The herbicide glyphosate, commonly sold under the brand name Roundup, is steadily becoming less effective as weeds develop resistance to it.

Glyphosate was first registered in 1974, but it was the introduction of “Roundup Ready” crops genetically modified to resist the herbicide in 1996 that really led to an explosion in its use, particularly in North America – the US and Canada alone apply more than 130 million kg each year.

For many years, some farmers relied almost exclusively on glyphosate for weed control, says Peter Sikkema, an agricultural scientist at the University of Guelph. That led, inevitably, to the rise of resistance among the weeds targeted. Since 1996, there have been 354 confirmed cases of glyphosate resistance in 57 weed species around the world.

“Because glyphosate is so effective the introduction of Roundup Ready crops set in motion weed management practices that created intense selective pressure for the evolution of resistant weeds,” he says.

Sikkema and his colleagues examined herbicide evaluation trials on seven species of weed from across the US and Canada between 1996 and 2021. They found that, over time, the effectiveness of weed control declined, while the variability of control increased when glyphosate was used after crops emerged from the soil, so the amount of glyphosate required to achieve the same level of weed control went up over time. But if the fields were treated first with a pre-emergence herbicide before crops sprouted, as well as glyphosate after, there was no decline in control or increase in variability.

Francois Tardif, who studies herbicide resistance at the University of Guelph but was not involved in this study, says he is not surprised that the effectiveness of glyphosate declined when it was the only weed control method employed. “If you want to avoid resistance, you need to make life complicated for the weeds,” he says.

Tardif recalls meeting representatives from Monsanto, the company that developed the Roundup Ready system, in the 1980s and 90s and warning them that there would be resistance and weed shifts if the system was widely adopted without other weed control measures. But that went against the company’s marketing message, which heavily implied that resistance would not be an issue.

“Farmers were not told to combine it with anything in the beginning,” says Tardif. “When the Roundup Ready system came out, the message from Monsanto was the best herbicide to add to Roundup was more Roundup.”

With such a simple and effective system on offer, Roundup became a victim of its own success, says Tardif. The first cases of resistance arose in weeds within just a few years of the introduction of the system. “Biology happens,” he says.

The message from herbicide manufacturers has since changed, and studies like this make clear the need to introduce more diversity into weed control, says Sikkema. Along with herbicides, growers need to include crop rotation, tillage, cover crops and new weed control technologies in their farming practices.

“If we rely too much on any one weed control tactic there will be negative consequences in the future,” he says.