US farmers rely on paraquat to manage glyphosate resistant weeds

Glyphosate tolerant pigweed growing in soybeans in Mississippi, USA Paraquat is the non-selective herbicide that powered the growth of no-till farming, especially in the US and other countries in the Americas.

Its unique properties, especially its very broad spectrum of weed control, contact-only action and excellent rainfastness also led to its wide use in many other weed control sectors.

Now it plays a vital role in managing weed resistance.

Spread of glyphosate resistant weeds

US farmers are being severely challenged by something once thought to be highly unlikely: glyphosate resistant weeds. A population of rigid ryegrass (Lolium rigidum) in a Californian orchard was the first recorded case of glyphosate resistance in the US in 1998¹. Since then, eighteen different resistant species have been recognised and the problem has spread across the country¹.

In recent years, weed resistance to glyphosate^2,3 has meant that many growers in the US have returned to using paraquat to combat this increasingly serious threat to food production and their livelihoods. This has been in response to the development of diversified approaches to managing weed resistance, which include the use of herbicides with different modes of action⁴. For example, a survey of cotton growers in Georgia showed that the use of paraquat as a burndown treatment before planting cotton had doubled over the five years before and after the discovery of glyphosate resistant Palmer amaranth (Amaranthus palmeri) in 2006⁵.

Pigweeds, especially Palmer amaranth, are generally considered to be the most serious and tenacious weeds with widespread resistance to glyphosate. A survey of more than 500 fields conducted across ten corn and soybean growing states in the Midwest in 2016 revealed that 77% of the Palmer amaranth or waterhemp (Amaranthus tuberculatus) populations sampled were resistant to glyphosate⁶.

Paraquat controls weeds glyphosate now misses

Paraquat is a very effective burndown herbicide for controlling Palmer amaranth. For example, greenhouse experiments showed that low rates gave 100% control of 10 cm tall plants, regardless of whether they were from populations susceptible or resistant to glyphosate⁷. Results from a field trial in Kansas conducted in wheat stubble on glyphosate-resistant Palmer amaranth showed that the recommended rate of paraquat provided complete control⁸. Similarly good control by paraquat was reported from Tennessee, where seed return to the soil was completely prevented⁹. University extension services such as Mississippi State¹⁰ and North Carolina¹¹ recommend the use of paraquat to rapidly control emerged Palmer amaranth when preparing fields for planting cotton and other crops.

The first case of glyphosate resistant weeds in field crops in the US was resistant marestail (Conyza canadensis, also known as horseweed) found in soybeans in the state of Delaware in 2000¹. Ohio State University has recommended paraquat as a replacement for glyphosate in burndown programmes (e.g. in mixture with 2,4-D and optionally metribuzin) to control marestail¹². Further south, the University of Arkansas has pointed out the efficacy of paraquat for controlling marestail before planting cotton, corn or soybeans, with the additional benefit that seed can be planted immediately after spraying¹³.

Another important weed species with glyphosate resistance issues is kochia (Kochia scoparia), which primarily infests cereal crops in the Great Plains. In Montana, paraquat has been recommended as a very effective herbicide for controlling glyphosate resistant kochia with 90 – 95% control expected three weeks after treatment¹⁴. In addition, this was noted as being one of the most cost-effective herbicide options. In Nebraska, kochia and Palmer amaranth in winter wheat stubble are controlled by paraquat, increasing spray volume to deal with larger plants or thicker stands¹⁵.

When new herbicide modes of action are hard to find, US farmers are fortunate to still be able to rely on the fast and reliable broad spectrum weed control from paraquat, used in a strategic approach to combatting glyphosate resistant weeds.

References

1. International Survey of Herbicide Resistant Weeds
2. Heap, I and Duke, S O (2018). Overview of glyphosate-resistant weeds worldwide. Pest Management Science, 74, (5), 1040-1049
3. Livingston, M et al. (2015). The economics of glyphosate resistance management in corn and soybean production. US Dept. Agriculture Economic Research Service report 184
4. Norsworthy, J K et al. (2012). Reducing the risks of herbicide resistance: best management practices and recommendations. Weed Science, 60, 31-62
5. Sosnoski, L M and Culpepper, S A (2014). Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) increases herbicide use, tillage, and hand-weeding in Georgia cotton. Weed Science, 62, 393:402
6. Bissonnette, S (2017). University of Illinois Plant Clinic Herbicide Resistance Report
7. Chandi, A et al. (2013). Interference and control of glyphosate-resistant and -susceptible Palmer amaranth (Amaranthus palmeri) populations under greenhouse conditions. Weed Science, 61, 259-266
8. Peterson D E et al. (2017). Alternatives to glyphosate for Palmer amaranth control in wheat stubble. Kansas Agricultural Experiment Station Research Reports, 3, (6), article 23
9. Crow W D et al. (2015). Evaluation of post-harvest herbicide applications for seed prevention of glyphosate-resistant Palmer amaranth (Amaranthus palmeri). Weed Technology, 29, (3), 405-411
10. Bond, J (2017). Control Palmer amaranth early. Mississippi State University Extension
11. Cahoon, C (2018). Planning for 2019: pre-emergence herbicides. North Carolina State Extension
12. Loux, M (2017). Rethinking Gramoxone at a reduced price. Corn Newsletter. Ohio State University Extension
13. Barber, T (2018). Horseweed causing problems in some areas. University of Arkansas Research and Extension
14. Preshant, J et al. (2013). Glyphosate-resistant kochia in Montana: herbicide recommendations and best management practices for growers. Montana State University Extension
15. Klein, R (2018). Post-harvest weed control in winter wheat. University of Nebraska-Lincoln Institute of Agriculture and Natural Resources