It may have been fun while it lasted, but the days of planting your crop, spraying your fields a couple of times with a postemergence herbicide and getting acceptable weed control appear to be over, weed scientists are saying.

The growing number of documented cases of glyphosate resistance – in six weed species, including: horseweed, giant ragweed, common ragweed, johnsongrass, Italian ryegrass and Palmer amaranth (pigweed) – in the southern U.S. is forcing growers to move away from what has been most convenient weed control.

“Now that we’re into this glyphosate-resistance era, one of the things I’ve been telling growers is that the era of total postemergence weed control, particularly in soybeans, is over, and I don’t know if we’re ever going back to it,” says Larry Steckel, University of Tennessee Extension weed scientist in Jackson. “And, quite frankly, I don’t know if we should because if we over-rely on one herbicide too long, we’re going to develop resistance to it, just like we’re seeing with glyphosate resistance now.

“It looks like we lead the world in glyphosate-resistant weeds, in the number of species and number of acres infested. It’s in the millions of acres now.

“In a state like Tennessee, we don’t have a row-crop acre that doesn’t have at least one resistant weed in it,” says Steckel.

Glyphosate resistance arrived in a field in Lauderdale County, TN, in 2001. A team of weed scientists determined horseweed plants in the field were resistant to glyphosate.

Since then giant and common ragweed have also been confirmed resistant.

“The most recently found glyphosate-resistant weed species are johnsongrass and Italian ryegrass, which were confirmed in Arkansas and Mississippi,” Steckel notes. “Of most concern in the South, due to its competitive nature, is Palmer amaranth.”

After being discovered in two counties in Georgia and two in North Carolina in 2005, glyphosate-resistant pigweed has spread over 120 counties in eight southern states. The movement has occurred in a number of ways including flooding, via equipment and in fields where gin trash was spread.

“However, no discernable pattern could be seen with some glyphosate-resistant Palmer amaranth infest-ations,” Steckel says. The recent research reported from Georgia showed the glyphosate-resistant trait in Palmer amaranth can be moved by pollen. This may help explain the spread to many of these fields.”

Research at southern universities indicates residual preplant- and pre-emergence-applied herbicides appear to provide the best chance for successful pigweed control without relying on glyphosate.

“With glyphosate-resistant Palmer amaranth, we’re going to have to manage with a residual herbicide up front,” Steckel says.

He recommends Fomesafen or Flexstar applied 14 days before planting in the Midsouth or pre-emergence in the Southeast for control; or flumioxazin or Valor applied 21 days before planting has provided good control if activated by rainfall.

In dryland production for both cotton and soybeans where the success of a pre-emergence-applied herbicide is dictated by rainfall, the addition of a preplant residual herbicide application would provide the best chance to catch an activating rainfall prior to Palmer amaranth emergence. Postemergence Palmer amaranth control in soybean and cotton can be successful with PPO herbicides in soybean or glufosinate in soybean or cotton.

“The biggest change is we’re going to have to have eyes on the field because when that residual gives out, and we get that first flush of pigweeds, we’ve got to be Johnny-on-the-spot with a postemergence application whether you’re in the Liberty Link system and spray Ignite or you’re going to spray a PPO like Flexstar,” says Steckel.

“Timing is critical; good control can only be obtained if those herbicides are applied on Palmer amaranth less than 4 in. tall. A major concern is that over reliance on the PPO herbicides will promote resistant populations of Palmer amaranth to this chemistry,” he says.