ILSOYADVISOR POST

It’s Never too Late in the Fight Against Weeds

When Roundup Ready® soybeans were introduced, times were good. You could schedule your spray application from the calendar and not think twice about whether your fields were going to stay clean. Mother Nature has since prevailed, showing scientists who is boss and showing us what we’ve been taking for granted. In the perfect scenario you would see a weed-free field with an ideal spring, but that’s not so common anymore. We now must take extra time and effort to find that right weed control program. 
 
Waterhemp escapes in a soybean field. Photo
courtesy of University of Illinois.
There is a silver lining, though. We have more soybean trait technology options now for growers to choose from. Choosing your plan of attack on troublesome weeds means you need to first find what management practice fits best.
  • Burndown  
  • Pre-plant incorporated
  • Pre-emergence 
All great places to start, but what if you didn’t have time? Torrential downpours and muddy fields sound more like what we’ve experienced in recent years, so you are left with a post-emergence situation. What are the best ways to manage this type of system?
  • Plant soybean resistant varieties with more than one mode of action
  • Do not cut the labeled rate for effective control
  • Apply post-spray as soon as possible, before canopy
  • Scout – escaped weeds might need a second application
Herbicides, like Group 4 growth regulators, have been around since the good old days, but it’s not until recent years that we have seen a soybean able to metabolize and resist their effects. Growth regulators have great control on a wide variety of weeds, but they’re no silver bullet. That’s the reason for the introduction of soybeans resistant to multiple modes of action. Glyphosate still has its place with good control on many grasses and works well for a burndown coupled with a Group 4 herbicide. Now that option is available for a post-emergence spray. 
 
Marestail coming back to life after
herbicide application. Photo courtesy
of University of Nebraska-Lincoln.
Glufosinate herbicides have good broad-spectrum control of small annuals like marestail and will become more common tank mixes for sprayers in the future. Yes, there are other effective post-spray options like the ACCase, ALS, and shoot-growth inhibitors, but with tougher weeds, options are good to have. 
 
Saving money now might cost you down the road. Choosing to cut corners and cut rates of a post herbicide can just make weeds angry. We have all seen this with weeds, like marestail, that will look dead and then magically show signs of life to only green up and poke through the soybean canopy. High-quality adjuvants also help the herbicide perform as it should, so use them and the full recommended rate to get the most out of an herbicide. 
 
Relying on a post spray still has urgency, because another one might be needed. The weather is unpredictable and causes herbicides to perform questionably at times. You might find a pop-up rain right behind the sprayer, temperature fluctuations that slow the translocation of herbicides, or a second pass could be warranted simply just to clean up some tough weeds. The smaller the better when trying to knock them down. Controlling the escapes will also help reduce some of the pressure for next year if they are stopped before producing seed. The weeds that make it through the initial pass won’t be found unless scouting is done, but late emerging weed seeds won’t be noticed until it is too late if you don’t look for them. 
 
Competition can be a good thing, but not in your soybean field. Start clean and stay clean, a common phrase that should be taken seriously, but that is not always guaranteed in today’s climate. Scouting early and continuing to manage difficult weeds if they get away could help grow the bottom line of a profitable soybean crop.  

Cody Pettit

Currently, Pettit is a field agronomist for the Pioneer brand with Corteva Agriscience, covering the east central part of Illinois. Prior to his current role, he was a district sales manager in the seed industry after graduating from the University of Illinois, where he earned a bachelor’s degree in crop sciences. Pettit has a passion for understanding new practices and solutions employed on a variety of farm operations, and is excited for the ever changing future of the agricultural industry. 



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