No. 15 Article 4/July 3, 2008

Glyphosate Use in Soybean: Alone or Tank-Mixed?

The ability of glyphosate to control a broad spectrum of grass and broadleaf weeds has resulted in its use as a "stand-alone" postemergence soybean herbicide more often than in combination with other postemergence herbicides. However, with the rapid adoption of glyphosate-resistant corn hybrids and weed spectrum changes in response to near-ubiquitous glyphosate use, the frequency of acres where glyphosate alone will be sufficient to manage weeds in soybean will continue to decrease over time. Soil-residual herbicides and tank-mix partners for postemergence applications of glyphosate will increasingly be needed to manage both the challenges currently faced by soybean farmers and those lurking beyond the horizon.

We've often discussed the merits of including soil-residual herbicides in glyphosate-resistant soybean systems, but we have discussed less frequently the advantages and disadvantages of tank-mix partners with glyphosate. Are there instances when that approach might improve overall weed control? Are there instances when tank-mixes might not be advisable? The answer to both questions is yes.

Advantageous Tank-Mixes

Volunteer corn. Volunteer corn is easily controlled by glyphosate--unless it carries the glyphosate-resistance trait. The number of acres planted with glyphosate-resistant corn hybrids in Illinois has been steadily increasing the past few years and will likely continue to increase into the foreseeable future, so soybean farmers will need to rely on an alternative herbicide to control volunteer glyphosate-resistant corn. Control can be accomplished through the use of certain soil-applied herbicides, but it is often more consistently accomplished by tank-mixing certain postemergence ALS- or ACCase-inhibiting herbicides with glyphosate.

Challenging annual broadleaf species. Annual morningglory species can be especially challenging to control with glyphosate alone. Soybean weed control practitioners are often frustrated when attempting to control morningglory postemergence exclusively with glyphosate. Current and past research at the University of Illinois has examined several glyphosate tank-mix partners that can substantially improve morningglory control compared to glyphosate alone. Products containing cloransulam, chlorimuron, 2,4-DB, fomesafen, lactofen, or acifluorfen may increase annual morningglory control over that achieved by glyphosate alone.

Some herbicide-resistant weed populations. Our colleagues in Indiana and Ohio have conducted extensive research on management options for glyphosate-resistant giant ragweed in soybean. Indiana and Ohio extension weed scientists have recommended applying the maximum allowable rate of glyphosate during the first postemergence application and then respraying (if needed) within 3 weeks. They also have reported some success controlling giant ragweed populations that are resistant to both glyphosate and ALS-inhibiting herbicides by combining glyphosate with Flexstar or Cobra/Phoenix, followed by a second glyphosate applications 3 weeks after the first application.

Tank-Mixes Not Recommended

Glyphosate-resistant waterhemp. As described in the Bulletin in "Glyphosate-Resistant Waterhemp in Illinois: Recommendations for Management" (issue 1, March 21, 2008), we do not recommend adding tank-mix partners to glyphosate in the 2008 soybean crop for postemergence control of waterhemp that may be resistant to glyphosate. What we do recommend is that the first application of glyphosate alone occur when waterhemp are 3 to 5 inches tall, followed with field scouting not longer than 7 days after application to determine treatment effectiveness. If scouting reveals that waterhemp control was inadequate and retreatment is necessary, we suggest you consider applying a PPO-inhibiting herbicide (lactofen, fomesafen, acifluorfen) at a full labeled rate and with recommended spray additives as soon as possible.

So why use glyphosate alone instead of tank-mixing with a PPO inhibitor? There are several justifications for this recommendation:

1. Glyphosate-sensitive waterhemp can be adequately controlled with 0.75 to 1.0 lb ae glyphosate per acre when applied to plants 3 to 5 inches tall. For these sensitive populations, there would be little to no increase in control from the tank-mix component. However, if waterhemp plants in the recommended size range do survive this rate of glyphosate applied alone while other weeds in the field appear to be adequately controlled, you should remain very attentive to these plants, as they may be glyphosate-resistant.

2. We simply have very limited experience with these types of tank-mixes applied specifically to glyphosate-resistant waterhemp populations. Within the next couple of weeks, we will have sprayed several tank-mix experiments on a known glyphosate-resistant waterhemp population, but to date these types of data are lacking. Lingering unanswered questions with glyphosate + PPO inhibitor tank-mixes applied to glyphosate-resistant waterhemp include these:

3. We have confirmed more populations in Illinois of PPO-resistant waterhemp than glyphosate-resistant waterhemp. There would be little to no benefit in control of PPO-resistant waterhemp from the PPO component of these tank-mixes. Additionally, if a PPO-resistant waterhemp population is treated with a tank-mix of glyphosate and PPO inhibitor, will the glyphosate adequately control this population given the concerns outlined previously?

In summary, glyphosate tank-mix partners can help improve control of certain problem weed species over that achieved with glyphosate alone. However, in other instances we suggest that tank-mixes may not be the most advisable recommendation. As we gather more data from experiments designed to answer these lingering questions, we'll be sure to keep you abreast of our latest findings.--Aaron Hager

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