How to go after marestail, waterhemp

Soybean Pest Beat: Residual herbicides are a key part of the solution to controlling these tough weeds. 
Dec 31, 2019
We had lots of marestail escapes. Waterhemp came in late. Should we stay with Roundup Ready soybeans or try Xtend, Enlist or Liberty beans? 
The panel of Indiana certified crop advisers answering this question includes Steve Gauck, Beck’s sales agronomist, Greensburg; Andy Like, Syngenta, Vincennes; and Jeff Nagel, Ceres Solutions agronomist, Lafayette.
Gauck: The great thing about new soybean technologies is that you have options; the tough part is determining what you need. Stop thinking about postemergence herbicides as the only answer. Start looking at your weed concerns and build a program that controls them with residual products. 
Start early with a strong burndown or tillage. Be sure fields are clean at planting. At planting, use residual products with multiple modes of action. Your goal is three different modes of action that target problem weeds. Spray post early — not by weeds present, but by the calendar. Target this post application 21 to 25 days after soybeans emerge.
Use a Group 15 herbicide with your post product. If you do this before canopy, you’ll build a layer of protection that will last until soybeans shade the ground. Either Liberty, Xtend or Enlist will work because you’re relying on residuals to do the heavy lifting. This also makes post products better because weeds are smaller, and you can get better coverage.
Like: Assuming marestail and waterhemp are resistant to glyphosate, I would look at changing to Liberty, Xtend or Enlist beans to head off escapes and improve your postemergence control options. When you change to a new traited platform, you would still use a residual chemical in both the preemergence/burndown and post application passes. Applying a residual with both passes lessens pressure on the post application and preserves the technology for the future. Selecting the right trait can be as simple as finding a variety you like and using the trait that comes with it.
Call your neighbors and find out what they’re using to minimize off-target issues.
Nagel: Switch to a different herbicide trait platform. Roundup Ready 2 Xtend, Enlist E3 and Liberty or LibertyLink GT27 soybeans are all viable options. Waterhemp and Palmer amaranth are game-changers and should be aggressively managed. Select soybean varieties that have strong yield potential with the disease package you need. Apply these five principles:
1. Consider fall burndown. This is especially true in conservation-tillage systems. Marestail is the primary target weed, along with winter annuals. If burndown did not get done this past fall, dicamba gives you the best option for overwintered marestail control in the spring.
2. Start clean. Either tillage or an effective burndown program will work.
3. Use residuals. Use a soil-applied residual herbicide on target weeds like waterhemp at or near planting time. Don’t omit this step! Omitting it puts all the pressure on postemergence herbicides, leading to quicker selection for resistant populations.
4. Watch weed size. Make the post application on weeds less than 4 inches tall. XtendiMax, Engenia or Fexapan, or Enlist One or Liberty, all will control small waterhemp but will struggle on larger weeds.
5. Add residual again. Tank-mix a Group 15 residual herbicide with the post application to help minimize additional waterhemp emergence. Check manufacturer websites for approved tankmixes.