Weed resistance has become a major issue for crop production, threating global food security.
The root of the problem is the difficulty in finding new herbicide modes of action.
The International Survey of Herbicide Resistant Weeds collates information from researchers worldwide on the spread of weed resistance. Today resistance is present in more than 250 weed species infesting nearly 100 crops and affecting 23 of the 26 known herbicide modes of action
Frequent application of a herbicide or herbicides with the same mode of action inevitably exerts selection pressure on weed populations. If surviving weeds are fit enough to reproduce and establish a population then resistance arises.
Weeds can become resistant to herbicides by several mechanisms:
- Target site mutations: the active site on the enzyme inhibited by the herbicide does not allow a good fit
- Translocation and/or sequestration mutations: the herbicide does not reach the site of action
- Enhanced metabolism mutations: the herbicide is metabolised before it can exert its effects
- Over-expression of the target enzyme necessitating a higher concentration of herbicide to be effective
Cross-resistance occurs when a single resistance mechanism confers resistance to more than one herbicide. The most common type is target-site cross-resistance and is the result of an altered target site conferring resistance to other herbicides that inhibit the same enzyme in the same way at that target site. Multiple resistance occurs when more than one resistance mechanism occurs within an individual plant. This is usually the result of sequential selection of resistance mechanisms by herbicides with different sites of action or through accumulation of resistance genes via pollen flow.
Resistance to important modes of action
The first well-documented case of herbicide resistance, triazine-resistant Senecio vulgaris, was in 1968. The progress of recorded cases of unique populations for three leading modes of action is shown in the chart.
Resistance to ALS inhibitors (e.g. sulfonylureas) and ACC-ase inhibitors (mainly ‘fop’ and ‘dim’ classes of grass herbicides) arose only a few years after they were commercialised. Glyphosate, however, had been very widely used for over twenty years before the first case of resistance was observed. Experts had generally believed that glyphosate's mode of action was exceptionally robust and that it would not follow the path of other herbicides.
This was proved to be utterly wrong after the introduction of GM crops tolerant to glyphosate allowed it to be used as a selective herbicide. Due to the technology’s incredible success and the massive increase in the use of glyphosate, the selection pressure for resistant individuals was vastly increased.
Strategies to avoid weed resistance involve a multi-facted approach, integrating different methods, chemical and cultural. However, herbicides with new modes of action are essential. New herbicide chemistry from existing modes of action may give some advantages such as time of application window, or use across more crops, but will not overcome resistance.
References and further reading
Herbicide Resistance Action Committee (HRAC): the industry body monitoring and coordinating action against herbicide resistant weeds.
International Survey of Herbicide Resistant Weeds: records cases of herbicide resistance around the world. These cases have been officially verified by universities, but there are undoubtedly many more.
Peterson, M A et al. (2017). The challenge of herbicide resistance around the world: a current summary. Pest Management Science, 74, (10), 2246-2259.
Science (May 2018 special issue): The rise of resistance.