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Practical and economic consequences of herbicide resistance

The first cases of herbicide-resistant plants started to appear by the 1970s [1], and have become increasingly common since then. This problem is mainly caused by the widespread and unregulated use of herbicides that leads to the evolution of complex resistance mechanisms in plants [2] (Image 1). Interestingly, resistance is often reported in areas where genetically-engineered herbicide-resistant crops are commonly grown. Herbicide-resistant crops (e.g. glyphosate) provide benefits such as avoiding crop rotation, reducing managerial intensity, adopting low- or no-till practices, and narrow-row spacing [3]. However, in very particular cases resistance to herbicides can also be transferred (via gene flow) from these crops to weeds, further amplifying the issue [2].

Image 1. The natural selection process of herbicide-resistant plants. Credits: AgroTex Global 2021


In many European countries, herbicide-resistant weeds, commonly grass species such as ryegrass and blackgrass, infest many cereal crops. In Europe, almost 9 million ha of wheat and rapeseed cropping systems are infested with resistant weeds, 0.2 million ha in the case of maize, 0.14 million ha in rice and more than 0.22 million ha in perennial crops [1].

The global economical cost of weed herbicide resistance is estimated to be about €215 per ha in the UK (Image 2) [4]. Similarly, weed scientists in the US have estimated that, depending on region and crop, farmers struggling with glyphosate-resistant weeds may incur increased costs in some cases of up to €230/ha, primarily due to the necessity of applying various herbicides with different modes of action [3]. In Canada, herbicide resistance costs producers up to € 1 billion annually due to both increased herbicide use and decreased yield and quality.

Image 2. The impact of black-grass herbicide resistance. Credits: Rothamsted Research 2019.


The pressure on herbicide users has caused the adoption of pre-emergence herbicides, along with greater use of other practices such as deep tillage, to reduce the chances of the emergence of resistance and decrease the number of dormant seeds in the soil. Nonetheless, these practices may increase soil vulnerability to erosion, carbon dioxide emissions and pesticide leaching [1].


As monocultures and widespread use of a few herbicides continue, resistance cases are only likely to increase in the near future. However, since there is no ‘silver bullet’ to tackle this problem, it is necessary to promote and implement diversity into cropping systems through the adoption of integrated weed management programs. These programs encourage alternative strategies such as 1) innovative planting and cultivation tools, such as equipment to minimize soil disturbance; 2) diversity in crop rotation/cultivation, such as the inclusion of summer crops to disrupt winter weeds; 3) adoption of cover crops to improve soil structure and drainage; 4) favour more competitive crop varieties; and 5) enhancing herbicides' performance in various ways (i.e. improved application methods, regulating application conditions and design of bioherbicides that can reduce the chances of resistance) [5].

Using such strategies may reduce farmers' net profitability by 4-24%, but that’s not nearly the cost of resistance development in the long term [1]. In fact, it was estimated in the UK that implementing some of these strategies represented half the cost of managing resistant weeds and reduced yields (Image 2) [4]. Likewise, in North America, economic models have shown that resistance management reduces profits in the first year of implementation but increases profits in the second and subsequent years. For corn and soybean, resistance management pays for itself by the second year [6].

Commonly, weed control decisions are heavily influenced by economics and utilize systems that favour easily measured near-term economic returns. Although proactive resistance management will come at a short-term cost, it will certainly enhance long-term profits and benefits to the environment.


Green PRAXIS investigates alternatives to manage vegetation, including weed control, using novel bioherbicides, chosen seeding and other innovative techniques.

We are currently testing these solutions, both in the laboratory and in situ, stay tuned to read more about our findings!



References

  1. Peterson, M. A., Collavo, A., Ovejero, R., Shivrain, V., & Walsh, M. J. (2018). The challenge of herbicide resistance around the world: a current summary. Pest Management Science, 74(10), 2246-2259. DOI: 10.1002/ps.4821.

  2. Délye, C., Jasieniuk, M., & Le Corre, V. (2013). Deciphering the evolution of herbicide resistance in weeds. Trends in Genetics, 29(11), 649-658. DOI: 10.1016/j.tig.2013.06.001

  3. Carpenter, J. E., & Gianessi, L. P. (2010). Economic impacts of glyphosate-resistant weeds. Glyphosate Resistance in Crops and Weeds: History, Development, and Management, 297-312. DOI: 10.1002/9780470634394.ch16

  4. Orson, J.H. (1999). The cost to the farmer of herbicide resistance. Weed Technology, 13, 607-611.

  5. Harker, K. N., & O'Donovan, J. T. (2013). Recent weed control, weed management, and integrated weed management. Weed Technology, 27(1), 1-11. DOI: 10.1614/WT-D-12-00109.1

  6. Livingston, M., Fernandez-Cornejo, J., & Frisvold, G. (2016). Economic returns to herbicide resistance management in the short and long run: the role of neighbour effects. Weed Science, 64 (S1), 595-608. DOI:10.1614/WS-D-15-00047.1


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