A study featured in the most recent issue of the journal Weed Technology shows that properly timed post-emergence herbicides can play an important role in the control of glyphosate-resistant Palmer amaranth and waterhemp in new soybean cropping systems. 

Twenty-five states have confirmed populations of glyphosate-resistant Palmer amaranth, while 16 have confirmed populations of glyphosate-resistant waterhemp. Next-generation herbicide-resistant soybean crops hitting the market soon will offer growers broader options for managing these resistant weeds using combinations of glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole and mesotrione.

Scientists at seven universities decided to explore the effectiveness of various pre-emergence and post-emergence herbicide programs that are compatible with these new soybean traits. They conducted field experiments in Arkansas, Illinois, Indiana, Missouri, Nebraska and Tennessee.

Five of the experiments involved pre-emergence herbicides only. Ten involved pre-emergence treatments followed by a post-emergence treatment applied 3-4 weeks later or 6-7 weeks later.

The results showed pre-emergence herbicide treatments combined with a post-emergence herbicide applied 3-4 weeks afterwards delivered the best results — producing 94% 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, a professor at the University of Arkansas and one of the authors of the study, says that despite the success of a combined pre-emergence/post-emergence herbicide program, growers should adopt an integrated, best practices-based approach to weed management. 

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

Full text of the article, “Herbicide Program Approaches for Managing Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) and Waterhemp (Amaranthus tuberculatus and Amaranthus rudis) in Future Soybean-Trait Technologies,” is now available in Weed Technology Vol. 29, Issue 4, October-December 2015.