Based on the number of papers dealing with glyphosate-resistant weeds at this week's North Central Weed Science Society meeting in St. Louis, you could draw one of two conclusions. Either the weed scientists have all the major weed control issues sorted out already, or else glyphosate-resistant weeds are a genuine threat. Depending on who you talk with, you could make an argument either way.

A symposium on glyphosate-resistant weeds attracted an overflow crowd on Thursday morning to hear updates on glyphosate-resistant marestail in the DelMarVa area, west Tennessee, Kentucky, southern Indiana and southern Ohio. Marestail (horseweed) is a big concern because it is a prolific seed producer and the seeds are easily blown around by the wind.

Although marestail is the first row-crop weed to develop glyphosate-resistance in the US, there are several other species being watched. Weed scientists in Iowa and Missouri have been testing waterhemp from a handful of fields that seems to have more tolerance to glyphosate. In addition, there have been more complaints about marginal control of velvetleaf, ivyleaf morningglory and lambsquarters control with glyphosate.

The big question is whether it makes more sense to try to avoid resistance problems or simply to deal with them if they show up. Although both claim to be proactive, as you might expect, Monsanto and Syngenta are taking slightly different approaches to that question.

Monsanto is making recommendations for resistance management in areas where glyphosate-resistant marestail has already been found as well as in the surrounding area. Monsanto has submitted a supplemental label to the EPA with modified requirements for using 2,4-D tankmixes for burndowns in the area surrounding the locations with known resistance.

Syngenta is taking a more aggressive resistance management approach based on herbicide rotations. In corn or soybean-based rotations, they recommend no more than two applications of glyphosate in a field ov er any two-year period. They recommend rotating Roundup Ready crops with conventional crops, and using full rates of glyphosate to minimize tolerant weed escapes.

Iowa State University weed scientist Mike Owen questioned whether proactive resistance management is economically feasable in the short run. He expects growers to continue to use low-cost, effective weed control tools such as glyphosate until they are forced to switch to alternative programs.

With few, if any, new blockbuster chemicals in the pipeline, the question may become whether there will be alternative programs to switch to if glyphosate loses its effectiveness.

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