New developments and understanding in resistance mechanisms and management

Author: Christopher Preston, University of Adelaide | Date: 28 Feb 2017

Take home messages

  • There are new weeds with resistance to paraquat, glyphosate and the Group I herbicides.
  • False negative results in resistance testing can occur due to inappropriate sampling or conditions at testing being different to those in the field.
  • Temperature affects the performance of many herbicides in Group A, Group C, as well as paraquat, glyphosate and glufosinate.
  • Understanding cross resistance patterns can help determine which herbicide products might still work.

Recent developments in herbicide resistance

The continual reliance on herbicides for weed control is inevitably resulting in the evolution of more herbicide resistance. This is complicating weed control, because other or additional tactics need to be brought in to manage the resistant weeds. Where there are several different weeds with resistance to different herbicides on the same farm, the complications increase.

The main new developments in herbicide resistance over the past 2 years have been the identification of new species with resistance to glyphosate, paraquat and 2,4-D. In addition, there has been the identification of resistance in annual ryegrass to Group J herbicides.

New species with resistance to glyphosate are sweet summer grass (Brachiaria eruciformis), feathertop Rhodes grass (Chloris virgata), red brome (Bromus rubens), common sowthistle (Sonchus oleraceus), prickly lettuce (Lactuca serriola) and tridax (Tridax procumbens). These have occurred in several situations including summer fallows, crops, road sides and orchards. It is clear that continued reliance solely on glyphosate in fallows, horticulture and road sides will lead to more glyphosate resistant weeds appearing.

Paraquat resistance has been identified in flaxleaf fleabane (Conyza bonariensis), crowsfoot grass (Eleusine indica), black nightshade (Solanum nigrum) and Pennsylvania cudweed (Gamochaeta pensylvanica) from horticulture and viticulture. We generally consider paraquat resistance harder to select for than glyphosate resistance, but this shows that over-reliance on paraquat will lead to resistance.

A concerning issue has been the identification of Group I resistance in common sowthistle (Sonchus olreaceus) and capeweed (Arctotheca calendula). These have come from intensive in-crop and fallow uses of Group I herbicides. Currently, common sowthistle has resistance to Group B, I and M only not all in the same population. These comprise the majority of the herbicides used for fallow weed control.

In 2015 resistance to Group J herbicides triallate (Avadex® Xtra) and prosulfocarb (Arcade® and also the main ingredient in Boxer Gold®) was identified in annual ryegrass. Groups D, J and K are the main pre-emergent herbicides for grass weeds and annual ryegrass now has resistance to two of these.

Temperature effects on herbicide resistance and implications for resistance testing

Prevailing environmental conditions often influence herbicide efficacy in the field. This can result in perceived herbicide failure in the field that is not the result of herbicide resistance. Additionally, the opposite can occur where resistance in the field can be tested with a false negative test result. Temperature is a key environmental factor that can affect herbicide performance as well as herbicide resistance test results. Temperature can influence many aspects of herbicide performance including absorption, translocation, metabolic degradation and the development of symptoms. It is the interrelationship between these effects and the herbicide resistance mechanism that determine whether temperature will affect test results.

Case study: Glyphosate resistance in barnyard grass

Temperature has a significant impact on the response to glyphosate of many herbicide resistant barnyard grass populations (Figure 1). At 20oC, most populations are less resistant to glyphosate than at 30oC. This has implications for both the use of glyphosate and testing for resistance. Glyphosate resistant barnyard grass populations are more likely to be controlled when prevailing conditions are mild than when conditions are hot.

Figure 1. Effect of higher temperature on response to glyphosate of resistance and susceptible barnyard grass populations. A) Dose response at 20oC; B) Dose response at 30oC. Almost all of the glyphosate resistant populations are more resistant to glyphosate at 30oC compared with 20oC.  Source: Nguyen et al. 2016.

Figure 1. Effect of higher temperature on response to glyphosate of resistance and susceptible barnyard grass populations. A) Dose response at 20oC; B) Dose response at 30oC. Almost all of the glyphosate resistant populations are more resistant to glyphosate at 30oC compared with 20oC.  Source: Nguyen et al. 2016.

More importantly, testing for resistance to glyphosate in barnyard grass can give conflicting results. For populations with lower levels of resistance, test results may indicate little or no resistance at the field rate if tested under cooler conditions. However, farmer experience in the field may be a complete failure of the herbicide.

There are several things going on in this case. As a general rule, glyphosate tends to be less effective under high temperature conditions. In part this is because glyphosate is one of those herbicides whose activity limits its own translocation. The more rapidly glyphosate symptoms appear, the more likely herbicide is to be trapped in the leaves, rather than being translocated around the plant. This means that while symptom appearance is more rapid under summer temperatures, plant kill is less complete. Also as temperatures get very high, absorption of glyphosate through the leaf cuticle reduces, leaving more of the herbicide outside the leaf. So in general, increasing glyphosate rates for summer applications will improve weed kill.

In the case of barnyard grass, this is then compounded by the mode of action of the herbicide and the resistance mechanism. It is the build-up of shikimate pathway intermediates that leads to death of plants, so any resistance mechanism that can keep the shikimate pathway operating is likely to lead to plant survival. This means that relatively weak resistance mechanisms, such as target site mutations, can be effective in these summer-growing weeds, as less glyphosate is getting to the target enzyme.

Case study: Clethodim resistance in annual ryegrass

Clethodim is frequently used to control established grass weeds, particularly annual ryegrass, in June to July. Conditions are often cool during this period and frost can be common. Resistance to clethodim is common in annual ryegrass and growers have increased the rate used to attempt to control resistant populations. Cold weather, and particularly frost, can dramatically change the performance of clethodim. Research looking at the impact of frost on clethodim performance showed that twice as much clethodim was required if frost had occurred in the 3 days prior to clethodim application on susceptible annual ryegrass (Figure 2). An even greater difference was observed for some clethodim resistant populations under frost conditions.

Clethodim is a herbicide that is markedly more effective under warmer conditions than under cooler conditions. This means that resistance testing undertaken during warm conditions could lead to false negative results. It also means that resistance populations in the field become much harder to control during extended cold periods. The current most common practice of using clethodim on tillered resistant grass weeds in the coldest part of winter does not play to its strengths. Avoiding frosty periods will improve control.

Figure 2. Effect of frost on response to clethodim of three resistant (A, B and C) and one susceptible (D) annual ryegrass populations. Plants were exposed to frost for 3 nights prior to clethodim application (FBS) or 3 nights after clethodim application (FAS) compared with no frost (NF). Some resistant populations became much more resistant with frost. Source: Saini et al. 2016.

Figure 2. Effect of frost on response to clethodim of three resistant (A, B and C) and one susceptible (D) annual ryegrass populations. Plants were exposed to frost for 3 nights prior to clethodim application (FBS) or 3 nights after clethodim application (FAS) compared with no frost (NF). Some resistant populations became much more resistant with frost. Source: Saini et al. 2016.

Rules of thumb

Quite a number of herbicides have varying performance at different ambient temperatures. Most herbicides do not work well if the temperature is too low, conditions are not that often encountered in Australia, except for night spraying in winter. Table 1, therefore provides rules of thumb that differentiate between average winter spraying and spring or summer spraying of herbicides. For example, Group A fop herbicides typically become less effective in summer compared to winter use. This means that more herbicide will be required for the same level of control. In contrast, the opposite tends to happen with Group A dim herbicides.

Where weed populations have low levels of resistance, these seasonal variations can create a disconnect between observations in the field and formal resistance testing. Most resistance testing only uses one or two herbicide rates, typically starting at the field rate. Testing in a season different to when the herbicide was applied can result in false negatives in these cases. Better results may be obtained through the whole plant Quick-Test™ than with a seed test.

Table 1. Rules of thumb for the effect of summer compared with winter application on herbicide efficacy on weeds

Herbicide Group

Example herbicide

Herbicide efficacy in summer compared with winter

A fops

Diclofop-methyl

Reduced

A dims

Clethodim

Increased

C

Atrazine

Increased

L

Paraquat

Reduced *

M

Glyphosate

Reduced

N

Glufosinate

Increased**

* While higher temperatures make Group L herbicides less effective, resistant populations also become less resistant at higher temperatures.

** However, efficacy of glufosinate is greatly reduced by conditions of low humidy.

Herbicide resistance mechanisms and cross resistance

In general resistance mechanisms can be divided into target site mechanisms and non-target site mechanisms. Target site mechanisms include point mutations in the target site protein that reduce herbicide binding or greatly increased amounts of the target site protein that soak up the herbicide. Non-target site mechanisms include everything else and usually these mechanisms reduce the amount of herbicide reaching the target site. Commonly increased detoxification of the herbicide is involved, but reduced herbicide absorption, reduced herbicide translocation or herbicide sequestration can also be involved.

Target site mutations often give some level of resistance to other herbicides of the same mode of action, but no resistance to herbicides of other modes of action. Non-target site mutations can also give some level of resistance to other herbicides of the same mode of action, although it is often less extensive than for target site mutations, and can provide unexpected resistance to herbicides of other modes of action.

There is frequently confusion that assumes high levels of resistance are due to target site resistance and low-levels due to non-target site resistance. This is true of some modes of action (Group C resistance for example), but not others. Understanding common resistance mechanisms can be useful for determining rules of thumb for using herbicides. In annual ryegrass, target site resistance to Group A fop herbicides was common. The most common mutations gave no or low resistance to dim herbicides. This allowed dims, and particularly clethodim, to be used after resistance to fops had occurred. However, in other grass weeds different patterns emerge. In wild oats/black oats (Avena spp.) non-target site resistance to Group A herbicides is common, giving cross-resistance to Mataven®, but meaning Targa®, VerdictTM and Axial® may still work despite resistance to Topik®.

These differences result from a combination of selection history and the biology of the weed species. Where weeds are polyploid (more than one genome) it is more difficult for high level target site resistance to evolve as more than one point mutation is required. This tilts the balance of selection pressure in favour of non-target site mechanisms. Feathertop Rhodes grass (Chloris virgata), a diploid species, has evolved resistance to glyphosate through target site point mutations, whereas its close relative windmill grass (Chloris truncata), a tetraploid species, has evolved resistance through gene amplification. In addition, different herbicides within the same mode of action may be preferred for use against different weeds, also altering the selection pressure.

While rules of thumb can be developed for resistance to herbicides in some modes of action, these are likely to be somewhat species specific. Herbicide resistance testing is the only way to work out which herbicides might still work on any one population. Even then, it is important to remember that there might be more than one resistance mechanism present in a field and test accordingly.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

References

Nguyen, T.H., Malone, J.M., Boutsalis, P., Shirley, N. and Preston, C. 2016. Temperature influences the level of glyphosate resistance in barnyard grass (Echinochloa colona). Pest Management Science 72: 1031-1039.

Saini, R.K., Malone, J., Preston, C. and Gill, G.S. 2016. Frost reduces clethodim efficacy in clethodim-resistant rigid ryegrass (Lolium rigidum) populations. Weed Science 64: 207-215.

Contact details

Dr Christopher Preston
University of Adelaide
Ph: 0488 404 120
Email: christopher.preston@adelaide.edu.au

® Registered trademark
TM Trademark