Fight tall waterhemp in soybeans

Soybean Pest Beat: Here are tips to get a plan rolling to control waterhemp in 2021. 
Oct 03, 2020
Tall waterhemp gave us fits in 2020. What type of weed control plan can we develop now to head it off in soybeans in 2021?
The Indiana certified crop advisers answering this question include Steve Gauck, regional agronomy manager for Beck’s, Greensburg; Andy Like, independent crops consultant, Vincennes; and Dan Ritter, agronomist with Corteva Agriscience, Rensselaer.
Gauck: Weed issues are always changing, and we must adjust weed control programs to match weed issues. First, we must understand the weed we are targeting. Tall waterhemp is an aggressive weed that produces a large seed bank and will germinate all growing season. The key to controlling waterhemp is residual herbicides and not relying on postemergence applications. Waterhemp has so many growing points that it’s extremely hard to get herbicide coverage when post spraying. Plus, it has resistance to many different types of post herbicides.
So, start with understanding that you cannot cut cost on residual herbicides. Begin the season with residuals that contain at least two, but preferably three, modes of action against waterhemp. To get good control in a heavy pressure area, also plan an early post application that contains another residual herbicide.
The key to in-season application is not to wait until canopy! You must get the residual herbicide to the ground for it to control weeds. Ideally, target applying your residual with the post spray 20 to 25 days after soybean emergence. Use a post herbicide with the residual in season to clean up any small weeds that escaped. Canopy is your best weed control later, so consider planting in 15-inch rows or narrower.
Like: I would focus your weed control plan around multiple layers of residual herbicide, with your goal being to not let a waterhemp emerge after soybeans are up. That will be difficult.
In a no-till system, I would include residual herbicides with a burndown that has good activity on small-seeded broadleaves. They should have two modes of action. Use robust rates. One example would be sulfentrazone (Spartan) tank-mixed with metolachlor (Dual) and the burndown herbicides.
For conventional tillage, I would apply the same residual products less the burndown after planting and before soybeans and waterhemp emerge. Depending on rainfall, this should give you three to four weeks of control and allow you to use another residual that can be applied when making a postemergence application. One example would be pyroxasulfone (Zidua) tank-mixed with the appropriate post herbicide that matches your trait platform. The idea is to minimize pressure on the post herbicide. If weather doesn’t cooperate on post application timing, you have some breathing room and aren’t trying to kill waterhemp beyond labeled size.
Ritter: Hit them hard from as many angles as possible. One shot just doesn’t cut it today, especially with tall waterhemp. It’s also just a smart resistance management strategy. I would put a solid preemergence herbicide down that works relatively well on tall waterhemp. Follow with a postemergence program.
Admittedly, the program I may recommend will be a bit biased. Consider the Enlist E3 soybean program, which gives you options of using Enlist One (2,4-D choline), Enlist Duo (2,4-D choline and glyphosate) and glufosinate. I may plan on a two-pass system, given the fact that you’re dealing with a known weed issue. With this plan, you’ve successfully used three modes of action at three different timings.