The More Herbicides Used on Weeds, the Stronger the Weeds Become

CC BY 2.0. Black-grass in a barley crop. (Photo: WIkimedia Commons)

The rampant use of agricultural chemicals is creating superweeds that laugh in the face of chemicals; which is bad news for crop productivity and food security ... but may have a silver lining.

First there were superbugs, the stalwart bacteria that have outsmarted antibiotics, getting stronger and stronger as they evolve to resist their adversaries. They have become one of the world’s most pressing public health problems.

And now we’ve got superweeds.

Researchers in a group study led by the University of Sheffield explored the unnerving case of herbicide-resistant weeds in the UK. Their conclusion?

Herbicides can no longer control the weeds that threaten crop productivity and food security in the UK because the plants have evolved resistance, and future control must depend on management strategies that reduce reliance on chemicals.

The team looked at the UK’s major agricultural weed, black-grass, mapping the density of it across 70 farms in England and collecting seed from 132 fields. They also gathered historical management data for all fields to ascertain which practices are driving the weed’s 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," says Paul Neve, a weed biologist from Rothamsted Research, one of the study’s collaborators.

Resistance was correlated with the frequency of historical herbicide applications, suggesting that evolution of resistance is primarily driven by intensity of exposure to herbicides, according to the study.

"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," says Neve.

They also found that when farmers switched the chemicals and/or applied them cyclically, resistance still endured, despite those being strategies for preventing the evolution of resistance.

While the idea of plants – and yes, even weeds – outwitting their chemical executioners may be admittedly attractive in a “just deserts” kind of way, the bottom line is that we need to be using other methods to keep weeds from threatening food supplies. And if the “poisoning the planet” argument isn’t enough to quell the use of herbicides, maybe the simple fact that they don’t work, will.

And indeed, the researchers conclude that current industry advice urgently needs to change to reflect the fact that current strategies aren’t working, and other methods using herbicides will likely meet the same fate. Instead, they recommend that “farmers switch to weed-management strategies that rely less on herbicides, as it is inevitable that weeds will overcome even new agents.”

(BRB, investing in teams of weed-eating goats.)

The study was published in the journal Nature Ecology and Evolution.