Weed resistance

More modes of action needed to support glyphosate tolerant crops

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.
However, relying heavily on a single herbicide mode of action to control weeds, especially in no-till, encourages the evolution of resistant populations. Although glyphosate had been widely used for more than 20 years before the first isolated cases of resistance from areas of heavy use appeared, the further expansion of its use in GT crops soon led to the appearance of many more outbreaks.

Paraquat wipes out superweeds

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

Weed wipers have been developed to apply glyphosate to tall weeds standing above crop canopies.

Malaysian weeds need mixed modes of action

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.

Weeds resistant to paraquat are rare

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.

Paraquat protects glyphosate in Aussie Double Knock

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.
The world’s first case of weed resistance to glyphosate was recorded in Australia in 1996 when a population of annual ryegrass (Lolium rigidum) in Victoria failed to be controlled2. By 2010, confirmed cases of glyphosate resistance in annual ryegrass had reached 981.

US growers must fight glyphosate resistance

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
Palmer pigweed (Amaranthus palmeri) is a huge problem in southern US states.

Paraquat is glyphosate's bodyguard

A paraquat-based herbicide has become an essential land preparation tool for Brazilian farmers to stem the spread of glyphosate resistant weeds.
The extensive adoption of GM soybean varieties tolerant to glyphosate has led to farmers using this non-selective herbicide for weed control too much and too often. Although glyphosate is encouraging the continued adoption of no-till, with all the benefits to soil conservation that brings, excessive use is also encouraging glyphosate resistant weeds1. Up to three million hectares in Brazil are now estimated to be infested with five weed species which are no longer controlled by glyphosate.
However, an integrated weed control system has been developed to ensure the benefits of glyphosate can be preserved. This involves continuing to spray glyphosate for burndown, but following just before or just after planting the crop with an application of a paraquat-based herbicide. This contains a second active ingredient, diuron. Diuron is a soil residual herbicide, but at the low rates used in this product it has little or no residual effect, but enhances the activity of paraquat.
Like paraquat, diuron works by interfering with photosynthesis, but in a different way. So-called PSII inhibitors like diuron slow photosynthesis.  (Read more about herbicide mode of action in the Knowledge Bank).

Dilemma for Brazilian farmers

Soybean growers in Brazil are facing a dilemma. More land under no-till is saving its soil and reducing energy use, but many no-till farmers are now finding that glyphosate resistant weeds are threatening future success.
No-till cropping systems have now been adopted on around 70% of cultivated land in the country, particularly for soybeans. Brazil and the US lead the world in no-till farming, but this is threatened by the rise of glyphosate resistant weeds. A recent article highlighted how worried American farmers are becoming (read more).
This is the first of two features looking at the problem in Brazil – and a success story involving a paraquat-based herbicide.
No-till and GM crops
Soybean production without soil cover is estimated to cause the loss of 55 million tonnes of topsoil in Brazil every year1. No-till cropping has been very successful in curtailing soil erosion, with its roots in the 1970s when the British company ICI began experimenting with the use of paraquat for weed control. Later, glyphosate was introduced and, with its advantage of controlling perennial weeds, has been used increasingly, year-on-year, for three decades.

First case of glufosinate resistance recorded

The first weed species to develop resistance to the non-selective herbicide glufosinate has been recorded by researchers in Malaysia.
Preliminary experiments have confirmed concerns that an aggressive grass weed is developing populations which are no longer controlled by glufosinate.
Weed scientists from the University of Malaya have been investigating reports of weed control problems in an oil palm nursery in the state of Pahang. The weed in question is goosegrass (Eleusine indica), a globally important weed of many warm climate crops.
In Malaysia it is a particularly serious problem in oil palm and rubber plantations, and on smallholdings growing fruit and vegetables.
Goosegrass has already developed resistance to several herbicide modes of action (MOA) in a number of countries around the world. It was the second weed species to be recorded as becoming resistant to glyphosate, also in Malaysia1. To keep weed resistance at bay, and avert the threat it poses to food production, it is vital to use integrated weed management practices. These involve using herbicides with different MOA. Like paraquat and glyphosate, glufosinate has a very distinctive MOA.

Paraquat fights glyphosate resistant Palmer amaranth

This season has seen a redoubling of efforts to fight the spread of glyphosate resistant Palmer amaranth (Amaranthus palmeri) in US crops. The key to resistance management is to use herbicides which have different modes of action. When it comes to achieving a broad-spectrum effect like glyphosate, the choice of alternatives is extremely limited. Paraquat’s unique mode of action1 together with its broad-spectrum weed control and fast action, make it an invaluable tool in the fight against weed resistance to glyphosate.
Palmer amaranth is one of several important weed species in the same genus (Amaranthus) commonly known as pigweeds or waterhemps. It is a major problem in many southern states, particularly in Georgia, where it is considered to be the number one weed problem in cotton.
Weed scientists at the University of Georgia estimate that an average of just two Palmer amaranth plants in every 6 m (about 20 feet) length of cotton row can reduce yield by at least 23%. Furthermore, when a single plant can produce an alarming 450,000 seeds, you can bet the problem will be worse the next season.
Already, as much as 400,000 ha (one million acres) in Georgia and South Carolina are estimated to have become infested with glyphosate resistant Palmer amaranth since it was first noticed in 2005.