The spread of multiple herbicide-resistant weeds across the Midwest of the United States has reached dramatic levels, according to the 2016 University of Illinois Plant Clinic herbicide resistance report. For the report published in January, the scientists analysed samples from 10 states across the Midwest. In the 2016 planting season, 593 field samples representing approximately 2,000 waterhemp or palmer amaranth plants were tested for herbicide resistance. 76,8 per cent of these samples (456) were resistant to glyphosate, the active ingredient in Monsanto’s herbicide Roundup. Around 62.5% of the weed samples (371) tested positive for a resistance to PPO inhibitors, herbicides which kill weeds by destroying cell membranes. Almost half of all samples (291) tested by the at the University of Illinois Plant Clinic showed resistance to both PPO inhibitor herbicides and glyphosate herbicides. “Fields with plants that are positive for both glyphosate and PPO inhibitor resistance are of particular concern, due to the limited possibilities for control of these weeds,” said plant diagnostic outreach Extension specialist Diane Plewa.

The majority of samples tested came from Illinois. Of the 378 samples, 74 per cent were resistant to glyphosate and 64.5 per cent to PPO inhibitors. 48 per cent were resistant to both glyphosate and PPO inhibitors. The scientists received samples from 52 counties in Illinois that had at least one sampled field with waterhemp or palmer amaranth plants that tested resistant to both glyphosate and PPO inhibitors. The researchers said that until the 2016 season, palmer amaranth in Illinois was not even known to be resistant to PPO inhibitors, but as the results show this is no longer the case.

Resistant weeds have become a major problem for farmers in the US. According to figures by the United States Department of Agriculture, 70 million acres of American farmland were infested with glyphosate resistant weeds in 2013, double the area in 2009. The Union of Concerned Scientists (UCS), a US-based nonprofit science advocacy organization, says glyphosate-resistant weeds have arisen largely due to the overuse of this herbicide in fields of crops genetically engineered to resist it. When farmers use Roundup exclusively, resistance develops more quickly. The organisation says the widespread use of glyphosate also led to the neglect of other weed control measures, encouraging farmers to abandon a range of practices that had been part of their weed control strategy. UCS also blames an outdated system of farming called monoculture that relies on planting huge acreages of the same crop year after year. They say this system has provided especially good habitat for weeds and pests and accelerated the development of resistance.

UCS says there are solutions to avert the looming superweed crisis for farmers and the environment. “Farmers can control weeds using practices grounded in the science of agroecology, including crop rotation, cover crops, judicious tillage, the use of manure and compost instead of synthetic fertilizers, and taking advantage of the weed-suppressing chemicals that some crops produce,” they wrote in a policy brief in 2013. “Such practices have benefits beyond weed control: they increase soil fertility and water-holding capacity, reduce water pollution and global warming emissions, and make the farm and its surroundings more welcoming to pollinators and other beneficial organisms.” (ab)