Important Thoughts About Weed Resistance

Ask questions before you decide on a plan.

Published on: Mar 15, 2010
The annual No-Till Breakfast in Ripley County, now sponsored by soil and water conservation districts throughout southeast Indiana, is a good place to get questions answers. Someone asked Barry Fisher, who moderated the exchanges, about glyphosate-resistant weeds. Fisher is state agronomist for the Natural Resources Conservation Service. 

Some farmers in that part of the state now report they have both glyphosate- resistant marestail and resistant giant ragweed in the same field. One early step is to add 2,4-D ester and see if that helps control, Fisher says.

One farmer asked, "Why can't I just double to rate if I have a resistant weed?" surely that much extra chemical would knock out the weed, the farmer proposed.

Without debating the merits of such a proposal too deeply, Fisher pointed out a couple holes in the theory. "Some of these weeds have been tested and are resistant up to at least 10 times the rate of glyphosate normally recommended and applied," he notes.

"Besides, most crops aren't labeled to withstand more than a double rate. Even though they are Roundup Ready crops, there is no license to start applying what ever you like without a license. At some point, corn hybrids will show injury symptoms to overdoses of the herbicide.

Larry Huffmeyer, a farmer in the area, and a chemical rep, alerted farmers as to what they could expect form such a situation. Not every plant in the population will be totally resistant, he says. For example, if a field where the farmer says marestail is resistant, glyphosate may still kill about one-third of the weed population. There will likely be another one-third that will be impacted, perhaps stunted, although it will likely recover. Finally, about one-third of the plants will likely be killed outright.

"Rotating corps can help on preventing weed resistance as much as anything, Fisher says. Of  course, it heps if you also rotate herbicide chemistries at the same time.

Factors as seemingly insignificant as what type of water goes in the tank may also influence how good of kill you get in any particular situation. Indiana is noted for having water that tends to interfere somewhat with glyphosate applications.

Some additives that you can buy at your local fertilizer and chemical dealer can likely correct the situation. Talk to your dealer to see what his recommendation is for additives to go with glyphosate.