Managing Mid Season Weeds in Soybeans

Soybeans flowering is in full swing in many areas, which marks the tail end of when soybean herbicides can be applied.

It is getting to point in the season where we hopefully have controlled our weeds in full season soybeans. Soybeans have moved into the reproductive phase with flowering in full swing in many areas, which is often the tail end of when soybean herbicides can be applied. The application of herbicides during pod set can have the potential to reduce pod numbers, so consider this before making an application. Some herbicides simply have a “days to harvest” restriction which is based on tolerable residue amounts that may enter the grain. In addition, most of the weeds are getting big, so the effectiveness of the herbicide will be greatly reduced. Glyphosate (Group 9) and ALS-resistant (Group 2) weeds are a particular concern because these will generally require a contact or PPO (Group 14) product. The PPO or contact products will burn and stunt the soybeans, so later season applications can potentially reduce yield. There may be some benefit to “hurting” the weeds and thereby reducing competition and seed production, but complete control will be difficult (see accompanying image). Of course we would also be remiss without mentioning that spraying big weeds with sub-lethel rates of herbicide is also a great way to hasten the selection for new herbicide resistant weed biotypes which of course in general, we do not support. So, there will be trade offs as whether to spray or not and the decision will need to be made whether the weeds are severe enough to warrant the application. Below are a brief list of post-applied soybean products, the herbicide group, and their lateness of application guidelines.

Table. Selected post-applied soybean herbicides, the herbicide group or site of action, and lateness of application guidelines.

Dwight Lingenfelter is an Program Development Specialist in Plant Science with the Pennsylvania State University.  William S. Curran Ph.D.is a Professor of Weed Science with the Pennsylvania State University. This article originally appeared in the Penn State Extension’s website and has been reposted with permission.