‘Are weeds winning the war against herbicide resistance?’ This question was posed in a recent article in the journal Scientific American.1 In a similar vein, a special edition of the peer-reviewed journal Science has addressed the alarming issue of resistant pathogens and pests in human health and agriculture.2
Here, we review the problem of tackling resistant weeds and explore how paraquat can help.
Weeds no longer controlled by herbicides that were once very effective have been known for decades, but the problem has been rapidly intensifying as global food production relies on fewer and older herbicide active ingredients. In certain US states, the point has been reached where some of the most high-tech growers in the world are having to abandon trying to control the aggressive weed Palmer amaranth (Amaranthus palmeri) with glyphosate and resort to hand weeding or using flame throwers.1
Origins of the problem
No new herbicide modes of action have been commercialised since the 1980s and most of the products introduced since that time come from just a few classes of chemistry. This lack of diversity has lead to serious problems of weed resistance worldwide. When consistently faced with the same mode of action, resistant plants may evolve in weed populations and become established through natural selection.
Factors influencing how quickly weeds become resistant include the frequency of genes for resistance; whether plants that have these particular genes are ‘fit’, i.e. can reproduce and thrive; and the ‘selection pressure’ from methods of weed control (e.g. whether a herbicide is active in the soil for significant period). When weed populations susceptible to a herbicide are reduced in size and competitiveness, populations of a resistant biotype can quickly fill the niche left available.
Moreover, the past two decades have seen glyphosate dominate approaches to weed control in many regions following the introduction of glyphosate tolerant crops. Weed resistance to glyphosate was once thought to be unlikely, and it took more than 20 years to appear, but ultimately the sheer intensity of use proved these predictions wrong.
Scale of the problem
Weed resistance is a serious issue for many of the most widely used herbicides including ACC-ase inhibitors, ALS-inhibitors, glyphosate and PSII inhibitors.3 The progress of the rise of resistant weed biotypes (i.e. distinct populations of a particular species) for these herbicides and paraquat are shown in Figure 1.
One example illustrating the severity of the problem comes from a recent study in the US that looked at the presence of glyphosate resistance in roadside populations of palmer amaranth as they spread across the Mississippi Delta.4 This weed species was found in about 40% of 500 randomly chosen sampling sites and 73% of populations were resistant to glyphosate. A further 20% showed reduced susceptibility. Moreover, some 95% of populations were resistant to glyphosate and pyrithiobac, another herbicide commonly used in this area where crops include cotton, rice and soybeans.
More information on glyphosate resistant weeds can be found here.
Paraquat resistant weeds
Paraquat has been one of the world’s most widely used herbicides since 1962, but over all that time and all those acres of crop and non-crop land, relatively few cases of resistant weeds have been recorded, as indicated in Figure 1.
Why are there not more cases of paraquat resistance? This is unknown, but it could be because the genes for resistance are very rare, or because individual plants that possess them do not survive well in many cases. The fact that paraquat is generally not used as the only means of weed control certainly helps. Often, other herbicides are mixed with paraquat, or selective herbicides are used later in the growing crop, so that weeds are faced with more than one herbicide mode of action.
Fighting back against weed resistance
Best management practices to reduce the threat of herbicide resistant weeds include using diverse elements in the weed management regime that integrate chemical, biological, mechanical and cultural methods.
Multiple herbicide modes of action should be employed in the most effective herbicide programme possible. Paraquat is a good choice for the basis of such a programme to prepare land for planting into clean seedbeds.
Crop rotation should be practiced to change the timing of cultivations (e.g. coincident with the germination of different weed species) and use of selective herbicides.
Care should be taken to avoid the dispersal of weed seed or vegetative propagules within and between fields, especially when harvesting crops.
Finally, prevention is better than cure and regular monitoring of weed populations is important so that swift action can be taken should problems arise.
1. Borel, B (2018). Are weeds winning the war against herbicide resistance? Scientific American, 18 June 2018 /
2. Science (May 2018 special issue). The rise of resistance.
4. Bagavathiannan, M V and Norsworthy, J K (2016). Multiple-herbicide resistance is widespread in roadside palmer amaranth populations. PLOS, published: April 12, 2016 https://doi.org/10.1371/journal.pone.0148748