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.

The growing number of documented cases of glyphosate resistance — in six weed species in the southern U.S. now — is forcing growers to begin to move away from what was the most convenient weed control most of them have ever known.

“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, Extension weed scientist with the University of Tennessee in Jackson.

“And, quite frankly, I don’t know if we should because if we over-rely on one herbicide too long, you’re going to develop resistance to it, just like we’re seeing with glyphosate resistance now.”

Steckel, a speaker at the Pan-American Weed Resistance Conference in Miami Jan. 20, gave weed scientists attending the conference sponsored by Bayer CropScience an update on the weed resistance situation in the southern U.S. The picture isn’t too rosy.

“It looks like we lead not only the United States, but probably the world in having our problems with glyphosate-resistant weeds,” he said. “Not only in the number of species — we have six or seven different weed species that are resistant to glyphosate — but also in the number of acres that we have infested. It’s in the millions of acres now.

“You get in a state like Tennessee, and we do not have a row crop acre that does not have at least one resistant weed in it,” said Steckel, who was speaking for a group of 10 weed scientists from the southeastern U.S. who were listed as collaborators on the presentation.

The scientists said six weed species have been identified as being resistant to glyphosate in the southern United States: horseweed, giant ragweed, common ragweed, Johnsongrass, Italian ryegrass and Palmer amaranth or pigweed.

Although it was discovered earlier in Delaware, glyphosate resistance arrived in the Mid-South in a field in west Tennessee’s Lauderdale County in 2001. A team of weed scientists led by Bob Hayes, now resident director of the West Tennessee Experiment Station, determined horseweed plants in the field were resistant to glyphosate.

Since then two Ambrosia species have also been confirmed to be glyphosate resistant: A. trifida (giant ragweed) and A. artimisiifolia (common ragweed). Of these two, A. trifida has proven to be the biggest problem in cotton and soybean production in fields adjacent to the Mississippi River in Arkansas and Tennessee.

“The most recently found glyphosate-resistant weed species are Sorghum halapense (Johnsongrass) and Lolium multiforum (Italian ryegrass) which were confirmed in the southern U.S. states of Arkansas and Mississippi,” Steckel notes. “The GR weed that is of most concern in the southern U.S., due to its competitive nature, is Amaranthus palmeri (Palmer amaranth).”

After being discovered in two counties in Georgia and two in North Carolina in 2005, glyphosate-resistant Palmer amaranth has spread over 120 counties in eight southern U.S. states. The movement has occurred in a number of ways.

“First, field observations would indicate that spring floods moved glyphosate-resistant Palmer amaranth seeds. Second, glyphosate-resistant Palmer amaranth could be found in fields where gin trash had been spread. Third, glyphosate-resistant Palmer amaranth field infestations appeared from their placement to have been mechanically moved by field equipment from field to field,” he notes.

“However, no discernable pattern could be seen with some glyphosate-resistant Palmer amaranth infestations. The recent research reported from Georgia that showed the glyphosate-resistant trait in Palmer amaranth can be moved by pollen 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 Palmer amaranth control without relying on glyphosate.

“As far as managing resistance going forward, particularly in cotton and soybeans and in particular with glyphosate-resistant Palmer amaranth, we’re going to have to manage it with a residual herbicide up front,” Steckel said in an interview.

Fomesafen or Flexstar applied 14 days before cotton planting in the Mid-South or pre-emergence in the Southeast as well as flumioxazin or Valor applied 21 days before planting has provided good control if activated by rainfall or irrigation.

In soybeans, those same herbicides or premixes with those herbicides have also provided good weed control if activated by rainfall or irrigation. 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 cotton or soybean.

“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 LibertyLink system and spray Ignite or whether you’re going to spray a PPO like Flexstar.”

Growers must be prepared to spray those herbicides when Palmer amaranth is about 4 inches tall. “Timing is critical as good control can only be obtained if those herbicides are applied on Palmer amaranth less than 4 inches tall. A major concern of weed scientist is that over reliance by cotton and soybean growers to the PPO herbicides will promote resistance populations of A. Palmeri to this class of chemistry.

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