Back to the Basics for Weed Control

The saying ‘all good things must come to an end’ is proving true for weed control in soybeans. Gone are the years of simply using Roundup to take care of everything (if they weren’t already for some acres). I confess that I was skeptical of weeds working around Roundup but should have known better since nature is geared to find a way to endure and overcome. Just like in the animal kingdom, survival depends on self-preservation. The same has happened with weeds, shifting their genetics to survive being sprayed with Roundup. 

The ‘Easy Button’ for weed control is over, not that we should ever have relied on straight Roundup as our sole option. We must now return to solid practices and ensure we use multiple modes to control problem weeds, which will help to stack the deck against further weed resistance and keep our herbicide options viable. 

Statistically, it is highly unlikely for any weed to simultaneously develop resistance to multiple herbicides. For example, say there is a 1 in 1,000,000 chance (much higher than the real world) for resistance to Roundup with a similar probability for resistance to 2-4D. The odds of finding both resistances in the same plant is 1/1,000,000 x 1/1,000,000 = 1/1,000,000,000,000, far worse than the minuscule odds of winning the lottery. 

The more logical path to multiple resistance is the weed population becoming resistant to one herbicide until the trait is locked in, and then using a non-resistant herbicide until that weed population develops resistance to that as well. 

In reflecting on the path to multiple resistance, we should hopefully see the error of single or even double modes of action. Using Xtend or Enlist soybeans on weeds that are already resistant to Roundup increases the danger of developing multiple resistance. With essentially one mode of control (the Roundup is doing nothing), you risk selecting plants resistant to Roundup and 2-4D or Dicamba. 

You should attack these weeds aggressively. Multiple modes of action will hopefully drive the population to nothingness, while using ineffective treatments will only allow them to spread to other fields. Even if you have no resistant weeds, it is still a strong policy to use multiple modes of action as a preventative measure in your fight against weeds. Remember that this is not a one-year fight; weed seed can sit in the soil for multiple years and come back to haunt you.

The basic program for weed control is starting with pre-emergence herbicide(s). This package can have several modes of action and prevent weeds from establishing in the first place. This will help the soybeans to get ahead of the weeds and allow post emerge spraying on a timely basis which eliminates having to come in when many of the weeds have grown beyond the size of the label. 

The University of Wisconsin has a very nice table of herbicides and their modes of action. It can be found at https://ipcm.wisc.edu/download/pubsPM/Herbicide-Mode-of-Action.pdf. It is important to match modes of action to the weeds you are trying to control. A good example is that group 2 (ALS Inhibitors) are not effective at controlling water hemp. 

After emergence, there are now several soybean options to control problem weeds: Xtend (Dicamba), Enlist (2-4D, Liberty), Liberty GT27 (Liberty) and coming next year XtendFlex (Dicamba, Liberty). Dicamba is probably the most effective and delivers some residual control and also has a great respray program for weed escapes. Unfortunately Dicamba has restrictions placed on its use due to some offsite movement during unfavorable conditions. 2-4D is a good spray and more forgiving but, just like Dicamba, can have the same offsite movement if the label is not followed properly. Liberty is my third choice as it is expensive and somewhat finicky when spraying. 

Though all three can be very effective, I lean towards Xtend as they have the clear lead in genetics and overall weed control. Enlist is next with its three modes of action that can be sprayed. I do like Liberty GT27 because they are genetically strong and allow for a mode of action beyond GT (Roundup). 

I cannot stress enough to follow the label on all three. These herbicides and soybean platforms are key weapons in our fight against weeds. There are many misinformed people that want to take them off the market in their zeal to ‘protect the environment’ and we don’t want to give them any reason to cite misuse. 

The easy days of Roundup being the magic bullet are gone but, with very minimal changes in our spray program, we can maintain the level of weed control we are accustomed to.

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