Weed resistance

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.
The US Cotton Belt stretches across 17 southern states, from Virginia and the Carolinas in the east to California, Arizona and New Mexico in the west.1
States where the largest areas are grown include Texas, Georgia and Arkansas. Cotton varieties genetically modified to be tolerant to glyphosate were first planted in the US in 1997, following GM soybeans, which were introduced one year earlier.2 This article looks at the impact that changes in weed control regimes have had and the advantages of using paraquat, drawing on work by researchers at the University of Arkansas who have been monitoring the advance of glyphosate resistant weeds in cotton for many years. Recently, a paper was published in the leading scientific journal Pest Management Science reviewing their research findings and discussing the impact of glyphosate resistant weeds and future control strategies.3
Weed control in US cotton
Cotton is a poor competitor to weeds and is grown in climates where there are multiple flushes of new weed germination, especially when irrigated.
Officially, there are now 31 weed species around the world confirmed as having biotypes resistant to glyphosate. Over the past four years, new cases of glyphosate resistant populations have been confirmed in Argentina, Australia, Brazil, Canada, Greece, Italy, Japan, Mexico, New Zealand, Portugal and the USA. This is, no doubt, just the tip of iceberg, because once a species has several resistant populations in a region, a new one is no longer a surprise and the emphasis shifts from confirming new outbreaks to fighting the problem. 
First confirmations of glyphosate resistant populations of six new species have been added to the list of resistant species maintained by the International Survey of Herbicide Resistant Weeds1 since the last update by The Paraquat Information Center (Table 1).
Table 1. New species of glyphosate resistant weeds confirmed since 2011.
Weed species
Amaranthus quitensis 
Mucronate pigweed 
Amaranthus spinosus 
Spiny amaranth
Bidens pilosa 
Using paraquat is an increasingly important part of successful peanut production. It has three key advantages: it controls a broad spectrum of weeds; it can be used selectively to keep crops weed-free during the critical early growth period; and, it is effective against herbicide resistant weeds.
Peanuts are low-growing crops that do not compete well with weeds. Growers must use weed management strategies to keep their crops free from weeds until the rows meet and close the canopy if they are to achieve good yields. Unfortunately, US growers, in particular, now face the additional challenge of weeds that have become resistant to many of the herbicides at their disposal.
Weed control options
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.
Paraquat has an important role in fighting the increasing problem of glyphosate resistant weeds. These now threaten not only cost-effective weed control in many crops, but also the future of sustainable farming systems.
An in-depth article on the topic of glyphosate resistant weeds has been added to the Paraquat Information Center’s Knowledge Bank. You can read this article here.
Why glyphosate is important
Glyphosate is by far the world’s most widely used crop protection chemical and has been called a ‘once-in-a-century’ herbicide because of its unique combination of high efficacy and low environmental impact1. Like paraquat, glyphosate controls a very broad spectrum of weeds and is deactivated in soil. Glyphosate is especially important because it controls perennial weeds. These properties mean that glyphosate has been instrumental in the increasing adoption of no-till and other conservation tillage systems. Chemical weed control means that fields do not need to be plowed. This has many benefits, including reducing soil erosion, improving soil health and fertility, and reducing greenhouse gas emissions. 
Glyphosate resistance issues
The introduction of glyphosate tolerant (GT) crops has been the one of the most significant technological changes in agriculture, along with mechanization and hybrid seed breeding. Farmers around the world are reaping the benefits of a simplified weed control system, saving costs, time and labor and increasing operational flexibility. Adoption has been rapid, particularly in the Americas. In the most important soybean growing US states, Brazil and Argentina around 90% of crops are GT. Globally 148 million ha were sown to GM crops across 29 countries in 2010 of which close to 90 million ha had a herbicide tolerance trait1. Three crops dominate this technology adoption, soybeans, cotton and corn.
The technology has also spurred-on growers to further simplify their farming by reducing plowing and taking-up conservation tillage practices such as no-till. Abandoning the plow enhances soil quality and reduces erosion, while increasing biodiversity.
Paraquat has been found to be an effective ‘fire fighting’ treatment for large glyphosate resistant Palmer amaranth, sometimes called a ‘superweed’. Researchers at the University of Georgia, USA, have found that applying paraquat through weed wipers and similar implements can very effectively kill Palmer amaranth plants up to 5ft (1.50 m) tall1. While paraquat is very effective on smaller weeds, glyphosate’s systemic action generally makes it a better option on large weeds. However, where there are glyphosate resistant weeds an innovative approach is needed. Paraquat has been approved for application through weed wipers to control weeds in peanuts in Florida, Georgia, and in North and South Carolina.
Since 2005, Palmer amaranth (Amaranthus palmeri, also known as Palmer pigweed) has become an increasing problem for farmers in Georgia and neighboring states2,3. It has become common for gangs of laborers to be used to remove weeds with hoes and machetes often at a cost of over $100 per acre ($250/ha).
Benefits of using paraquat through weed wipers Prevents spread of glyphosate resistant weeds
More effective application of late fungicides
Easier harvesting
Much cheaper than hand-weeding
Malaysian agriculture has had a wake-up call to the threat posed by herbicide resistant weeds. Paraquat has an essential role to play in avoiding this potentially huge problem. A report by the Paraquat Information Center of the discovery of populations the noxious weed goosegrass (Eleusine indica) resistant to the non-selective herbicide glufosinate has now been confirmed by publication in a scientific journal1.
A vegetable farmer in Malacca state and planters at an oil palm nursery in Pahang state had suspected that glufosinate was failing to control goosegrass. Subsequent investigations have now confirmed that one population shows a two-fold resistance and the other an eight-fold resistance.
These are the first cases of weed resistance to glufosinate to be recorded anywhere in the world after more than 25 years of use. This is reminiscent of the fall of glyphosate to the first resistant weeds in the mid 1990’s. Escalating use of glyphosate over a similar period ultimately resulted in resistance. The second weed species to have confirmed glyphosate resistance was also goosegrass in Malaysia2. Even paraquat has had a few isolated populations become resistant, but overall it has had a very good record with respect to weed resistance.
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.
The recognised authority for recording all outbreaks of weed resistance (www.weedscience.org) currently states that there are 25 weed species with a total of 43 different paraquat resistant biotypes in 13 countries. These figures include the recent observation of a resistant population of annual ryegrass (Lolium rigidum) in South Australia.
Not until 18 years after commercial use began was the first patch of paraquat resistant weeds recorded.
When major classes of herbicides are compared, the progress of weed resistance to paraquat has been unusually slow. The table and chart below illustrate how resistance to some herbicides has been observed very quickly. Resistant plants have rare genes which counter the normal susceptibility in a given species: usually by producing enzymes which eliminate foreign molecules or they have biochemical variations at the site of herbicide action which render them insensitive. Mode of action First sales
First record of resistance
No. of resistant biotypes
No. of species
No. of countries
Australian researchers have found that spraying paraquat in a ‘Double Knock’ system is a very effective way to restrict the evolution of glyphosate resistant weeds. Survivors of glyphosate burndown are sprayed with paraquat up to two weeks later. The Double Knock ensures that weeds are hit with two different modes of action.
The Double Knock system has been modified over the years, with different options being developed, but each retains the principle of diversity in weed control.  It was pioneered when paraquat was first introduced as a burndown herbicide in Australia. A spray was followed by soil cultivation. Nowadays, with the extensive use of glyphosate and the popularity of no-till, an alternative option is for the Double Knock to be given as glyphosate followed by paraquat. The system works best when glyphosate is given a little time to translocate through weeds to the roots; when paraquat is applied while weeds are still green; and when early rains encourage a new flush of germination1.