Eric Koetz, NSW DPIRD, illustrates the hormesis effect in glyphosate-resistant awnless barnyard grass treated with a low-dose of glyphosate (right), compared to the nil-glyphosate control (left).
By Cindy Benjamin
Hold tight - we’re checking permissions before loading more content
Some weeds can really thrive on lower-dose applications of herbicide, producing more biomass and more seed according to research conducted by Dr Asad Asaduzzaman and Eric Koetz, examining weed populations with resistant and susceptible individuals to investigate this phenomenon further.
Eric, a weed research agronomist with NSW Department of Primary Industries and Regional Development, says sub-lethal applications are known to promote herbicide resistance in weed populations by selecting for individual plants which 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 which 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,” Eric explains.
“Research at the University of Queensland has also measured the hormesis effect in sowthistle treated with glyphosate,” he adds.
In the awnless barnyard grass (ABG) experiments, low-rate glyphosate applications (34g and 67.5g 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 the hormesis response exists naturally in ABG.
In contrast, the glyphosate-resistant ABG plants produced more biomass for longer, extending through to the reproductive stage.
Spikelet production peaked at doses of 100g to 540g 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,” Eric says.
“However, if glyphosate is used and the dose reaching 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 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 62.5g to 125g 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,” Asad says.
“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,” he says.
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 also 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,” Eric says.
Research by Dr Asad (CSU) with resistant tall fleabane shows the hormesis effect at low doses of paraquat (middle) compared to higher doses (right) and the nil-herbicide control (left).Picture: CSU
“Any practice leading to an increase in seed production is a genuine risk to long-term profitability and herbicide sustainability in cotton and grain production systems,” he says.
The WeedSmart Big 6 is an integrated weed management strategy applying 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 which includes herbicide and non-herbicide tactics where possible.
For more information about fence-line weed management, visit the WeedSmart website: www.weedsmart.org.au
10°C