Controlling problem weeds post-emerge in soybeans begins with pre-emerge planning.

The past few days have been filled with many service calls on both waterhemp and marestail control. These weeds are becoming major challenges in our crops and this season has proven to be one of the tougher one I have encountered in my career. The task of killing these weeds is already a challenge, but this season has been especially challenging. Here’re are few tips on controlling these weeds.

Waterhemp
Glyphosate-resistant waterhemp can be controlled by PPO inhibitors such as Cobra^® or Flexstar^® or Liberty^® if Liberty Link^® soybeans were planted. Please keep in mind, however, that these herbicides don’t translocate in the weed and performance can be inconsistent once the weed height exceeds 5 inches.

This field had a bad outbreak of waterhemp since a pre-emergent product wasn’t applied. I have sprayed the field with Liberty and it seems to be doing a good job at this point. Below are photos taken before spraying and four days after, along with a close-up of a dead waterhemp plant.

Marestail
I find marestail to be even more challenging than waterhemp. Our company agronomists have talked with growers, fellow agronomists and chemical reps to find the best solution to controlling large marestail in soybean fields. We all concluded there is no right or easy answer. We have decided to test a variety of different approaches based on opinions from across the country. In a few days we will evaluate these four approaches and see which works best:

• Firstrate^® at 0.5 oz./acre
• Butyrac^® 200 at 2 oz./acre
• Classic^® at 1 oz./acre with 2 oz./acre of Butyrac^® 200
• Classic at 1 oz./acre with 4 oz./acre of Resource^®

As you see in the above image, marestail was knocked back earlier but now weeds are coming back and will be harder to kill. It seems the conditions this spring have not been favorable for good control of marestail. Many growers who would have normally applied 2,4-D did not do so because of the seven-day interval required before planting. Cool and wet conditions this spring caused a lot of uncertainty in planting and herbicide application windows.

In a few days, we will look at the applications made and evaluate the best mix. In the end, the bottom line is that there is no post-emergence herbicide that will consistently control marestail and waterhemp. The best advice I can give for post-emergence application is to spray early when weeds are small and keep a very close eye on those fields, or consider moving to a Liberty Link system.

Dawn Kielsmeier is an agronomy sales specialist with Pearl City Elevator in Baileyville, Ill. She has a B.S. in dairy science and an M.S. in agronomy, both from the University of Illinois and has been a CCA since 1993. She is a 2017 Illinois Soybean Association CCA Soy Envoy.