by Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist)
Excessive and constant rainfall through the season thus far across southern Wisconsin has challenged timely herbicide applications in our experimental plots. If not raining, the wind was often blowing or the soil was too wet to even backpack spray. After spending several years in Nebraska, I learned that one can’t complain about rain, but come on! Still, we have managed to get most of our herbicide treatments out (thanks to a dedicated and hard-working research team that is literally not afraid of getting some mud on their boots!). So those out there waiting for the right opportunity to get the rig out and spray your soybean fields, I understand your pain.
Nonetheless, the soybeans keep on growing and if they haven’t already, soon they will start flowering (UW-Madison Agronomist Dr. Shawn Conley has observed the first flowers on his early-planted high-yield soybean plots at Arlington Ag Research Station this week). With the onset of flowering (R1 growth stage), comes the end of legal application window for several commonly used POST-emergence soybean herbicides such as Pursuit (Group 2), Liberty (Group 10) in Liberty Link soybeans, and Dicamba (Group 4) in Xtend Soybeans. Glyphosate (Group 9) in Roundup Ready soybeans, FirstRate (Group 2), and Cadet (Group 14) are examples of herbicides that can be sprayed up to full bloom (R2 growth stage).
If your soybeans are close to flowering but still have a ways to go until reach full canopy, adding a herbicide with soil residual activity to the POST program may be a viable strategy, particularly in fields with known waterhemp infestation or where small-seeded late-season weeds are still actively emerging. Assuming soybeans are passed the 3rd trifoliate (V3 growth stage), herbicide options with soil residual activity become Outlook (Group 15; up to V5 growth stage), Warrant (Group 15; up to R2 growth stage), and Warrant Ultra (Groups 15 + 14; up to R2 growth stage; see label for geographic restrictions).
Though the application window is getting shorter, wait for the right opportunity to spray and adjust herbicide program if necessary (don’t spray under marginal conditions).
For assistance identifying soybean growth stages, see: “A Visual Guide to Soybean Growth Stages”.
For assistance with herbicide selection, see: “Pest Management in Wisconsin Field Crops (UWEX Bulletin A3646)” and “Wisconsin Herbicide Mode of Action Chart”.
Always read the herbicide label for product rate, application window, and restrictions.