SPRAYING WEEDS with chemicals has always been costly – but now it is both costly and ineffective.

According to English researchers, the growing problem of herbicide resistance means that a new strategy to protect crops needs to be developed that reduces farmers' reliance on chemicals.

A nationwide epidemiological assessment of the factors that are driving the abundance and spread of the major agricultural weed, black-grass, has been the focus of collaborative work led by the University of Sheffield, with Rothamsted Research and the Zoological Society of London.

The team mapped the density of black-grass populations across 70 farms in England, collecting seed from 132 fields. They also collected historical management data for all fields to address the question “which management factors are driving black-grass abundance and herbicide resistance?”

“At Rothamsted, we used glasshouse bioassays to determine that 80% of sampled populations were highly resistant to all herbicides that can be used for selective black-grass control in a wheat crop,” said weed biologist and leader of Rothamsted’s strategic Smart Crop Protection programme, Paul Neve.

“Field monitoring indicated that the level of resistance to herbicides was correlated with population density, indicating that resistance is a major driver for black-grass population expansion in England,” he noted.

“We found that the extent of herbicide resistance was primarily dictated by the historical intensity of herbicide use, and that no other management factors had been successful in modifying this resistance risk.”

The research team also surveyed farmers about their use of herbicides, and about how much their different approaches cost them. The team found that the increased weed densities lead to higher herbicide costs and lower crop yields, resulting in significant losses of profit.

Increasing resistance was linked to the number of herbicide applications, and it was found that mixing different chemicals or applying them cyclically did not prevent resistance developing.

The researchers concluded that current industry advice urgently needed to change to reflect these factors, and recommended that farmers switch to weed-management strategies that relied less on herbicides, as it was inevitable that weeds will overcome even new agents.