Glyphosate resistant weeds

Glyphosate resistant weeds threaten the crop ... and glyphosate's future Glyphosate resistant weeds, once believed unlikely to occur, are now threatening not only cost-effective ways of controlling weeds, but also sustainable farming systems. Glyphosate has been called a ‘once-in-a-century’ herbicide because of its unique combination of high efficacy and low environmental impact¹. It has followed paraquat as a major enabling factor driving the expansion of no-till farming, replacing the need to plow.

No-till reduces soil erosion and increases the health and fertility of the soil. It provides habitats for pest predators and havens for wildlife. Less fuel is required to grow a crop and greenhouse gas emissions are reduced.

However, farmers who use glyphosate too often should heed the warnings of both weed resistance experts and fellow farmers who have experienced resistance problems. Australian Professor Steve Powles, director of the Western Australia Herbicide Resistance Initiative and one of the global leaders in weed science, has warned: “… glyphosate will be driven to redundancy in large parts of North America and South America, unless growers diversify weed control now”².

Paraquat, glyphosate and glufosinate have been the only commercially successful non-selective herbicides in the whole history of the chemical crop protection industry. Glyphosate is the only one to control perennial weeds. So it is essential to food security to ensure that glyphosate remains an effective option for the world’s farmers. This means understanding and overcoming the problem of glyphosate resistant weeds.

State of the problem

Introduced in 1974, glyphosate was free from weed resistance issues until 1995³. The first recorded case of resistance was in Lolium rigidum (annual ryegrass) in a field in Australia where glyphosate had been used repeatedly over 15 years.

Figure 1. Annual data on confirmed numbers of cases of glyphosate resistant weeds Soon afterwards, two events led to a huge increase in sales of what was already the biggest selling herbicide in the world by far. GM glyphosate tolerant crops were introduced in 1996 and glyphosate’s US patent expired in 2000. Since then, glyphosate resistant weeds have evolved in glyphosate tolerant crops including soybeans, cotton, canola and corn where glyphosate had often been the only herbicide used. The annual numbers of confirmed cases of glyphosate resistance are shown in Fig 1.³

The International Survey of Herbicide Resistant Weeds collates information on the subject and recognised 42 different species with glyphosate resistant biotypes in September 2018. The distribution of these around the world is presented in the following tables.

Why have glyphosate resistant weeds arisen?

To find solutions to the problem of glyphosate resistant weeds, the biochemistry of resistance and the ways in which resistant biotypes are selected need to be appreciated.

Biochemistry of resistance

Weeds can become resistant to herbicides by three common mechanisms⁴:

• 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 metabolized before it can exert its effects

Currently, examples of resistance to glyphosate by the first two means are known, but not the third. Resistance mechanisms identified in particular species are noted in Table 7.

Glyphosate blocks the biosynthesis of aromatic amino acids (phenylalanine, tryptophan, tyrosine) by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Knowledge gained about the genetics and biochemistry of this enzyme in the development of GM glyphosate tolerant crops has been applied to understanding resistant weeds. A common target site mutation in EPSPS is the substitution of the amino acid proline for serine at a key position in the molecule. This changes the shape of active site so that glyphosate can no longer bind effectively⁴. The normal catalytic function of EPSPS is unaffected, so that plants with this mutation are as ‘fit’ as the rest of the population.

Glyphosate is readily translocated from leaves to growing points of shoots, roots and rhizomes. This is fundamental to its ability to control perennial weeds. Most studies on resistant weeds showing reduced translocation have demonstrated that in these biotypes glyphosate is rapidly sequestered in cell vacuoles and unavailable for translocation⁴. A few others have indicated that glyphosate is poorly absorbed into the leaves of resistant plants⁴.

One other type of glyphosate resistance has been investigated, although the mechanism is unknown. In some glyphosate resistant populations of Ambrosia trifida, the leaves of treated plants rapidly desiccate and fall off, so limiting the amount translocated⁴.

Selection for resistant biotypes

Glyphosate had been widely used for 20 years before the first case of resistance was recorded. Given that resistance to herbicides such as sulfonylureas and ACC-ase inhibitors was soon widespread within a few years of their commercialisation, any mutated genes for resistance to glyphosate were thought to be very rare and individual plants possessing them likely to be poor competitors. In addition, having a unique mode of action and no soil residual activity means that the selection pressure from any one application is low and only applied to the cohort of weeds present at application. Later emerging flushes are unaffected.

Before the introduction of GM glyphosate tolerant crops, glyphosate was generally only used as one part of a diverse weed management system involving other control methods: mechanical, cultural and a wide variety of selective herbicides. Using different methods ensures that any weeds surviving glyphosate are killed by other means⁵. However, in situations where glyphosate is used several times in one crop, and to the exclusion of other means of weed control, the selection pressure is much greater. An illustration on what has happened in glyphosate tolerant cotton in the US is presented as a case study.

Solutions: How paraquat helps

Glyphosate resistant weeds have come to dominate certain populations because of intensive use of glyphosate for weed control. Including a diversity of approaches to weed control is essential to avoiding the shift to resistant biotypes. An integrated system is best, taking advantage of opportunities to rotate crops and use herbicides with different modes of action, as mixtures or sequential applications. Cultural techniques such as the use of cover crops can also help⁹.

Some of the ways in which paraquat is helping to fight glyphosate resistance can be discovered in these case studies: 

Paraquat protects glyphosate in Aussie Double Knock

Spraying paraquat in a ‘Double Knock’ system is a very effective way to prevent resistance to glyphosate developing. Survivors of glyphosate burndown are sprayed with paraquat up to two weeks later to hit them with two different modes of action …  

  
Paraquat is glyphosate’s bodyguard

In Brazil, glyphosate resistant weeds are estimated to infest three million hectares. To help ensure the benefits of glyphosate can be preserved, farmers continue to spray glyphosate for burndown, but follow just before or just after planting with a paraquat-based herbicide …

Paraquat fights glyphosate resistant weeds in US cotton

Glyphosate resistant weeds are a major headache for US cotton growers. Using paraquat adds to the all-important diversity of mode of action necessary for successful weed control programs… 

References

1. Duke, S O and Powles, S B (2008). Glyphosate: a once-in-a-century herbicide. Pest Management Science, 64, (4) 319-325
2. Agriculture.com (2010)
3. International Survey of Herbicide Resistant Weeds
4. Shaner, D L et al (2012). What have the mechanisms of resistance to glyphosate taught us? Pest Management Science, 68, (1), 3-9
5. Powles S B (2008). Evolved glyphosate–resistant weeds around the world: lessons to be learnt. Pest Management Science, 64, 360-365
6. US Department of Agriculture
7. Green, J M (2012). The benefits of herbicide-resistant crops. Pest Management Science, 68, (10), 1323 - 1331
8. Robertson, R (2010).  Palmer amaranth super weed.  South East Farm Press, 18 October 2010. 
9. Beckie, H J (2011). Herbicide-resistant weed management: focus on glyphosate. Pest Management Science, 67, (9),1037-104