Read time: 4 minutes

Resistance amplifies glyphosate + 2,4-D tank mix survival rate

Take resistance into account when planning tank mixes.

You can listen to this article being read above!

WeedSmart extension agronomist, Peter Newman is urging growers to think twice before mixing glyphosate with 2,4-D when treating weeds with low to medium levels of resistance to glyphosate.

Antagonism between the two products has been widely known for some time and in most circumstances careful product choice and a slight rate adjustment is all that is required to compensate for the compromised performance of glyphosate.

New research from the Australian Herbicide Resistance Initiative (AHRI) has shown that high label rates of glyphosate can often control grass weeds with low level glyphosate resistance; but adding 2,4-D amine or ester to the glyphosate can result in these weeds surviving the spray.

Once low to medium glyphosate resistance has established in a population of awnless barnyard grass, using a tank mix of glyphosate and 2,4-D is not going to work.

With investment from the Grains Research and Development Corporation and others, AHRI researcher Jingbo Li and others studied two populations of awnless barnyard grass with relatively low-level glyphosate resistance and found this phenomenon is due to the 2,4-D dramatically affecting uptake and translocation of glyphosate from the tank mix.

“For awnless barnyard grass a susceptible population was 100 per cent controlled using 0.5 L/ha glyphosate 540 while 11 per cent survived when the same rate of glyphosate was mixed with 1 L/ha of 2,4-D amine 700,” says Peter. “For the low-level resistant population, a higher rate of 1 L/ha of glyphosate was required to achieve 100 per cent control but when this rate of glyphosate was mixed with the 1 L/ha of 2,4-D amine, 90 per cent of the weeds survived. A similar result was found using 2,4-D ester.”

Survival of awnless barnyard grass seedlings with low level resistance to glyphosate. Left: Zero survival from 1 L/ha Glyphosate 540 application. Right: 85 per cent survival to 1 L/ha Glyphosate 540 + 1.03 L/ha 2,4-D Ester 680 mixture.

In another, more resistant, population of awnless barnyard grass the same scenario played out, albeit with an even higher rate of 3 L/ha of glyphosate to achieve 100 per cent control. In this population the survival rate was 77 per cent for the tank mix.

“What this means for growers is that once glyphosate resistance has established in a grass weed population, using this particular tank mix is not going to work,” says Peter. “A grower with glyphosate resistant grass weeds would be better served by applying the higher rate of glyphosate on its own, or perhaps with a different mixing partner, to achieve maximum control. It is then necessary to look at building in additional tactics to keep weed numbers low into the future with less reliance on glyphosate.”

While not examined in this study, 2,4-D antagonism of glyphosate is reported on several other species including Johnson grass, wheat, barley and wild oats. 2,4-D is also reported as antagonistic of Group A herbicides on species such as wild oats and annual ryegrass. Although mixing these two herbicides can provide a valuable multi-shot control of both grass and broadleaf weeds, the pros and cons need to be carefully evaluated.

“The other thing to remember is that the maximum level of control when using glyphosate is achieved when the best formulation is applied to young weeds at higher label rates,” says Peter. “These factors are generally within the grower’s control, even if they cannot control the weather conditions or plant stress levels, which also impact on glyphosate efficacy.”

To keep glyphosate as a viable option into the future Peter also recommends applying a double knock tactic after each application of glyphosate. He says following glyphosate with paraquat has been an effective double knock for many years but there are other options to consider, including strategic tillage and alternative herbicides.

Other resources

Related Articles

Related Articles

View all
Case Study
Case Study

Slarke family, Lake Grace WA

The Slarke family implement an integrated weed management program to minimise the impact of herbicide resistance on their farming business. Read More...
Article
News

Efficient farm layout benefits weed control

Their block farming layout allows the Slarkes to complete major operations like seeding, spraying and harvesting very efficiently. Read More...
Article
News

Don’t let weeds call the shots in your business

With good advice and the WeedSmart Big 6 checklist, you will soon be on track with an integrated plan to keep weed numbers low. Read More...

Webinars

View all
Video
Webinar

Combating resistant annual ryegrass in northern farming systems

In this webinar, we discuss the practical strategies to prevent and manage incursions of resistant annual ryegrass populations in northern cropping systems. Read More...
Video
Webinar

The fate of herbicide residues in soil – why it matters and what research is telling us

Learn about the effect herbicide residues may have on soil microbial activity and on the establishment and growth of crops following the fallow, even after the plant back period. Read More...
Video
Webinar

Combat Velocity® resistant wild radish with the WeedSmart Big 6

AHRI researcher Dr Roberto Busi explains in this webinar how he discovered Velocity® resistant wild radish populations in two paddocks in the northern Wheatbelt of Western Australia. Read More...

Videos

View all
Video
Webinar

Combating resistant annual ryegrass in northern farming systems

In this webinar, we discuss the practical strategies to prevent and manage incursions of resistant annual ryegrass populations in northern cropping systems. Read More...
Video
Webinar

The fate of herbicide residues in soil – why it matters and what research is telling us

Learn about the effect herbicide residues may have on soil microbial activity and on the establishment and growth of crops following the fallow, even after the plant back period. Read More...
Video
Webinar

Combat Velocity® resistant wild radish with the WeedSmart Big 6

AHRI researcher Dr Roberto Busi explains in this webinar how he discovered Velocity® resistant wild radish populations in two paddocks in the northern Wheatbelt of Western Australia. Read More...

Factsheets

View all
Fact Sheet

Understanding post-emergent herbicide use

Understand how post-emergent herbicides work and how resistance evolves and is expressed, in order to maximise herbicide performance. Read More...
Fact Sheet

Pulse width modulation (PWM) sprayers

Pulse width modulation sprayers can provide more consistent performance, individual section control and turn compensation. Read More...
Fact Sheet

Soil behaviour of pre-emergent herbicides in Australian farming systems

A reference manual covering the factors influencing the performance and break down of pre-emergent herbicides in soil. Read More...

Subscribe to the WeedSmart Newsletter