The journal of Weed Technology recently published a study showing the proper application of post emergence herbicides are an important factor in controlling glyphosate-resistant Palmer amaranth and waterhemp in new soybean cropping systems.

Glyphostae-resistant Palmer amaranth has been confirmed to challenge growers in twenty-five states, and glyphostae-resistant waterhemp is present in 16.  Soon growers will be offered options combining glyphostae, glufosinate, dicama, 2,4-D, isoxaflutole and mesotrione for their next generation soybean crops.

The study was conducted in six states at seven universities.  Scientists looked at the effectiveness of various preemerge and postemerge herbicide compatible with the new soybean traits.

Five of the experiments involved preemergence herbicides only. Ten involved preemergence treatments followed by a postemergence treatment applied three to four weeks later or six to seven weeks later.

The results showed preemergence herbicide treatments combined with a postemergence herbicide applied three to four weeks afterwards delivered the best results – producing 94 percent or greater control of Palmer amaranth and waterhemp. Scientists also determined that precipitation matters. In situations where there is little or no rainfall in the weeks following the application of residual herbicides, a second application may be needed for season-long control.

Jason Norsworthy, Ph.D., a professor at the University of Arkansas and one of the authors of the study, says that despite the new traits the most effective approach to weed management is still an integrated, best practices-based approach.

“There are already Amaranthus spp. weed populations resistant to many of the herbicides used in the new soybean herbicide programs,” he said. “For sustainable results over the long-haul, it is vital that we rotate herbicide mechanisms of action and incorporate appropriate nonchemical controls as well.”

You can read the full article, “Herbicide Program Approaches for Managing Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) and Waterhemp (Amaranthus tuberculatus and Amaranthus rudis) in Future Soybean-Trait Technologies,” in Weed Technology Vol. 29, Issue 4, October-December 2015.