Glyphosate resistance triggers weed control warning

• First case of resistant ryegrass in England

• Experts call to protect future for herbicide

• Growers and agronomists remain vigilant

The discovery of glyphosate resistant Italian ryegrass has strengthened calls for better weed control practices to protect the herbicide’s long-term efficacy.

ADAS weed science principal consultant John Cussans – who made the discovery – said it was vital to find a middle ground between catastrophising the discovery and dismissing it as a one-off incident.

Wake-up call

But resistance was still a wake-up call, said Mr Cussans. The loss of glyphosate efficacy on a field scale was a real possible in the UK – and the industry must reduce its reliance on the active for weed control.

“If we look at European cases, there was one case in 2006, two by 2008 and three by 2012 – there are still only three cases today, he told last month’s Association of Independent Crop Consultants (AICC) annual technical conference at Towcester.

“If you look at those as an indication of how this will go in the UK, we don’t expect a massive increase. Nevertheless, that’s not a reason for complacency and we still need to have a high level of management and focus.”

Impact assessment

To assess the potential impact of glyphosate loss from a regulatory ban or the evolution of resistance, Helen Metcalfe at Rothamsted Research conducted a modelling study in 2024.

The study simulated a simple cereal and oilseed rotation over ten growing seasons, incorporating various weed management practices, including pre- and post-emergence herbicides and different tillage techniques.

The model considered varying weather conditions and weed communities. It sought to establish a “business-as-usual” baseline with glyphosate, then compared it to scenarios without glyphosate and scenarios with alternative weed control.

Rothamsted Research ecologist David Comont said results revealed a significant decrease in yields and profitability across all scenarios excluding glyphosate, emphasising the herbicide’s role in farming systems.

Vulnerable position

While the reduction wasn’t as dramatic as initially feared, it highlighted the vulnerability of farmers relying solely on glyphosate, he said.

The study also analysed alternative strategies, including increased frequency of grass leys, delayed drilling of winter crops, ploughing instead of glyphosate, and increasing the frequency of spring cropping.

Ploughing emerged as the most effective alternative for weed control in the absence of glyphosate, physically destroying weeds and burying seeds. But ploughing also disrupts soil structure and biodiversity – and emits greenhouse gas.

Delayed drilling and increased grass leys showed varying degrees of success, found the study.

But none matched the effectiveness of glyphosate while also presenting their own sets of drawbacks concerning reduced yields and environmental impacts.

‘Sharpen up’ to avoid further cases

Sub-optimal application of glyphosate is a contributory factor in resistance development and users must sharpen up practices to avoid more cases, says AICC agronomist Sean Sparling..

“The simplest possible message I could give farmers now is to treat glyphosate like you would a flag leaf fungicide spray in winter wheat. It might not be costing you as much, but it probably gives you a greater benefit,” he adds.

In Northamptonshire, AICC’s Luke Wheeler has expertise in implementing regenerative systems. The resistance case is concerning, he says. But even in plough-based systems with tough grassweed populations, growers rely heavily on glyphosate pre-drilling.

“Yes, it’s certainly more of a worry for regen systems, but it’s a concern for everyone and is something we need to face up to together.

“We can manage the issue by following WRAG guidelines and a key message is to use good glyphosate products responsibly at the right rate, the right time and in the right conditions.”