Herbicide resistant weeds are causing widespread concern for farmers on the United States’ arable producing great plains, and have knocked confidence in the world’s most widely used herbicide, glyphosate.

In Kansas, palmer amaranth and other species of ‘pigweed’ that are fast growing and can shrug off applications of post-emergence herbicide once they grow above around 10 cm tall are a major nuisance in soybeans this year. Palmer amaranth plants can reach over six foot in height and experts from University extension programmes across the Midwest have warned that the pigweed seed bank in arable soils could cause problems for years to come.

What is more, the resistant weeds seem to be expanding their range. Growers in the country have been advised to keep a close watch on their fields to try and halt the weeds’ spread.

This has become such a high profile problem that the US House of Representatives will discuss the issue later in the year.

The high pressure from resistant weeds this year has led to renewed discussions on strategies for coping with these ‘superweeds’. Famously resistant to glyphosate (sold by Monsanto as RoundUp) there has been debate over the last few years as to whether the increasing use of genetically-modified herbicide-resistant crops and has exacerbated the problem or whether tough weeds have exposed the limitations of current management practices.

In response to the pressures, major seed houses and ag-chem companies have slated the release of new chemistry, combining glyphosate with 2,4-D (Dow) and Dicamba (Monsanto). Companies have also promised new varieties of soybeans with stacked resistance to more herbicides, though the shifts to these varieties will be dependent on approval from the authorities in the United States’ trading partners.

However, some experts have said that evidence of weeds showing resistance to multiple herbicides mean this strategy may not buy farmers much time. Kansas State University weed scientist Dallas Peterson told Reuters this week that the combinations of active ingredients in new herbicides still struggled to control palmer amaranth in tests conducted by the university

Three years ago, a paper by Professor Charles Benbrook of Washington State University revealed that herbicide use in the production of three crops (cotton, soybeans and corn) in the United States had risen by 183 million Kilograms since 1996. Prof Benbrook put this increase down to the emergence of resistant weeds.

Prof Benbrook said at the time, “Resistant weeds have become a major problem for many farmers reliant on [GM] crops, and are now driving up the volume of herbicide needed each year by about 25 percent.”