Tackling herbicide resistant weeds with technology

A leading Australian weed expert has warned broadacre crop farmers and horticulturalists across southern Australia and WA to be aware of paraquat-resistant species such as annual ryegrass and fleabane.

Dr Peter Boutsalis is an Adelaide plant scientist and consultant with Plant Science Consulting, and the University of Adelaide.

He says in the last two years there have been sporadic detection of paraquat-resistant ryegrass (Lolium rigidum) but recently there have been more outbreaks, and he warned growers to be aware and to use other strategies to control them.

Ryegrass resistant to Glyphosate and Paraquat from Broadacre paddocks becoming more prevalent. We have detected at least 20 samples from all southern states. We first started detecting paraquat resistance about 5 years ago. @WeedSmartAU pic.twitter.com/DE2VjlLoap

— Peter Boutsalis (@PBoutsalis) August 2, 2023

Ryegrass is a vigorous weed which competes with crops, reduces yield, and adds cost to the farm input. The WA Department of Primary Industries also says annual ryegrass toxicity (ARGT) is an often fatal poisoning of livestock that consume ryegrass infected by the bacterium  Rathayibacter toxicus  (formerly known as Clavibacter toxicus).

Infected ryegrass remains toxic even when it has senesced and dried off. Hay made from toxic ryegrass will also be toxic. All grazing animals are susceptible, including horses and pigs.

Boutsalis says traditionally farmers in the past dealt with weeds by tilling the soil which often resulted in poor soil structure and dust storms removing the energy rich topsoil. New methods of minimum tillage conserves the soil structure and microbial activity by leaving crop stubble in place till next season.  

But as farmers throughout Australia have adopted minimum tillage practices, which leaves the top soil intact, they have come to rely on glyphosate to kill weeds.

Over time, when the weeds start to show signs of herbicide resistance, farmers use a different herbicide, usually paraquat, to directly target the small number of weeds which are resistant to glyphosate. Paraquat, like glyphosate, is a comparatively low cost option which Boutsalis says breaks down rapidly in the soil and leaves no residue.

But now there is evidence that in a small number of regions, the weeds are developing resistance to paraquat.

“If you let the herbicide resistant weeds grow, they are very competitive, reducing crop yields,” Boutsalis says. “And next season, there’ll be more resistant plants from the seeds that were set. So it’s in the farmers’ interest to try and control those.

“Farmers have been using this (paraquat) very successfully.

“We’ve only seen it (paraquat resistance) in 10 to 20 cases but that was the purpose of the tweet just to remind farmers that they shouldn’t only rely only on another herbicide, they have to think about other tactics as well.

“It usually occurs in higher rainfall areas like central and southern Victoria, Southern Western Australia. We haven’t seen any in South Australia just yet but that’s only a matter of time. They’re few and far between but nevertheless, it shows the power of a weed to develop resistance, through what is called selection pressure.

“We have seen resistant weeds also occurring in horticulture such as in vineyards and tree crops.”

Boutsalis urges farmers to include other weed control tactics whether they have resistance or not. The only way to prevent resistance is to use multiple tactics to lower the risk of selecting for glyphosate resistant weeds. This includes sending weeds to a lab to be tested for herbicide resistance so the farmers know which herbicides are still working.

“And the chance of resistance is greater when you’re spraying millions of seedlings rather than spraying, hundreds or 1000s in a field. The chances of those plants that have survived glyphosate being paraquat resistant is really low. Even so, we have now detected ryegrass with resistance to paraquat.

“But there are a lot of other herbicides that can use to control them, and they work in completely different ways with a different mode of action, and they will control ryegrass that’s resistant to glyphosate and paraquat as well.”

Boutsalis says the new products can be very expensive: “because it costs a great deal of money to develop a herbicide.”

Protecting the top soil (and saving money on fuel, labour and machinery) means farmers strive to reduce the number of times machinery travels over a paddock.

Innovative solutions are needed to deal with the issues.

Weeds are a persistent problem in all cropping areas, as illustrated by this social media post from Ben Boughton of Agriobs. The company specialises in agricultural satellite imagery, including of weed dispersion before and after herbicide spraying.

Seeing more of this, unfortunately. These milkies have survived a glyphosate + valor spray. pic.twitter.com/GTA4Atr6Gy

— Ben Boughton (@BenBoughton1) August 20, 2023

Boughton says the nature of the resistant weed population clustering enables satellite imagery to capture the weed density. “An individual weed would not be detected. Our farm management response would usually be to follow up with a selective camera sprayer with  different herbicide mode of action to kill the weed before it sets seed.”

The Single family of Coonamble, in the central west of NSW, is among those who have developed a drone to attack the problem. In their case their drone named “Singleshot” maps every weed in a paddock, and the map is then used to direct a spray boom to accurately pin-point the exact area the weed is found.

Robotics expoerts helped John Single and his sons Tony and Ben develop the weed detection sensor which they say is capable of mapping 200 hectares an hour, consistently picking up weeds as little as 4cm.

Tony won an agricultural award for his innovative approach.

Tony Single says controlling and eliminating weeds which have escaped from a herbicide application at very low numbers is vital to the continued effective use of herbicides in the agricultural industry. “The aim is to stop every weed from setting seed.”

South Australian company Erizon, used a drone to target weeds from the air over a 4.7hecatre plot in Penrith, and although its waiting for final data on weed destruction, it’s confident in its process.