Weed resistance

Glyphosate resistant weeds, once believed unlikely to occur, are now threatening not only cost-effective ways of controlling weeds, but also sustainable farming systems. Glyphosate has been called a ‘once-in-a-century’ herbicide because of its unique combination of high efficacy and low environmental impact¹. It has followed paraquat as a major enabling factor driving the expansion of no-till farming, replacing the need to plow.
No-till reduces soil erosion and increases the health and fertility of the soil. It provides habitats for pest predators and havens for wildlife. Less fuel is required to grow a crop and greenhouse gas emissions are reduced.
However, farmers who use glyphosate too often should heed the warnings of both weed resistance experts and fellow farmers who have experienced resistance problems. Australian Professor Steve Powles, director of the Western Australia Herbicide Resistance Initiative and one of the global leaders in weed science, has warned: “… glyphosate will be driven to redundancy in large parts of North America and South America, unless growers diversify weed control now”².
Paraquat, glyphosate and glufosinate have been the only commercially successful non-selective herbicides in the whole history of the chemical crop protection industry.

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 states^2,3. 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.

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.

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.