19.02.2018 |

Herbicide use drives the evolution of herbicide-resistant weeds, study

Black-grass in a barley crop (Photo: CC0,,

Spraying weeds with chemicals has always come at a high cost, both to farmers and the environment. But using herbicides to control weeds is also driving the evolution of herbicide-resistant crops, new research shows. According to a study, led by scientists from the University of Sheffield, farms that use a greater volume of herbicide have more crop resistance to herbicides. Future control of weeds must depend on management strategies that reduce reliance on chemicals, the researchers argue. For the study, published in the journal Nature Ecology and Evolution, the research team mapped the density of the UK’s major agricultural weed, black-grass (Alopecurus myosuroides) across 70 farms in England, collecting seed from 132 fields. They also collected historical management data for all fields to find out which management factors are driving black-grass abundance and herbicide resistance.

The researchers surveyed 24,824 quadrats in the UK – small areas of habitat selected at random as samples for assessing the local distribution of plants and animals. They found that, black-grass has become a widespread weed present in 88% of the quadrats monitored by the researchers. 32% of quadrats contained high or very high densities of black-grass. It has spread northward in recent years and the scientists found the weed in areas where it had not been found in previous decades. “The driver for this spread is evolved herbicide resistance: we found that weeds in fields with higher densities are more resistant to herbicides,” said the lead author of the study, Professor Rob Freckleton from the University of Sheffield. “Once resistance has evolved it does not seem to go away: two years later, fields with high densities still had high densities, despite farmers employing a suite of different management techniques. This is confirmed by co-author Paul Neve, a weed biologist at Rothamsted. “Resistance is a major driver for black-grass population expansion in England,” notes Neve. “80% of sampled populations were highly resistant to all herbicides that can be used for selective black-grass control in a wheat crop,” he added.

The researchers found that increasing resistance is linked to the number of herbicide applications. “The results were simple: farms that used a greater volume of herbicide had more resistance,” said Professor Freckleton. Diversifying management and the range of chemicals used did not prevent resistance developing, the team report. “A major imminent threat to food production is the growing reliance on glyphosate as a weed management tool,” the researchers warn in their study. “Resistance to glyphosate is already present in eight different countries. How long it will take for resistance to glyphosate to become near universal is uncertain, but in evolutionary terms it is inevitable unless standard management practices change.” The researchers stress the importance of reducing the evolution of resistance. They recommend that farmers switch to weed-management strategies that rely less on herbicides, as it is inevitable that weeds will overcome even new products. (ab)

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