It’s best to expect that more than one postemergence application will be needed to control giant ragweed, Hager says. “Fence lines tend to harbor giant ragweed that, if left uncontrolled, will continue to provide seed to infest fields,” he says. “So scout 7 to 10 days after an initial postemergence application and look for a second flush of weeds.”

Annual morningglory weeds also can congregate along field borders. “Since farmers typically have more problems with morningglory on the border edges of the field, rather than in the middle of the field, a spot treatment on the outer edges later in the season can be an effective strategy,” Hager says.

Timing is also extremely important with annual morningglory control, Green notes. “It can get away from you very quickly,” he says. “Morningglory is not a weed that we find in every field in the state, but where it’s found, it’s not as effectively controlled with glyphosate as other weeds typically are. So when using glyphosate for weed control where annual morningglory is present, you’ll need to use a soil residual herbicide at planting time or tank mix a product with glyphosate that offers good control for morningglory.”

Another concern for the future in the southern Corn Belt is johnsongrass, which has already been reported to be glyphosate resistant in South America, Mississippi and Arkansas, Green says. In Kentucky, it has been confirmed resistant to herbicides classified as ALS and ACCase inhibitors, he adds.