Stay one step ahead of mounting resistance issues by knowing how to identify and control
With almost four decades of farming under his belt, Sam Spellman knows the pain of dealing with stubborn weeds. He likens today’s herbicide issues to the weed problems he dealt with early in his career.
Look at the group number, not product name, to determine if herbicides will be effective against your weeds.
In 1996, rigid ryegrass made history as the first glyphosate-resistant weed (found in Australia). Now, there are 151 resistant weeds documented in the U.S., and 13 are resistant to glyphosate.
True resistance occurs when a plant changes internally to withstand a herbicide that’s typically lethal. After repeated use, some weeds have mutated to withstand heightened herbicide rates from multiple chemical families.
“Farmers kind of created problems [with resistance] by reducing rates,” admits Spellman, who grows corn and soybeans near Woodward, Iowa. “A 95% kill isn’t good enough.”
Weeds steal nutrients, sunlight, water, oxygen and space from crops, which can result in yield loss. With resistance spreading, researchers are looking for new solutions.
“It has been really easy to just spray glyphosate,” says Mike Owen, Iowa State University (ISU) professor of weed science. “We need to be more diligent and timely with scouting.”
Instead of broadcasting a Band-Aid solution, typically glyphosate, consider a more systematic, season-long approach to weed control.
“Rule No. 1 is to start with a clean field,” says Bob Hartzler, ISU Extension weed scientist.
Planting into a field where weeds have a head start gives them the advantage all season, he adds. In no-till, consider fall burndown, and in no-till and conventional tillage, pre-emergent herbicides can start off crops right.
As the season progresses, scouting is essential to manage weed control and plan for next year.
““Divide fields into smaller units, investigate problems and address them individually,” Owen says.
While scouting, determine if your herbicide program is effective and if you have resistant weeds.
Unfortunately, if you have a highly infested field, it might be too late, Hartzler says. At that point, take note of the fields with bad pockets to help plan for targeted herbicide or mechanical methods next year.
When taking notes, record location, types of weeds, herbicide program and when the weeds started growing. If they germinated before spraying and continued to grow, it could be a sign of herbicide resistance. If they sprouted after application, you might need to add a residual.
Late-season weed control can impact profit and weed control for next year, even if you see no immediate response in this year’s crops.
“Farmers have learned late-season waterhemp doesn’t frequently impact yield, so many don’t worry about it,” Hartzler says. “The real cost is allowing weed seed banks to increase.”
Above all, effective weed control protects plants from extra competition.
“Remember, the reason you use herbicides is not to kill weeds but to protect yields,” Owen says.
This story is the first in a three-part series on the billion dollar bind. The next two installments will take a look at how weed resistance developed and how it hurts yields and profits.