Dicamba comes under scrutiny by regulatory agencies responding to off-target drift concerns or injury to sensitive crops near fields where dicamba is applied over the top to resistant crops. Stanley Culpepper says congressional staffers and other policy influencers sometimes ask this question: “Why do your cotton growers really need dicamba?” Timing has a lot to do with it.

To help illustrate that point, the University of Georgia Extension weed specialist and his team in two years of study compared a timely and not so timely dicamba-based program to a timely and not so timely glufosinate-based program. Palmer amaranth was the target weed for each program in the study.

So, what does timely mean? For both timely programs, the test plots were weed-free at planting and two pre-emergence residual herbicides went behind the planter’s press wheel at planting. The first post-emergence herbicide application from each respective program hit weeds before they reached three to four inches tall. A second application was applied two weeks after the first one, along with an additional layby application.

The timely dicamba program maintained zero Palmer amaranths per acre. The timely glufosinate program resulted in only 50 Palmer amaranths per acre. “Both programs are very effective in controlling Palmer amaranth if growers can get into their fields in a timely manner and apply that important first post-emergence application,” he said.

In the real world, weather, equipment or labor issues don’t always allow growers to make that timely first application.

They looked at what would happen if that first herbicide application for each program was delayed by just seven days, which shifted the programs one week later than the timely check programs. In each delayed program, the pigweeds reached 10 inches to 12 inches tall before the first post application was applied.

In the one-week delayed dicamba program, 386 pigweed per acre survived. In the one-week delayed glufosinate program, 1,895 pigweed per acre survived.

“What our research is showing us, and the reason so many of our growers enjoy the value of the dicamba system compared to the Liberty system is when you can't get to every field on time every time. The delayed dicamba system is more effective, offering more flexibility,” he said.

What was it about the delayed dicamba system that suppressed more pigweeds than the delayed glufosinate program? Though one week delayed, that second application in the dicamba system knocked those taller pigweeds down harder, laying them out closer to the ground, which allowed the layby application to finish off more of those taller weeds.