Control of marestail, waterhemp, and other resistant weeds has been an especially difficult problem for growers this season. Extension officials have few recommendations left at this point in the season. A combination of a wet spring and spreading glyphosate resistance in waterhemp and marestail have posed a weed control issue with few answers. Soybean and no-till fields have been especially hard hit. With many of these weeds now 6 to 12 inches tall, there are few options left. Aaron Hager, Extension weed specialist at University of Illinois, has been in touch with his counterparts at Purdue and Ohio State and they have only come up with one option, “To literally try and mow off the tops of the marestail plants. This is not going to control the plants — they will regrow, but the purpose is to try and reduce the number of seeds these plants make.” Hager says marestail seeds carry easily in the wind and spread quickly.
Hager says doing nothing is also not really an option, “If we don’t do anything to try and control these marestail plants, it will not just be a problem in a field. But if these plants area allowed to make seed, will become a problem in an entire region.”
This late in the season there are no chemical options that are going to provide growers with the kind of control over marestail or waterhemp for which they are looking, “If they’re over 12-inches tall — and that’s pretty likely this late in the season — then there is no way to control them with any herbicide or herbicide tank mix.” Hager warns that too strong of a chemical application can cause more problems than it solves, “We need to be aware that this late in the calendar we could cause a good deal of damage to the soybean crop.” He added that slowing down plant growth in soybeans is not something we want to do now, because we need to get the canopy established as soon as possible. A mechanical option seems to be about the only approach growers can consider now.