Weed resistance

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 palmerii) 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, sold as brands such as Basta and Liberty. Glufosinate is a foliar herbicide, slower acting than paraquat, but faster than glyphosate. It is the herbicide used in LibertyLink GM cropping systems.
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.

How do herbicides work?

Knowing how a herbicide works in detail - its 'mode of action' (MOA) - is important to understanding how to use it most effectively. Herbicide MOA is a major factor in weed control spectrum, crop selectivity and weed resistance.
Herbicides control weeds by interfering with how they grow. Different MOAs all ultimately either stop seeds from germinating or establishing as seedlings; prevent plants from making essential carbohydrates, proteins or lipids (oils and fats); or desiccate leaves and stems.
Paraquat’s MOA involves diverting the flow of energy captured from sunlight in photosynthesis to produce highly reactive free radicals which destroy cell membranes to quickly desiccate leaves. This happens within hours in bright sunlight because of the high levels of energy running out of control. Almost all green plants are affected by paraquat making it a broad-spectrum, non-selective herbicide.

Mode of action: how herbicides work

Introduction
Herbicides control weeds by interfering with how they grow. This is achieved by a number of different ‘modes of action’ (MOA) which all either ultimately stop seeds from germinating or establishing as seedlings; prevent plants from making essential carbohydrates, proteins or lipids (oils and fats); or desiccate leaves and stems. Knowing a herbicide MOA is important to understanding how to use a herbicide most effectively. It is a major factor in both herbicide selectivity and weed resistance.
The symptoms observed on weeds sprayed with herbicides express the MOA. In herbicide R&D, when new chemicals are screened experts carefully observe the detail and timing of the appearance of symptoms to gain clues as to the MOA. Fully understanding a MOA may take years of research by plant physiologists, biochemists, molecular biologists and many other scientific disciplines. The precise MOA of paraquat is very well understood - for more information, click here.
The more that is known about MOA, the more safely and effectively herbicides can be used. There are even examples of pharmaceuticals being developed as a result of research into herbicide MOA . For example, the drug nitisinone has now replaced the need for a liver transplant as the first-line response to type 1 hereditary tyrosinaemia, a rare metabolic disorder in children.

Weeds and weed control

Paraquat is used to control a huge range of weeds worldwide, but to control weeds effectively and sustainably it is important to understand them.
Why does a plant become a weed? How can different types of weeds be described? What are the features of weeds and the way they grow which can be targeted by herbicides for successful control?

Integrated weed management in no-till systems

Integrated weed management and no-till are advanced agronomic tools with common aims to improve efficiency and profitabilty, while reducing the environmental impact of crop production. Although advanced in concept, these tools are straightforward and can be adapted for use in all cropping systems, from highly mechanised ones to subsistence farming, all around the world.
Tillage is a well proven means of controlling weeds, so are other methods good enough to use in an integrated approach to weed management in no-till systems? This article examines how farmers can reap the rewards of both techniques together.

Farmers around the world know just how hard it is to control weeds. They tend to come back with a vengance, especially when the many elements causing weed problems have not been appreciated and addressed. Aiming to manage weeds rather than control them is not only more realistic, but if Integrated Weed Management (IWM) is applied properly, it can reduce costs, protect the soil, and support pest and disease control.
No-till systems also provide economic and environmental advantages. However, in no-till, the traditional means of weed management by ploughing to prepare a field for cropping is not used.

No-till and biofuel crops

“America is addicted to oil” as President George W. Bush acknowledged in his 2006 State of the Union Speech. And, it is not just a US problem, nor is the addiction only to oil. Oil, coal and natural gas are the fossil reserves which power our planet, but now the spotlight is on crop biomass to provide a significant alternative source of energy and materials.
No-till farming and paraquat have a vital role to play in producing enough biomass while sustaining food production and protecting the environment.
At present, biofuels are manufactured from the parts of crops otherwise harvested for food, eg grain. This leads to two problems:

Not enough fuel
Potentially not enough food

The yield of fuel – biodiesel or bioethanol – from the oils or starch found in seeds is relatively low. With the economic and environmental motivation to grow more crops for biofuels, in future, they may take up valuable land that should be used for growing food, especially in poor Third World countries. Already, in Mexico the rising price of corn tortillas, a staple food for many poorer people, has been a problem. This has been due to the higher price of US corn, driven-up by the demand for ethanol.
To address both fuel and food issues, it would be much more attractive to use unharvested parts such as corn stover or wheat straw for biofuel production.