Glyphosate-resistant weeds found in multiple Indiana counties

As the summer drought continues, farmers across Indiana are struggling to control glyphosate-resistant weeds, says a Purdue Extension specialist.

Farmers are seeing marestail, giant ragweed, and common waterhemp, throughout their fields this year, and the hot, dry weather is making it more difficult to get a firm grip on controlling them.

According to Bill Johnson, marestail and giant ragweed are spread widely around the state. Waterhemp is mainly seen in southwest, northwest and east-central Indiana and glyphosate-resistant populations have now been confirmed in nine counties. Glyphosate-resistant Palmer amaranth has been confirmed in one county. Although glyphosate-resistant marestail is the primary concern in 2012 and can be found in most counties where soybean is grown, waterhemp and giant ragweed are of growing importance.

Johnson said marestail is extra problematic this summer because of the mild winter.

"We didn't get winterkill for the populations that emerged in the fall," he said. "The ones that emerged this spring emerged very early and, because of the warm weather conditions, were able to grow unabated, and quickly became too large to control with labeled herbicide rates."

Glyphosate-resistant broadleaf weeds are a greater problem in soybean fields than in cornfields because we have fewer herbicides to control broadleaf weeds in a broadleaf crop such as soybean. In addition, the drought is causing the weeds to dry out and curl up, making it less sensitive to herbicides.

Johnson said that as weeds show glyphosate resistance, farmers are turning to older herbicides - something he said is feasible but requires a lot of work and fine-tuning.

Although crops that have a resistance to older broadleaf herbicides such as 2,4-D and dicamba are being developed, Johnson said it will be a few years before we are able to use them.

Farmers in the southeast portion of Indiana saw a severe case of glyphosate-resistant weeds 10 years ago. The problem gradually spread. Five years ago, glyphosate-resistant weeds became a statewide problem.

Johnson said farmers should scout fields to determine where glyphosate-resistant weeds are worst, and know if the current treatment plan is effective.