Argentina, where crops engineered to be unaffected by glyphosate have been grown on a large scale for many years.
“But actually, since the first case was 30 years ago, this hasn’t exploded into a big problem,” says Neve.
It is a big problem on some farms where resistant weeds are present, says Neve, but weeds spread more slowly than, for example, insecticide-resistant pests. Taking precautions such as cleaning farm equipment to get rid of seeds can slow their spread.
Farmers also need to adopt a range of weed control measures and not rely solely on glyphosate, says Metcalfe. “We found that if farmers focus on weed control and implement all these alternatives to glyphosate, it’s possible for profits to start recovering after five to 10 years,” she says.
Cussans’ team increased surveillance in the UK in 2018, testing more than 300 samples of Italian ryegrass. He believes that the resistant plants on the farm in Kent almost certainly developed in situ, rather than being brought in from elsewhere.
He also believes that the fact that resistance appears to have taken much longer to develop in the UK than in other countries may be because farmers there are not growing crops that have been genetically modified or conventionally bred to be resistant to glyphosate. With such crops, glyphosate can be used to control weeds while the crops are growing as well as before seed is sown.
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