Agronomists and researchers with the University of Arkansas System Division of Agriculture said this week that they are continuing to push for expanded use of dicamba-based herbicides in cotton and soybean varieties that are genetically engineered to tolerate the modes of action.

The U.S. Environmental Protection Agency announced late last week that it will extend by 30 days the public comment period on Monsanto’s plea for expanded use of its M1691 dicamba herbicide, commercially known as Roundup Ready Xtend. The public comment period is now open until May 31, 2016.

Bill Robertson, Extension cotton agronomist, said cotton varieties that are dicamba tolerant have been commercially available in Arkansas and elsewhere since 2015, and many growers have incorporated them into their crop rotation. However, the herbicide itself is currently only labeled for use as a post-harvest “burn down” technology, and cannot be used during the growing season. The technology’s current label also doesn’t allow for its use in tank mixes of any kind.

“Basically, we can’t legally use dicamba any later than three weeks prior to planting cotton,” said Robertson, who plans to submit comments to the EPA on behalf of the Division of Agriculture.

When the new dicamba technologies were first introduced in late 2014, many growers and agriculture experts looked at it as the best new weapon against glyphosate-resistant pigweed, which have bedeviled growers in cotton and, increasingly, soybeans.

Tom Barber, university associate professor of weed science, said the prohibition on combining dicamba with other chemicals in tank mixes hobbles its effectiveness as a resistance management tool.

“We’re concerned that the proposed label that we’ve been made aware of doesn’t allow for any tank mixes with other modes of action herbicides,” Barber said. “Therefore, it would not be as efficient and effective a resistance management as we would like.”

To learn more about weed management in row crops, contact your local Cooperative Extension Service agent or visit www.uaex.edu.