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.
To become noticed, resistant plants have to become a significant part of the weed population. How quickly this happens depends on several factors, including rarity of the genes for resistance; whether plants which have these particular genes are ‘fit’, ie can reproduce and thrive; and on the ‘selection pressure’ from methods of weed control. Resistance arises quickly in weed species in which resistance genes are more common and do not affect normal growth and reproduction. When competing herbicide susceptible plants are wiped out they can quickly fill the niche left available.
The importance of selection pressure is well illustrated by glyphosate. More than 20 years after first sales there were no known cases of resistance. However, in 1996 the first cases began to appear. Typical situations have been in orchards and plantations where there has been long-term use of glyphosate, and in glyphosate resistant soybeans and cotton when several applications may have been made to a single crop.
Annual ryegrass is notorious for becoming resistant to herbicides. Producing a very large number of genetically diverse seeds every season, it is a relatively quick process to find resistant individual plants when put under heavy selection pressure. Relying on one herbicide mode of action to control ryegrass has caused problems in Australia in the past1. For instance, the grass weed
Mode of action
||First record of resistance
||No. of resistant biotypes
||No. of species
||No. of countries
|Triazine PSII inhibitors
The leaves are "narrow" as opposed to the "broad" leaves of broadleaved weeds. Also called 'monocots' having one seed leaf opposed to 'dicots' having two seed leaves.
Authoritative On-line References and Resources
The International Weed Science Society represents individual associations around the world. herbicide diclofop-methyl had selected populations of resistant ryegrass in cereals and canola less than four years after its introduction in the late 1970s. By 2000, there were estimated to be up to 5 million infested acres. Resistance to the sulfonylureas chlorimuron and metsulfuron-methyl was even quicker to establish. Annual ryegrass resistance to glyphosate was recorded after about 20 years of use, but paraquat resistant populations have taken nearly 50 years to be noticed.
Scientists who have studied the biochemistry and genetics of paraquat resistant weeds have discovered that the resistance mechanism is not one of the usual ones 2,3,4. Paraquat seems to be inactivated, possibly by tight binding as in soil, before reaching the site of action in chloroplasts. Others have suggested that resistant plants are less affected by the damaging free radicals generated by paraquat.
Why are there not more cases of paraquat resistance? This is unknown, but it could be because the genes for resistance are very rare, or because individual plants which possess them do not survive well in many cases. The fact that paraquat is generally not used as the only means of weed control certainly helps. Often, other herbicides are mixed with paraquat, or selective herbicides
First outbreaks of herbicide resistant annual ryegrass in Australia1
- 1982 diclofop-methyl: South Australia and Western Australia
- 1982 chlorosulfuron and metsulfuron-methyl: South Australia and Western Australia
- 1988 atrazine and simazine: South Australia and Western Australia
- 1996 glyphosate: Victoria
2010 paraquat: South Australia
A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops.
Authoritative On-line References and Resources
http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University. are used later in the growing crop. This is the crucial practical point in avoiding weed resistance to herbicides: integrate the use of different chemical and cultural approaches to control weeds. With a severe lack of new herbicide modes of action it is vital to effectively use and preserve those currently available. Paraquat's robust mode of action is invaluable to keeping weed resistance at bay.
- International Survey of Herbicide Resistant Weeds
- Fuerst, E P & Vaughn, K C (1990). Mechanisms of paraquat resistance. Weed Technology, 4, 150-156
- Powles S B & Howat, P D (1990). Herbicide-resistant weeds in Australia. Weed Technology, 4, (1), 178-185
- Qin Yu, Cairns A & Powles S B (2004). Paraquat resistance in a population of Lolium rigidum. Functional Plant Biology, 31, (3), 247 – 254
The brand name of the leading paraquat product is Gramoxone and the leading paraquat + diquat mixture in Australia is Spray.Seed.