North America

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.
Non-selective herbicides such as paraquat are essential to the efficient conservation of water for use by crops in dry regions. Low rainfall during the growing season means that in these regions crops rely mainly on stored soil moisture.
Ecofallow is a system of soil management developed in the dry mid- and western states of the US and Canada to conserve moisture throughout the crop rotation.
The ecofallow system (also known as ‘chem fallow’) is based on no-till cultivation and herbicides to ensure weeds do not deplete the reserves of soil moisture being accumulated during the fallow (uncropped) period between crops.1,2
Crops are thirsty
As a general rule, one litre of water has been used to produce one calorie of food. Considering average requirements for adults are 2000 – 3000 calories per day, this fact emphasises the importance of water to food security. In regions where rainfall is low and irrigation is not feasible, crop yields are restricted by insufficient water.
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
Volunteer plants – those that grow from seed shed by the previous crop – are weeds that bring the same problems as wild ones, or worse.
Volunteers can form a ‘green bridge’ from one crop to the next carrying insect pests and fungal diseases. Volunteers reduce yields and quality, and hinder crop management. They can be difficult to control, especially if they are growing in a new crop of the same species.
Options for controlling volunteers before planting the next crop can be particularly limited if they carry a herbicide tolerance trait to glyphosate, glufosinate or both. However, recently reported research has shown that paraquat boosted by the addition of a PSII inhibitor herbicide provides a good option to control volunteer corn (maize) before a new crop is planted1. More information about PSII inhibitors and herbicide modes of action can be found here.
Volunteer problems
Concerns that 2012 could be a bad year for volunteer corn were raised in the southeast US last year when lodged crops and late harvests from flooded fields pointed to significant grain loss2. This season there have, indeed, been reports of widespread problems in the US3,4.
Maize (corn) along with wheat and rice is one of the world’s top crops. Maize provides not only the fast-foods of western society - breakfast cereals, sweet corn and popcorn – but also the staple foods for much of the world’s population in developing countries where it is used to make porridge, bread and tortillas. All around the world, maize grain is a basic livestock feed, and the crop can be cut while still green to make silage as a winter feed. Also, over recent years maize has been increasingly used as a feedstock for the production of bioethanol.
Maize, perhaps more than any other crop, reaches both high and low extremes of sophistication, mechanisation and technology in crop production. All farmers, however, need to maximise the yield and quality of their produce, while saving the costs, time and labor needed to grow it. Protecting maize from weeds, pests and diseases is essential to avoiding heavy losses in yields and quality of grain. Weed control is usually most important. Paraquat is a non-selective herbicide which, when used in integrated weed management systems, can provide the solution to weed problems.
As maize is grown so widely, and often intensively, its production can create significant environmental impact.
US crop scientists are excited about the potential to protect soil in fields where corn is grown for biofuel production by the use of perennial cover crops suppressed by paraquat.
In the near future, the food versus fuel dilemma will be addressed by using ‘waste’ crop residues instead of grain as the feedstock for bioethanol. However, the concern is that removal of stover and chaff normally returned to the soil will cause serious erosion and fertility problems.
Iowa State University has conducted a three-year project that has demonstrated how carefully managed perennial cover crops can more than substitute for the biomass harvested for ethanol production1. The key to success is the use of paraquat and its fast, contact-only action. Paraquat is used to burn down the cover crop in spring, but because it is not translocated and is adsorbed by soil, the cover crop recovers to protect soil after harvest and over winter.
Second generation biofuels
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
Paraquat is in the front-line of US cotton growers’ defenses against glyphosate resistant Palmer amaranth, often described as a ‘superweed’.
This season an advanced design spray hood will enable weeds up to four feet tall (120 cm) to be controlled by paraquat, while shielding late growth-stage cotton from any leaf scorching. The hood has been designed for use at all growth-stages up until ‘layby’ (when the rows close).
Externally the hood is streamlined to glide easily between cotton rows without knocking fruit off more advanced plants.  Inside, the hood has been designed to maximise spray coverage. As the sprayer moves over the crop, weeds are guided into the hood and knocked down by a horizontal bar before being sprayed from three nozzles, positioned above and to each side1.
Conservation tillage
Glyphosate resistant weeds, like Palmer amaranth (Amaranthus palmeri, also known as Palmer pigweed), threaten not only the productivity and profitability of infested crops, but also the future of sustainable farming systems. Glyphosate, described as a “once in a century” innovation2, has been a major factor in the increased adoption of conservation tillage systems, especially no-till, because it enabled virtually all weeds to be controlled without plowing.
Farmers in the US have been aware for some time of the threat glyphosate resistant weeds pose to their crops and livelihoods. Now, the public is becoming more aware too after recent media attention following the publication of the US National Research Council’s report: Impact of Genetically Engineered Crops on Farm Sustainability in the United States1. The report confirmed the substantial economic and environmental benefits of GM crops, but warned that care was needed to preserve the value of the technology, especially with the threat posed by glyphosate resistant weeds. Farmers growing herbicide resistant crops must ensure that a diverse range of agronomic practices are used to control weeds and must not simply rely on one herbicide mode of action.
Unfortunately, for many growers the simplicity of glyphosate-based weed control is hard to resist – if they are yet to experience any problems. However, they should heed the warnings of both weed resistance experts and fellow farmers. Australian Professor Steve Powles, one of the global leaders in weed science, has warned that “… glyphosate will be driven to redundancy in large parts of North America and South America, unless growers diversify weed control now.”2