COLUMBUS – In an Ohio State University greenhouse test, it was confirmed that marestail in three Ohio and two Indiana fields are glyphosate-resistant, according to three OSU agronomy specialists.
Plants from these populations survived glyphosate rates up to 3.0 pounds acid equivalent per acre, according to Jeff Stachler, Mark Loux and Geoff Trainer.
The three Ohio populations are from west central Brown County between Mt. Orab and Georgetown.
The two Indiana populations are from Jackson County, southwest of Seymour and west of Interstate 65.
The Indiana populations are almost directly west of the Ohio populations, and researchers assume that many fields between these two areas are also likely to have glyphosate-resistant marestail populations.
More testing. The researchers are in the process of testing additional marestail populations.
Many of these fields have been continuous no-tillage soybeans and planted with Roundup Ready soybeans for at least the previous five years, with two or more applications of glyphosate per year.
Fields with glyphosate-resistant marestail are likely to have a combination of dead, severely injured and slightly injured plants occurring in patches at the time of harvest.
Further greenhouse studies will be conducted to determine if these populations are also acetolactate synthase (ALS) resistant. However, based upon previous greenhouse research and the fact that some of these fields were treated with a mixture of FirstRate plus glyphosate, these populations are most likely resistant to both glyphosate and ALS inhibitors, according to the researchers.
If these populations have multiple resistance, then herbicide control options become limited in soybeans, they said.
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