A report released in scientific journal Weed Science on Wednesday has suggested one of the world’s most widely used herbicides has been losing its effectiveness at combating herbicide resistant ‘superweeds’ which are causing crop losses across a swathe of the United States.

Superweeds are a growing problem in the United States in particular, where 10 million acres (40,000 km²) across 13 states are known to be affected. The growing problem is thought to have been exacerbated by the use of herbicide-resistant genetically modified crops, which do not require targeted spraying of herbicides and mean farmers can plant larger acreages of the same crop.

Food writer Michael Pollan explains, “Technologies don’t operate in isolation; they’re contextual, and the context of the GM crops we have today is monocultures. They are designed specifically to allow monocultures to get even bigger - more corn and soy – all the money goes into more corn and soy. [These crops] are basically band aids on monocultures.”   

Agchem companies have said that the rapid appearance of herbicide resistant weeds is the result of misuse of agricultural chemicals. They blame the emergence of weeds resistant to commonly used preparations on farmers who have not planted herbicide-tolerant crops in rotation, as recommended, meaning fields, and the species within them, are subjected to “intense selection pressure in the form of repeated use of a single herbicide.”

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Nevertheless, the rapid spread of such weeds has caused panic within the industry. Resistance was first reported to widely used herbicide glyphosate, which led agribusinesses to develop GM crops tolerant of both glyphosate and other chemicals, including 2,4-D. Dow AgroSciences is currently seeking approval to release 2,4-D and glyphosate-resistant crops onto the American market.

However, sceptics have suggested this drive could further increase resistance, and that other methods, including integrated weed management need to be adopted to tackle the growing problem. Critics suggest that observing crop rotations, planting break crops and introducing grazing, mechanical weed control and mulches would all reduce arable farming’s environmental impact and prove effective regardless of pesticide resistance.

Marcia Ishii-Eiteman, a senior scientist at North America’s Pesticide Action Network, said earlier this month that, “After 15 years of commercialization, and millions of dollars in publicly funded research, the agricultural biotech industry has yet to deliver on their promises. Rather than reducing the need for hazardous pesticides, herbicide-resistant seeds have driven a massive increase in herbicide use that has been linked to significant environmental and public health concerns.” She cited a study by the US-based Organic Center that showed GM crops were treated with 26 per cent more chemicals than conventional varieties in 2008.

This week’s report in Weed Science revealed waterhemp resistant to 2,4-D, commonly manufactured by the agrochemical arm of Olympic sponsor Dow Chemical, has been discovered in Nebraska. The report’s authors found that after ten years of treatment, waterhemp was no longer effectively controlled by 2,4-D; the highest doses of the chemical used on the site proved insufficient to control 50 per cent of the weed population, according to the Weed Science report. Greenhouse testing using seed from the Nebraska field site and a control population of waterhemp confirmed the Nebraska plant’s resistance.

2,4-D was developed during World War Two and is thought to be one of, if not the, most commonly used herbicides worldwide. The chemical, which kills broadleaf plants but is less effective on grasses, is licensed for use on crops in the EU. However, pesticide use figures from the UK show its use is declining on cereals, though it remains widely used in orchards and on pastureland.  

Earlier this month in the UK, campaign group GM Freeze stated that, “Weeds resistant to 2,4-D have already been recorded in North and South America, Europe Australia and Asia.” The group claimed Weed Science’s own International Survey of herbicide Resistant Weeds, reveals the number of known 2,4-D resistant species rocketed between 1990, when 13 species were known to be resistant, and today, when 31 cases of resistance have been reported.

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