FARGO – Cheap is not sustainable. Low commodity prices will tempt farmers to cut back on weed control, but it won't work to rely on a single mode of action for multiple years, said Jason Norsworthy, a University of Arkansas weed scientist, speaking recently at a Regional Weed Resistance Conference in Fargo.
"If a weed program lacks diversity, you will quickly develop resistance," Norsworthy said. "And it's nothing specific to the Roundup ready (glyphosate) program."
Farmers need a minimum of two effective modes of herbicide action in a soybean field in a given year, and then perhaps rotate to other modes for different crops the following year, said Norsworthy, who is part of a U.S. Department of Agriculture project that has engaged sociologists and agricultural economists on how to get through to farmers dealing with resistance.
Biological controls – not chemicals – likely are the "the future of weed control," but those breakthroughs likely won't come for another 15 to 20 years. Farmers have to be proactive to protect herbicides they have now, and the ones coming through the pipeline.
One of the latest solutions is dicamba, and colleagues have confirmed dicamba resistance in kochia in a greenhouse after only three growth generations.
The rate of glyphosate-resistant weeds spread continues to rise, Norsworthy said. Looking to 2020, scientists project 164.5 million acres with Roundup resistant weeds, and on the "vast majority of U.S. crop acres, at that point, we've had the loss of the world's greatest herbicide."
The best strategy for weed control is to create a quick crop canopy that shades the soil surface. The proper weed size for most postemergence herbicides is 2 to 4 inches. Spraying larger weeds or shaving application rates to save money expands the population of resistance.
Norsworthy said his own research shows 99 percent of waterhemp and Palmer amaranth weed seed goes through a combine at harvest. "If it goes through that combine, what are we doing with that seed? We're turning around and spreading it across our field," he said. "That's how resistance spreads."
Norsworthy showed photos of the Harrington Seed Destructor, a pull-behind machine that attaches to a combine to kill weed seeds. Developed by a farmer in Western Australia, the machine employs cage mills, spinning in opposite directions, to crack or pulverize weed seeds, which rot when they fall to the ground and are exposed to moisture.
If brought to the U.S., a seed destructor would likely cost roughly $220,000, Norsworthy said. "I expect within the next five years, when you go and purchase a combine, just like having an opportunity to put GPS or GIS on a combine, you'll have an opportunity to buy an in-the-combine seed destructor" attachment, he said. "I expect those costs to be $50,000 to $75,000."