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Weeds’ chemical resistance is a growing crop problem

By Bonnie Coblentz
MSU Ag Communications

MISSISSIPPI STATE – Mississippi row crop producers are facing a growing problem, as five common weeds have developed resistance to the primary herbicide used to manage them.

Roundup is the trade name for glyphosate, a powerful broad-spectrum herbicide that can kill a wide range of weeds in varying growth stages. But by the 2010 growing season, 19 weeds worldwide had become resistant to glyphosate, and five are found in Mississippi. These weeds are horseweed, Italian ryegrass, Johnsongrass, and Palmer amaranth and waterhemp -- both species of pigweed.

Crops genetically modified to be resistant to glyphosate were first marketed in 1996. Known as Roundup Ready seed, it allowed producers to apply glyphosate across a field, killing weeds but leaving the crop undamaged.

Tom Eubank, a Mississippi Agricultural and Forestry Experiment Station soybean weed scientist, said that before Roundup Ready crops were developed, weed control was accomplished primarily with residual herbicides and tillage.

“These are soil-applied herbicides that, upon activation by rainfall, prevent weeds from emerging,” Eubank said. “Most of the time, they were applied either just before or at planting.”

A few post-emergence herbicides were also available, and these controlled weeds that emerged during the growing season.

“Total post-emergence weed programs were not extremely effective before Roundup Ready cropping systems,” Eubank said. “With a total post program, you spray a group of weeds and while you may control those, an immediate flush of more weeds usually occurs after a rainfall. You’re also limited on how many treatments you can make in a season.”

When Roundup Ready seed was introduced, revolutionized production practices.

“Producers did not have to rely on tillage and residual chemistries to control weeds. They could delay treatments and wait until the weeds got a little size on them. They could make two to three applications of Roundup per season, and the fields would be clean,” Eubank said.

But those good days didn’t last long. Glyphosate resistance was documented in horseweed in 2000 in Delaware and in Mississippi in 2003. In 2005, glyphosate resistance was officially documented in Palmer amaranth in Georgia, and in Mississippi in 2008. By 2005, the state had glyphosate-resistant horseweed and Italian ryegrass, and then water-hemp in 2010.

John Byrd, a weed scientist with Mississippi State University’s Extension Service, said producers made a mistake by switching to the almost exclusive use of one chemical for weed control in every row crop but rice.

“Anytime you rely solely on one herbicide or one group of related herbicides, then you’re going to naturally select for those individual weeds in the population that have tolerance to that specific herbicide,” Byrd said. “Some plants will survive and produce seeds with similar resistance.”

Byrd said glyphosate is not the only herbicide to which plants have developed a resistance. Certain weeds are resistant to atrazine and 2,4-D, for example.

“There is no perfect herbicide that controls every weed 100 percent of the time,” Byrd said.

In the past, producers scouted during the growing season to determine what weeds were present in their fields. They calculated the chemicals and quantities needed to control these weeds and sprayed this mixture on their own fields.

“They had to come up with a recipe -- a little bit of this and a little bit of that -- to control what was out there. But with Roundup, you put it in the tank, and it controlled pretty much all of what was out there,” Byrd said.

Glyphosate’s effectiveness transformed a practice that used to be very complex into almost a one-step process.

“Weed control is a little bit like war,” Byrd said. “The more variety you have in your artillery, the more successful you’re going to be at winning that war.”

Byrd said the key to managing resistance is to rotate the use of herbicides that have different modes of action, or the process within the plant that the herbicide blocks to ultimately cause its death.

“Only by following that strategy will a farm be able to ensure that resistant populations don’t develop on their property,” Byrd said.

LibertyLink is a new herbicide-resistant crop technology that has been available for cotton since 2004 and soybeans since 2009. The herbicide used in LinbertyLink crops is Ignite, a broad spectrum herbicide that controls a wide range of weed species, and it is effective on some weeds that Roundup had difficulty with.

“I hope we can take the lessons we learned with the Roundup systems and not go down that path with the LibertyLink systems,” Eubank said.

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Released: Aug. 5, 2010
Contact: Dr. Tom Eubank, (662) 822-1964