Low-dose herbicide applications can accelerate resistance

Eric Koetz, weed research agronomist with NSW Department of Primary Industries and Regional Development, says that sub-lethal applications are known to promote herbicide resistance in weed populations by selecting for individual plants that can survive exposure to a particular mode of action.

“These sub-lethal doses cause an effect called hormesis, which results in a weed showing a fitness advantage in response to a low-dose herbicide application,” he says. “The hormesis effect can stimulate some weeds to grow more biomass and produce more seed than plants from the same population that have no herbicide applied.”

Stubble shading weeds, dust on the target leaves, spray drift, or poor boom setup are all examples of how low dosing can occur and potentially trigger a hormetic response in weeds.

Hormesis can be measured in both resistant and susceptible individuals in a weed population. If the weeds are resistant to the herbicide at higher rates, this fitness advantage can accelerate the spread of the resistant genes. This response to a low-rate application can bulk up the weed seed bank and put more selection pressure on future herbicide applications.

“In glasshouse studies, we have observed this effect in awnless barnyard grass sprayed with glyphosate and tall fleabane sprayed with paraquat,” says Eric. “Research at the University of Queensland has also measured the hormesis effect in sowthistle treated with glyphosate.”

In the awnless barnyard grass (ABG) experiments, low-rate glyphosate applications (34 and 67.5 g a.i/ha) boosted biomass production in both glyphosate-susceptible and glyphosate-resistant plants. As the glyphosate rate increased, biomass production in the susceptible population dropped quickly. This demonstrates that the hormesis response exists naturally in ABG.

The hormesis effect in glyphosate-resistant awnless barnyard grass treated with a low-dose of glyphosate (left), compared to the nil-glyphosate control (right).

In contrast, the glyphosate-resistant ABG plants produced more biomass for longer, extending through to the reproductive stage. Spikelet production peaked at doses of 100 to 540 g a.i/ha, where the glyphosate-resistant population produced 20 to 40 per cent more spikes per plant than the no-herbicide control.

“In practice, glyphosate-resistant awnless barnyard grass growing in a field situation would be less competitive than susceptible weeds in the paddock if no glyphosate is applied. However, if glyphosate is used and the dose that reaches the target is less than the label rate, the resistant biotypes would eventually gain the upper hand, aided by hormesis boosting seed production,” he says.

“It’s a different story on the field edges, where glyphosate-resistant plants may be exposed to low-dose applications from spray drift or sub-optimal fence-line glyphosate applications. These plants could express a hormetic response and rapidly produce a large quantity of glyphosate-resistant seed that is easily spread by machinery into the cropped area.”

Dr Asaduzzaman, Charles Sturt University conducted dose-response experiments on paraquat-resistant tall fleabane, where paraquat application doses ranging from 62.5 to 125 g a.i/ha, applied at the 4–6 leaf stage, stimulated a hormesis effect.

“The exposure to a low dose of paraquat stimulated these resistant tall fleabane plants to produce 30 to 60 per cent more buds/plant than untreated paraquat-resistant plants,” says Asad. “Even when label rates are applied in a field, paraquat can stimulate a hormetic response, possibly due to in-field dose variation due to plant or stubble shading or from spray drift.”

Fence line management continues to be a frontier in the evolution of herbicide resistance. These non-crop areas can allow weeds to flourish with full access to soil moisture and sunshine. While growers concentrate their efforts on effective in-crop spraying and herbicide selection, the crop borders are often given a lower priority in terms of timeliness and optimal spray application.

Spray drift is an environmental risk and can damage susceptible crops; however, the threat of spray drift causing low-dose application on weeds is often overlooked.

“As we know, weed control is essentially a numbers game,” says Eric. “Any practice that leads to an increase in seed production is a genuine risk to long-term profitability and herbicide sustainability in cotton and grain production systems.”

The WeedSmart Big 6 is an integrated weed management strategy that applies to both crop and non-crop areas on and around farms. Glyphosate and paraquat resistance often enters cropping fields from the borders, and prevention requires a dedicated fence-line management program that includes herbicide and non-herbicide tactics where possible.

More resources

• Redefining fencelines weed control options
• Managing fence line weeds and avoiding resistance (webinar recording)
• What are the options to manage glyphosate and paraquat resistant ryegrass?
• Mix up your approach to fencelines weeds
• Can I win against ryegrass with dual resistance to glyphosate and paraquat?