by Aaron Hager
No. 3 Article 4/April 10, 2009
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The utility of glyphosate for postemergence weed control in glyphosate-resistant crops has contributed to an unparalleled level of technology adoption. This technology has, in many respects, "simplified" weed control for soybean farmers in various ways. A single active ingredient--glyphosate--with application rate flexibility easily adjusted according to weed spectrum and size, controls most broadleaf and grass weed species without the need for tank-mix partners or changes in spray additive selection. This broad-spectrum weed control has resulted in glyphosate's use as a "stand-alone" postemergence soybean herbicide being much more common than its application in tank-mix combinations with other postemergence herbicides. While it is still relatively early in the game with respect to adoption of glyphosate-resistant corn hybrids in Illinois, observations to date suggest that a similar use pattern (i.e., glyphosate utilized as the sole postemergence herbicide) is developing in corn.
Several motifs are common in weed science. Some are intuitive and easily observed each season, such as the annual presence of weeds in the vast majority of Illinois crop acres. Other motifs, however, recur over much longer periods, such as the ability of weed species to adapt to widespread production practices. The time it takes for weed adaptations to occur depends on myriad factors. For example, many years elapsed following the introduction of atrazine before fall panicum became a significant and problematic grass weed species in corn, but only three to five years elapsed after the introduction of ALS-inhibiting herbicides before the first ALS-resistant weed biotype was reported. Prior experience also has demonstrated that weed adaptations occur faster in management systems that rely on a single practice or a limited number of them.
As biological systems (i.e., weeds) change in response to the widespread use of glyphosate, weed control practices utilizing glyphosate will undergo a concomitant change. The rapid adoption of glyphosate-resistant corn hybrids and weed spectrum changes in response to the near-ubiquitous use of glyphosate in soybean suggests the following thesis: the ability of glyphosate to be a stand-alone herbicide for weed management in soybean will (continue to) decline. In other words, the "simplicity" of glyphosate as a stand-alone weed management tool soon will be relegated to the annals of history. Soil-residual herbicides and glyphosate tank-mix partners will increasingly be needed to manage both current challenges and those lurking beyond the horizon.
Extension weed scientists often have discussed the merits of including soil-residual herbicides in glyphosate-resistant cropping systems, but they less frequently have discussed the advantages and disadvantages of including tank-mix partners with glyphosate. Are there instances when glyphosate tank-mixes might improve overall weed control? Are there instances when tank-mixes may not be advisable? The answer to both questions is yes.
Glyphosate-resistant volunteer corn. Volunteer corn is easily controlled with glyphosate--unless it carries the glyphosate-resistance trait. The number of acres planted with glyphosate-resistant corn hybrids in Illinois has been steadily increasing and will likely continue increasing into the foreseeable future. Soybean farmers thus will need to rely on an alternative herbicide to control volunteer glyphosate-resistant corn. This can be accomplished through the use of certain soil-applied herbicides, but control of this "new" weed often is more consistent by tank-mixing certain ALS- or ACCase-inhibiting herbicides with glyphosate.
Several postemergence herbicides provide excellent control of glyphosate-resistant volunteer corn. The ACCase-inhibiting herbicides (clethodim, quizalofop, fluazifop, sethoxydim) frequently are tank-mixed with glyphosate to control glyphosate-resistant volunteer corn. Be mindful that spray additive recommendations for ACCase inhibitors can vary depending on how the product is used (alone or in a tank-mix) or the type of glyphosate formulation with which it is tank-mixed. For example, additive recommendations can vary depending on whether a product is tank-mixed with a glyphosate formulation containing a "built-in" adjuvant system or if it is tank-mixed with a glyphosate formulation that itself requires additional surfactant.
Challenging annual broadleaf species. Several species of annual morningglory, including tall (Ipomoea purpurea), ivyleaf (I. hederacea), and pitted (I. lacunosa), occur in Illinois agronomic cropping systems. Soybean weed control practitioners often are frustrated when attempting to control morningglory with glyphosate alone, as these species are not as sensitive to glyphosate as are other species (such as giant foxtail). Glyphosate applied at 0.75 lb ae per acre is much more effective on small morningglory (about 1 to 3 inches) than it is on the large plants (8 to 12 inches) that result from delayed applications.
Three options that might improve morningglory control are increasing the glyphosate application rate from 0.75 to 1.5 lb ae per acre; making sequential glyphosate applications, spaced approximately 10 to 14 days apart; or adding a tank-mix partner to glyphosate. Field research conducted at the University of Illinois (as well as field research from several other universities) has demonstrated that each of these options can improve morningglory control over a single glyphosate application at 0.75 lb ae per acre. Tank-mixing glyphosate with herbicides containing cloransulam, chlorimuron, 2,4-DB, fomesafen, lactofen, acifluorfen, dicamba, or 2,4-D can increase annual morningglory control over that of glyphosate alone.
Enhanced control of certain herbicide-resistant weed populations. Weed scientists in Indiana and Ohio have conducted extensive research to define management options for glyphosate-resistant giant ragweed in soybean. Extension weed scientists in those states recommend applying the maximum allowable rate of glyphosate (1.5 lb ae per acre) during the first postemergence application followed by a second application (if needed) within 3 weeks. They also have reported some success controlling giant ragweed populations resistant to both glyphosate and ALS-inhibiting herbicides by combining glyphosate with Flexstar or Cobra/Phoenix, followed by a sequential application of glyphosate 3 weeks later. The recommendations for glyphosate-resistant giant ragweed demonstrate glyphosate tank-mixes can be advantageous, but other glyphosate-resistant weeds may warrant a different approach.
Tank-Mixes Not Recommended
Glyphosate-resistant waterhemp. University of Illinois weed scientists do not recommend prophylactically tank-mixing other soybean herbicides with glyphosate for control of waterhemp that might be resistant to glyphosate. If a waterhemp population is known to be resistant to glyphosate but sensitive to PPO inhibitors, applying a PPO inhibitor first followed several days later with glyphosate will likely result in better overall control. Tank-mixing glyphosate with a PPO inhibitor might also provide better control than glyphosate alone. Otherwise, unless the population is confirmed as glyphosate-resistant, recommendations are to make the first glyphosate application when waterhemp plants are 3 to 5 inches tall, followed by field scouting no more than 7 days later to determine treatment effectiveness. If scouting reveals waterhemp control was inadequate and retreatment is necessary, farmers are encouraged to apply a PPO-inhibiting herbicide (lactofen, fomesafen, acifluorfen) at a full labeled rate and with recommended spray additives as soon as possible.
Why recommend glyphosate be applied alone when the herbicide sensitivity/resistance profile is unknown instead of being tank-mixed with a PPO inhibitor? There are several justifications for this recommendation:
1. Glyphosate-sensitive waterhemp plants 3 to 5 inches tall can be adequately controlled with 0.75 to 1.0 lb ae glyphosate per acre. (Variability in control with glyphosate, even for glyphosate-sensitive populations, tends to increase as plants become larger.) Be alert to any waterhemp plants (in the recommended size range) that survive this rate of glyphosate, especially if other weeds in the field are adequately controlled.
2. Research that has evaluated these tank-mixes specifically for control of a glyphosate-resistant waterhemp population is limited. University of Illinois field research in 2008 represented our initial effort to evaluate these tank-mixes on a confirmed glyphosate-resistant waterhemp population. One year of field-generated data is not sufficient from which to draw many conclusions, and lingering questions remain about tank-mixes of glyphosate and PPO inhibitors, including these:
• How likely is it that antagonism will occur when combining glyphosate (a translocated herbicide) with PPO inhibitors (contact herbicides)? If antagonism occurs, will the control of waterhemp (sensitive, PPO-resistant, glyphosate-resistant) or other weed species be affected?
• What spray additive(s) should be included with these tank-mixes? PPO inhibitors generally perform better with COC or MSO, while most glyphosate product labels allow only AMS or NIS.
• What type of spray nozzle, spray pressure, and volume should be used in conjunction with these tank-mixes? PPO inhibitors require more thorough spray coverage of the target vegetation for effective control than do translocated herbicides.
• At what rate should each tank-mix component be applied?
3. More PPO-resistant waterhemp populations have been confirmed in Illinois than glyphosate-resistant populations. A tank-mix of glyphosate and a PPO inhibitor would not improve control of PPO-resistant waterhemp over that of glyphosate alone.
In summary, glyphosate tank-mix partners can improve control of certain problem weed species over that obtained by glyphosate alone. However, in other instances tank-mixes may not be an advisable recommendation. As more data are generated from experiments designed to answer these lingering questions, extension weed scientists will be able to further refine these types of recommendations.