Weed resistance

Paraquat is one of the most widely used herbicides in the world, marketed globally under the brand name Gramoxone by Syngenta. A list of other brand names under which paraquat is sold can be found here.
Paraquat is used in nearly 90 countries either to prepare the land for planting or for controlling weeds in more than 100 crops, including major food crops: corn, rice, soya, wheat, potatoes; major fruits: apples, oranges, bananas; beverages: coffee, tea, cocoa; and processed crops: cotton, oil palm, sugarcane, and rubber.
In this section you can find information about crops for which paraquat is an essential production tool and a selection of case studies illustrating the benefits from using paraquat in some very different cropping systems.
The crop reviews describe the crop, where it is grown and what it is used for; crop production methods and weed, insect pest and disease problems; and the key benefits from using paraquat. They are summarised here and more extensive articles can be found in the Knowledge Bank.
The case studies demonstrate why paraquat is an essential tool for use in sustainable agriculture, and one which many farmers around the world rely upon for their livelihoods and to support their families.

Paraquat has been a valuable tool for farmers all over the world for nearly 50 years. Its speed of action, rainfastness and now its role as a weed resistance fighter are unsurpassed.
However, paraquat’s importance and benefits go beyond simply weed control. Its unique property of being locked onto soil so tightly on contact, and the consequent biological inactivation, mean that it has become a key component in conservation tillage systems, especially no-till.
Paraquat plus reduced tillage improves soil structure and fertility, timeliness of operations, biodiversity and the overall profitability of farming. It is also unique as a non-selective herbicide in being able to be sprayed between the rows of growing crops without fear of crop damage.
In this section you can find out about the specific benefits paraquat brings on the farm, for the environment and in rural communities.
Key benefits of paraquat
On the farm For the environment In rural communities Controls weeds
Reduces [no-glossary] soil erosion
Creates opportunities
Acts fast
Increases soil organic matter
In 2008 the problem of weeds that have become resistant to glyphosate has continued to increase.
New outbreaks have been reported in Australia, Brazil, Paraquay and the USA, including one species previously unrecorded as being resistant1. Urochloa panicoides (liverseedgrass) was found in New South Wales, Australia infesting grain sorghum and wheat crops.
In addition, populations of Chenopodium album (lambsquarters, fat hen) with less than usual sensitivity to glyphosate have been noted in US soybean fields2.
Glyphosate has been called a ‘once-in-a-century’ herbicide because of its unique combination of high efficacy and low impact3. Its popularity ironically now threatens its key role in farming because of weed resistance, and a more diverse approach to managing weeds has been called for by leading weed scientists. This would include using cultural techniques and ensuring different herbicide modes of action are employed in an integrated approach to weed management.
Problems continue in 2008
Trends over recent years suggest that there will be more outbreaks of resistant weeds recorded before the year is out1. In 2006, 15 outbreaks were logged and in 2007 there were nine. These past couple of seasons have seen cases reported which well illustrate the general scope and scale of the problem.
Prior to the recent confirmation of glyphosate resistance in awnless barnyard grass (Echinochloa colona) in northern New South Wales (Australia), only annual ryegrass (Lolium rigidum) had been confirmed resistant to glyphosate in Australia.
A report dated 21 June 2007 showed in some tests with barnyard grass seeds and their progeny from a paddock sample from Belatta (NSW), 3 weeks after treatment, an almost total plant kill in non resistant weeds and high survival rates in glyphosate-resistant populations.
Four more weeds have been identified as at risk of developing resistance to glyphosate: wild oat (Avena fatua and Avena ludoviciana), sowthistle (Sonchus oleraceus) flaxleaf fleabane (Conyza bonariensis) and liverseed grass (Urochloa panicoides).
 Three common factors determined the high risk category for these weeds: Each produce large quantities of seed, resulting in large populations;
They have a history of herbicide resistance somewhere in the world;
They occur in minimum tillage or no-till farming systems where they are exposed to multiple applications of glyphosate.
The risk assessment was conducted as part of the Northern Glyphosate Resistance Project, by members of Weeds CRC, NSW Department of Primary Industries and Queensland Department of Primary Industries and Fisheries. 
Over the years in many areas of the United States, certain weeds have become resistant to some herbicides used, most recently to glyphosate since Roundup Ready crops have been grown on an increasing massive scale.
Horseweed / marestail (Conyza canadensis) is native to the United States, but it has recently become much more problematic to control in Roundup Ready soybean production. Horseweed is well adapted to no-till crop production. It has developed resistance to several herbicides including glyphosate. Horseweed was the first broadleaved weed documented to develop resistance to glyphosate in the United States. Since this initial report, glyphosate resistance has been reported in several other states.
Dr L. Steckel is a row-crop weed specialist in Tennessee, USA. He has statewide extension responsibility as a weed specialist for all row crops. His research programme focuses upon the study of the biology and management of weeds that are troublesome to Tennessee row crop producers.
He already alerted about horseweed and palmer pigweed (Amaranthus palmeri) resistance in 2006 in an article published by Delta Farmpress.
Glyphosate resistance was first documented in Australia in 1996 at a site in Victoria. Since then, there have been a number of other sites reporting glyphosate resistance. So far, only annual ryegrass populations had been reported with glyphosate resistance in Australia. The Glyphosate Resistance Register contains information regarding all the known glyphosate-resistant weed populations present in Australia. The register is managed by Dr Chris Preston of the Plant & Food Science department at the University of Adelaide.
In a news alert dated January 29, 2007 the Department of Primary Industries (DPI) has warned one of the state's most prolific weeds has developed resistance to glyphosate.
The herbicide is one of the last weapons farmers are able to use in the control of barnyard grass (Echinochloa colona), which competes with summer crops and fallows. One plant can produce up to 42,000 seeds. The mature plant grows taller than rice and competes with the crop for resources like sunlight and soil nutrients. It is also an alternate host for tungro (RTD) and rice yellow dwarf (RYD) viruses as well as for corn pests.
Paraquat is playing an increasingly important role in Integrated Weed Management (IWM) programs associated with glyphosate-resistant weeds and in other situations where glyphosate has been intensively used for long periods of time.
An article in AgriNews on 7 February 2006, highlighted rising concerns about the rapid increase of crops resistant to glyphosate and cited Purdue University weed scientist Bill Johnson as saying:
“The development of glyphosate resistance in weeds will be slowed dramatically – and, in turn, may not be important for at least another decade – if producers start instituting resistance-prevention strategies now.”
Resistance-prevention strategies are mentioned in the article including utilizing alternative modes of action on the worst weeds and when rotating crops.
The Resistance Fighter™ code, developed in the United States, for effective management is simple:  212  - no more than two applications of glyphosate on one field over a two-year period.
Paraquat’s unique mode of action makes it an ideal tool in an ongoing resistance-prevention strategy. Paraquat acts in the presence of light to desiccate the green parts of all plants with which it comes into contact. For more information on paraquat’s unique mode of action, view The Science of Paraquat .
What is Paraquat?
Paraquat is a herbicide (chemical weed killer) used to control a very broad range of weeds (unwanted plants) in more than 100 crops, including cereals, oilseeds, fruit and vegetables, growing in all climates. Weeds shade crops, take their water and nutrients, and make harvesting difficult. The leading manufacturer of paraquat is Syngenta, which (as ICI) developed the active ingredient (AI) in the early 1960’s. Since then, paraquat has made possible many innovations in sustainable farming systems, based on its simplification of crop production by effectively controlling weeds and, in doing so, removing the need for ploughing to bury them. This has freed up farmers’ time and also helped care for the soil. Paraquat is one of the most widely used herbicides in the world. It has been approved for use by authorities in nearly 90 countries. When used as recommended, paraquat is effective and safe to users, consumers and the environment.
Key facts about the safe and effective use of paraquat are noted below. This fact sheet also contains a list of referenced scientific papers and other publications and a summary of technical information.
Why Farmers Use Paraquat
Spraying paraquat lets millions of farmers grow better crops more easily, while respecting the environment. Paraquat has a unique set of characteristics: