Which is the correct postemergence soybean weed control system for you? With new options for 2021, the choice is different than it was a year ago.

Farm Progress asked Bill Johnson, a Purdue University Extension weed control expert, and Jeff Nagel, a longtime agronomist with Ceres Solutions, Lafayette, Ind., a series of questions related to selecting a herbicide-tolerant weed control system. Read the first part of this discussion in Explore options for herbicide-tolerant soybeans in 2021 and further discussion in Factor in weed resistance when selecting herbicide system.

Starting from scratch, how do growers begin to sort out which system they should choose?

Johnson: First, decide what soybean variety fits the field in terms of tillage, disease and soybean cyst nematode resistance, yield potential for the geography, and soil type and drainage characteristics. Then, based on what weeds are most problematic, choose the residual herbicide for the field. Then, base the soybean herbicide trait system on what weeds you think will break through the residual herbicide.

The residual herbicide is the backbone. I can’t stress that enough. The postemergence herbicide is just to clean up escapes from the residual herbicide, not the backbone of the weed control program.

Nagel: Establish whether you need dicamba, glufosinate or 2,4-D in your postemergence program to aid in the control of glyphosate-resistant weeds. Evaluate varieties that are available to you in those trait offerings. Focus on yield and agronomics like disease packages. In general, we’re seeing varieties with good yield potential in all trait platforms. Evaluate the varieties that are available in the maturity ranges needed.

Yield potential is important, but also pay attention to disease ratings. Many farmers will be planting some varieties they’ve never seen on their farm before. Some are even making whole-farm changes to a new trait platform and will have all varieties new to them.

Seed companies may not have a complete profile on disease ratings for newer releases. Look for the gaps in disease protection and implement management strategies to mitigate potential issues. Unexpected disease pressure like phytophthora root rot, white mold, sudden death syndrome or frogeye leaf spot can cause significant yield reductions. Focus variety selection on yield potential, agronomics and management.

Which issues are related more to the grower’s weed spectrum?

Johnson: What weeds are problematic in a geographical area? Which ones frequently are not controlled well by “labeled rates” of soil residual herbicides? These are the weeds you need to target with post herbicides.

Nagel: It’s pretty simple. If you have glyphosate-resistant weeds like marestail, waterhemp, Palmer amaranth or giant ragweed, you will want a herbicide trait platform that will allow the postemergence application of dicamba, 2,4-D choline or glufosinate. Waterhemp and Palmer are surfacing as the key driver weeds.

What issues might be related to a grower’s sprayer capability, volume of product to handle or other logistical issues?

Johnson: My view is that we do not have the sprayer capacity to spray post herbicides in soybeans at the right time in the eastern Corn Belt because of the high number of acres and small window of time when weeds are 4 inches or less and weather conditions are favorable. It’s especially true with the small windows of time available to spray dicamba or 2,4-D. That’s why we need to use residual herbicides — so there are fewer acres that will need to be sprayed at one time with post herbicides.

Nagel: Dicamba and 2,4-D choline are systemic herbicides, while glufosinate is a contact herbicide. Typically, systemic herbicides can achieve acceptable weed control with lower spray volumes. However, dicamba requires spray nozzles that produce ultra-coarse droplets and a drift-reducing agent. Thus, better control is being achieved with higher spray volumes.

Practically, we’re managing the herbicide systems the same. Spray volume is a minimum of 15 gallons per acre, and 20 gallons per acre is recommended for larger or heavier weed pressure canopies. XtendiMax, Engenia, Tavium, Fexapan, Enlist One and Enlist Duo all require users to check websites for labeled nozzles, approved tankmixes and adjuvant systems. Liberty labels specify recommended droplet sizes but provide more flexibility on nozzle choices.

Are there cost differences in the systems, or differences in overall economics?

Johnson: Regarding cost, one would need to check with the dealer to see what kind of bundling economic packages are available. If I were king for a day, I would put pressure on the basic manufacturers to bundle the right residual herbicides with these traits to ensure their longevity. Instead, what tends to happen is the manufacturers will guarantee re-spray of the post herbicide if it doesn’t work the first time. Unfortunately, this just leads us down the resistance pathway at a much faster rate.

Nagel: Herbicide costs will be relatively comparable across the trait platforms. Total cost per acre will vary depending on target weeds. For example, adding a Group 15 residual herbicide for extended control of waterhemp or Palmer amaranth will add $10 to $15 per acre to any program.