Yield losses mount as control measures dwindle
By Fran Howard and Rhonda Brooks
This little piggy keeps getting farther away from home. Scientists are convinced that Palmer amaranth, also known as pigweed, could soon blanket the Corn Belt. It’s just a matter of time.
Palmer amaranth is one of the few plants that actually thrive in the hot, dry conditions that prevailed across much of the country this summer.
"That’s when it’s in its wheelhouse," says Larry Steckel, University of Tennessee Extension weed specialist.
Steckel says he is concerned that glyphosate-resistant Palmer amarath is making strong inroads into farmers’ soybean fields in the lower Midwest. Glyphosate-resistant Palmer amaranth, which is prevalent throughout the Southeast and mid-South, has been found in farm fields as far north as southern Michigan.
"Palmer amaranth is not as widespread in Illinois as the other amaranth species, but we have some concern that it might not remain that way," says Aaron Hager, Extension weed specialist with the University of Illinois. "The growth rate and competitive ability of this species exceed that of other amaranth species."
"I’m seeing the same things in parts of the Midwest that we’ve had in the mid-South, where glyphosate isn’t providing adequate control any longer," Steckel contends. "We’ve got to get some other chemistries in the mix to control this weed."
Quick spread. In fall 2011, Jason Norsworthy, University of Arkansas agronomist, surveyed four southern states, including Arkansas. In Arkansas alone, 2.5 million acres were infested with glyphosate-resistant Palmer amaranth, representing about 80% of the state’s cotton fields and about 60% of its soybean fields. First seen in Arkansas in 2005, the weed spread throughout the state in just seven years.
"Across the state last year, soybeans sustained 5% yield loss caused by pigweed," Norsworthy says. Some fields were a total loss and thus abandoned. The average statewide yield losses represent $60 million to $70 million in lost revenue, he adds.
In addition to developing resistance to herbicides, Palmer amaranth grows at an alarming rate of 2" per day. "It is a challenge to kill this weed postemergence," Norsworthy says.
A resilient threat. The first step in controlling this highly problematic weed is to recognize it. The first leaf shoots of Palmer amaranth are relatively long compared with waterhemp, another amarath species. As Palmer amaranth matures, its leaf petioles (the stalks that attach the leaf blades to the stem) are two to three times longer than its leaf blade. It also has little to no pubescence and the plants are either male or female, which introduces a great deal of genetic diversity, Hager says.
Once the weed is identified, Hager and Norsworthy say that the best approach to controlling it is to use a combination of soil-applied herbicides, post-emergence herbicides and mechanical cultivation.
"Remember, once glyphosate-resistant Palmer amaranth grows larger than 4" tall, there are no herbicides that will control it in conventional or Roundup Ready soybeans," Steckel says. In the LibertyLink system, a very high rate of Liberty herbicide (36 oz. per acre) can control the weed up to about 6" tall, he says.
Norsworthy adds that tilling the soil to 10" with a moldboard plow aids in controlling the resilient weed.
"Tennessee, which is a predominantly no-till state, is starting to see a major push by its producers to begin tilling again," he notes.