Few would argue with the statement that the commercialization of glyphosate-resistant soybean varieties has dramatically changed how weeds are managed in soybean-production systems. When first commercially available during the 1996 growing season, there were only limited options with respect to which glyphosate-containing product could be used for in-crop applications. However, numerous glyphosate-containing products are now available for postemergence use in glyphosate-resistant soybean varieties as well as glyphosate-resistant corn hybrids. While all these options contain the same ingredient (glyphosate) that controls the weeds, other differences exist that may influence the level of weed control obtained. What are some of these differences and how would they influence weed control?|
Determining Equivalent Product Rates
Is 1 pint of glyphosate product X equivalent to 1 pint of glyphosate product Y? The simple answer is that it depends. Equivalent rates of herbicides containing the same ingredient are usually calculated based on equivalent amounts of active ingredient. With glyphosate-containing products, however, equivalent rates must be calculated on the basis of comparable acid equivalents. The acid equivalent represents only that portion of a herbicide formulation that is physiologically active in the plant. All glyphosate formulations commercially available for use in glyphosate-resistant crops are formulated as salts to enhance absorption into the plant. Once the formulation (acid + salt) is absorbed into the plant, the salt portion is cleaved off, leaving only the acid portion to block the site of action. The primary salts used in formulated glyphosate products include isopropylamine, monoammonium, trimesium, and diammonium. Different formulations may contain different salts. For example, Touchdown 5 contained the trimesium salt, whereas Touchdown contains the diammonium salt. Roundup Ultra Max contains the isopropylamine salt, whereas Roundup Ultra Dry contains the monoammon-ium salt. These different salts can influence the amount of active ingredient/acid equivalent contained in a gallon of formulated product because they have different molecular weights. Table 3 lists many commercially available glyphosate formulations along with the amount of active ingredient and acid equivalent contained in 1 gallon of formulated product. Equivalent product application rates based on acid equivalent are also presented. Some example calculations following the table are provided to show how to determine equivalent product rates based on acid equivalents.
Is a surfactant formulated with the product, or does a surfactant need to be added?
Several glyphosate products contain a surfactant already in the formulation so no additional surfactant is required. Other glyphosate products require the user to add the appropriate amount of surfactant. Surfactant selection may influence the level of weed control achieved, so choose a high quality surfactant based on past experience or per manufacturer recommendations. Each respective glyphosate-containing product label will indicate whether surfactant should or should not be added. Virtually all glyphosate products, however, recommend the addition of a spray grade ammonium sulfate (AMS).
The labels for most of these glyphosate-containing products are similar; however, some minor label differences may exist (for example, rain-free intervals). Even though these labels are similar, you should always consult the respective product label for specific product recommendations.
In many university trials, the various glyphosate-containing products have performed very similarly. If equivalent application rates are calculated based on acid equivalents and a high-quality surfactant is added when recommended on the label, few if any differences in weed control should result.-- Aaron Hager and Christy Sprague