By Darren Hefty

While traveling around the country this past year, one of the questions I was asked most was, “How do you stop Roundup-resistant weeds?”  As you can imagine, the answer is easy.  That’s why everyone is doing it, right?  All kidding aside, the answer to the problem may not be as difficult as you imagine it to be.  Here’s the catch, though.  With herbicide applications, timing is everything.  You have to beat Roundup-resistant weeds before they get started.  Then, you kill off the remainder when they are very small.  I’ll explain.

I don’t care what crop you’re raising.  Stopping weeds is one of the most important things you can do to increase yield potential.  It goes without saying that weeds rob water and nutrients from your crop, but exactly WHEN is the concern.  By the time most weeds are 1 to 2 inches tall, they are already a significant yield threat.  Here’s what you need to do.

FIRST: START WITH A GOOD PRE-EMERGE HERBICIDE PROGRAM

In soybeans, we recommend starting with three different sites of action.  Choose a product from each of the following categories.

  1. DNA or Group 15 – Treflan, Sonalan or Prowl have been the best choices historically.  We’d also throw Zidua into this category because it does similar things in the field.  Zidua plus Valor makes up Fierce herbicide, by the way.
  2. Metribuzin – This is what used to be called Sencor.  It’s a relative of atrazine, and you need to watch your rate closely (meaning lower it) in light soils and when soil pH is above 7.4.
  3. PPOs – Authority or Valor provide excellent residual plus some burndown activity on small-seeded broadleaf weeds like the pigweed family.  Sharpen falls into this category, too, and we love Sharpen, but you can only use 1 ounce safely in front of soybeans, which is not enough to get a great deal of residual control.  In other words, we want you to use Authority or Valor instead in this application.

In corn, we recommend increasing the rate on the pre’s you’ve been using.  We also promote using a premix with a broadleaf component where needed.

  1. Grass herbicides like Harness, Surpass, Outlook and Dual have been very good when used at full rates.  The reduced rates that have been used in front of Roundup are not enough to have season-long residual.
  2. Balance Flexx hasn’t been as good at controlling grasses as the herbicides listed above, but it is significantly better at controlling broadleaf weeds.  This makes Balance Flexx a very useful tool for stopping Roundup-resistant weeds.
  3. Premixes like Verdict, TripleFLEX, SureStart, Corvus and more have become popular due to the amount of broadleaf weed control they exhibit on tough Roundup-resistant weeds.
  4. For resistance management, we recommend avoiding using the same sites of action both pre- and post-emerge.  For example, if you use Balance Flexx pre-emerge, I’d recommend choosing something other than Callisto, Laudis, Impact or Armezon post.

SECOND: SPRAY POST-EMERGE PRODUCTS EARLY

If you don’t know a weed is resistant to Roundup and you find out only after you spray a post-emerge application, chances are your weeds are too tall for most, if not all, rescue treatments.  However, if you know that a weed is resistant going into it, your plan of attack must be to spray early.  Read the label of almost any conventional (not Roundup or Liberty) herbicide and you’ll see the weed height for control on most resistant weeds like pigweeds, ragweeds, marestail, etc. is around 1 to 4 inches tall.  You can’t wait until the weed is 6 inches, 8 inches, or a foot tall or more and expect to get 100% control.

THIRD: ADD RESIDUAL HERBICIDES TO YOUR POST-EMERGE APPLICATION

In grass crops such as wheat and corn, getting as close to 100% weed control as possible on tough broadleaves that are resistant to Roundup is very helpful.  In broadleaf crops like soybeans, adding a residual product post-emerge can help extend your control until crop canopy is complete.  Products like Warrant, Outlook and Dual are options.  They have no burndown activity on emerged weeds, but their soil activity can extend your control.

OTHER IDEAS

There are lots of other things you can do to fight Roundup-resistant weeds.  You may consider planting higher populations, using narrow row spacings, changing your crop rotation, and adding tillage when needed, using pop-up fertilizer and/or biological products to stimulate early crop growth.  Attend one of our free Ag PhD Agronomy Workshops this winter, and we’ll spend a little time talking about how to best manage these problem weeds on your farm.