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There has been a lot of discussion about horseweed/marestail management in soybeans in recent years. Historically glyphosate resistant horseweed was limited to the southeastern Pennsylvania down into the Delmarva and West in Ohio, but now it is much more common in central PA and has appeared in western PA as well. This weed is mostly a problem in no-till and more of a problem in soybean than corn. It is also a common sight along the margins of some fields where herbicide programs are usually more hit or miss.
Ohio and Indiana farmers are also battling horseweed and it is not uncommon for them to have both glyphosate and Group 2 or ALS-inhibitor resistant biotypes. The use of residual herbicides to control escapes has become more common in some areas and particularly in Ohio and Indiana where 6 to 8 weeks of residual control may be necessary to manage spring emerging horseweed. Here in the Mid-Atlantic, the need for residual herbicides for horseweed control is less clear as it appears that most of our horseweed emerges in the fall and early spring prior to burndown herbicide applications. However, be on the lookout as you scout fields this spring and early summer and residual soybean herbicides can help manage not only late emerging horseweed, but also all those other summer annual broadleaves that are problematic.
Here are the general guidelines that we continue to recommend for horseweed/maretail management in our area:
There are recommendations/restrictions for tank-mixing saflufenacil with other Group 14 or PPO herbicides due to the concern for soybean injury. According to the BASF labels, you must wait at least 14 days if you mix 1 fl. oz. of Sharpen (or 5 fl. oz. Verdict) with other Group 14 herbicides (More than 1 fl. oz. Sharpen requires 30 days).
Soil applied Group 14 herbicide include the following:
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