By Darren Hefty

When you think about the resistant weed problems you face in soybeans, almost every one of them is a broadleaf weed.  If you can solve the toughest broadleaf weed problems on your farm, your soybean fields will not only be able to reach their maximum yield potential, but you’ll be able to stand proudly in your fields because having a clean field is much more the exception than the rule these days.

If you are in an area of the country where you haven’t planted beans yet due to wet conditions or because you are double cropping, make sure you start with a GREAT pre-emerge program so you don’t have as many weeds to begin with.  If your beans are already in the ground, the next step is scouting and deciding exactly WHICH WEED is the primary one in each field and base your program around that.  If you have giant ragweed, marestail, kochia, pigweed, wild buckwheat, and cocklebur in your field, guess what?  NO SINGLE HERBICIDE IS GOING TO CONTROL ALL OF THEM.  You’re going to have to pick the one that works the best on your worst weed species.

Here are some key weeds and the best products to control them.  If you don’t see your primary weed on the list, please download the free Ag PhD Field Guide app, as we have an extensive list of problem weeds and the control solutions for each of them in soybeans, as well as corn and wheat.

Pigweed – Whether it’s Palmer or tall waterhemp, the choices are the same.  #1 is Flexstar, but you can only use it once per season, and it has a 10-month rotational restriction to corn.  My #2 choice is Cobra, but some prefer Cadet.

Marestail – Generally a winter annual, marestail control needs to happen early to be effective.  Big marestail is almost impossible to bring down.  #1 control option for me is FirstRate.  #2 is Marvel (Flexstar/Cadet).  Brian has always liked the Flexstar/Classic tankmix if your soil pH is below 7.4.

Kochia – Nothing is good on kochia in Roundup Ready crops if Roundup is no longer effective.  A full rate of Cobra would be my first choice, and I’d expect 50% to 60% control, maybe a little more if the kochia is smaller than 2 inches.  Cadet can sometimes be as good as Cobra, and Marvel would be another option.

Common and Giant Ragweed – I’ll group these together because the control options are similar.  If you don’t have ALS-resistant ragweed, FirstRate is our #1 choice.  Flexstar is #2 and an awfully good tankmix partner for FirstRate.  Cobra comes in third place.

Wild Buckwheat – Here’s a weed that not everyone is talking about yet but is growing in severity in a number of geographies.  Any of these vine-type weeds must be stopped before they vine out or you’re behind the 8-ball.  I like Pursuit or Raptor the best.  Flexstar is #2, but some would argue Harass is not too bad.  The problem with Harass is you can’t use a strong rate in soybeans like you can in wheat.

Lambsquarters – I haven’t seen Roundup resistance in lambsquarters yet, but tolerance is high.  Harass is #1, especially when you consider it only costs $1 per acre.  Resource may be #2 but is a distant second unless you use a high rate.  Raptor is slightly better than Pursuit, so that would be my third choice.

The key to getting all of these products to work is:

  1. Get excellent coverage with smaller spray droplets.
  2. Spray early.  Ideally you want the weeds 2 inches tall or less.
  3. Pick the right product for your worst weeds.  As you can see, that may be a different combination in each field.