Let’s cut to the chase on the subject of residual herbicides in soybeans – they’re necessary. Period. Going without them sets a grower up for weed escapes, headaches throughout the season and probable yield loss.
Don’t build up any false hope. We’re not going back to the days of blanket, over-the-top herbicide sprays that kill everything but the crop – even with new technologies that have come on the scene. Over-reliance on glyphosate-resistant technology put an end to those years of great convenience, and nobody wants to repeat that mistake with this next generation of products. If you’re looking for silver bullets, go to YouTube and search for those grainy episodes of The Lone Ranger.
Accept the reality that residuals are simply part of the game plan and be prepared to juggle more balls and make more decisions.
For one thing, product choices must be based on herbicidal modes of action (MOA). It’s necessary to keep rotating and switching them up so that weeds are exposed to multiple rounds of lethal chemistry.
Beyond that, residual MOAs must complement the MOAs used in post-emergence programs. Relying too heavily on the same MOA at different times of the season will open the way for new forms of herbicide resistance. This also means that growers must now embrace a basketful of older chemistries and approaches that actually were the mainstay for weed control before Roundup Ready came along.
Weed scientists at Mississippi State University and the University of Tennessee published articles and info sheets that closely focus on residual approaches. The material is worth reading and I’ve included links to the pieces. But to streamline things a bit, here are 13 key takeaway points gleaned from their research.
Herbicide Programs for Managing Glyphosate- and ALS-resistant Palmer Amaranth in Mississippi Soybeans
This info sheet, written by MSU weed scientist Thomas Eubank, actually dates back to 2013 – a time before auxin-based technology. But it still holds up well and provides a quick overview for formulating a residual approach, whether planting conventional, Roundup Ready or LibertyLink soybeans.
Lists of herbicides that can be applied preplant incorporated (PPI), preplant/preemergence (PRE) and postemergence (POST) are given. Here are 4 major points to consider when using any of the herbicide programs follow.
- Use of residual herbicides at full label rates is imperative for managing resistant weeds.
- The right herbicide applied correctly can provide 2 to 3 weeks of pigweed control.
- Size matters. Time POST herbicide applications when weeds are no more than 2 to 3 inches tall. That’s critical for controlling escaped pigweeds following application of PPI and PRE herbicides.
- Remember that some residual herbicides (e.g., Dual Magnum, Warrant, Zidua) can be applied in-season for additional residual control of pigweed.
Choices of Residual Herbicides in Mississippi Soybean
This article by Extension Weed Specialist Jason Bond also was written before auxin-resistant varieties rolled onto the scene. But like the article noted above, it remains quite valid.
Here are 6 major takeaways:
- Most weed management decisions for soybean production should be based on managing glyphosate-resistant (GR) Palmer amaranth. That’s only logical since it’s probably the most prolific and hardest-to-control resistant weed in the South and, increasingly, in the Midwest.
- Herbicide programs designed to manage GR pigweed should include herbicides with multiple modes of action (MOA) that are effective for controlling this and other resistant weeds. This is a critical step in developing such programs.
- A numbering system to designate a herbicide’s MOA allows growers and crop advisors to avoid sequential applications of herbicides with the same MOA.
- Diverse herbicide programs that are designed to control GR Palmer amaranth and other HR weeds begin with the inclusion of residual PRE herbicides.
- A list of the most effective PRE herbicides and herbicide mixtures is provided.
- None of the PRE herbicides provided complete control of GR pigweed, so a POST herbicide application is required to control escapes.
Soybean Residual Herbicide Consideration
This article by University of Tennessee Extension Weed Specialist Larry Steckel is the most recent of the three I’m citing here. Here are 3 major points:
- Growers must now be aware more than ever of herbicide MOA since they can no longer depend solely on a PPO residual herbicide like Valor to provide consistent long-lasting control of Palmer amaranth.
- On sites where Palmer amaranth has not yet selected for PPO resistance, PPO residual herbicides will provide 100% control of this weed for 3 to 4 weeks.
- In order to provide complete or near-complete residual control of PPO-resistant as well as PPO-susceptible pigweed, growers are advised to apply a mixture of herbicides with 2 effective MOAs [e.g. Authority MTZ (PPO inhibitor Group 14) and metribuzin (Group 5)] against pigweed.
The bottom line from all of the above is:
First, a weed control program for soybeans should include both PRE and POST herbicides to control/manage the increasing occurrence of HR weeds.
Second, ensure that a herbicide program includes multiple MOAs that are effective against both HR-resistant and susceptible targeted weed biotypes.
The era of using only POST glyphosate for weed control in soybean has long-since come and gone. Producers are now left with devising weed control programs similar to those that preceded herbicide-resistant technologies…with one difference.
They now have to contend with resistant weeds that will require a more integrated approach that includes consideration of herbicide MOA in the selection and use of both PRE and POST herbicides that are required to achieve acceptable control of problematic weeds in soybeans.