Palmer amaranth, also known as Palmer pigweed, has always been tough to control all season long. It grows aggressively and reproduces seemingly overnight. Combine those characteristics with resistance to glyphosate and you've got a real weed threat.

Glyphosate-resistant Palmer amaranth has been raising its threatening green head the last few years. And it's spreading fast.

Bo Stone is all too familiar with the weed. The Rowland, N.C., grower first identified small patches of glyphosate-resistant Palmer amaranth in his fields in 2007. “I knew immediately I had to change my weed-control strategy.”

Last year, he added a pre-emergence herbicide to his normal burndown application for the first time in years. To determine which product would deliver the best control when applied at planting, he compared DuPont's Envive to Prefix and Valor herbicides.

“The results were good for all three, but I got better control and longer control with Envive. It did a good job on the pigweed and morningglory, which are the only weed escapes I see from my Roundup program.

“I am shooting for 100 percent control of pigweed,” he says. “With this glyphosate resistance issue, I can't risk any weed escapes.”

A more aggressive weed control strategy is essential to getting a handle on the growing Palmer amaranth threat, says Trey Koger, Mississippi State University Extension soybean specialist. “We saw a lot more Palmer amaranth in fields last year than in previous years. We don't know if they were all glyphosate-resistant, but there was more pigweed pressure on crops in 2008.”

Glyphosate-resistant pigweed first appeared in Georgia in 2004 and was confirmed in 2005. Since then, populations have been discovered in North Carolina, Tennessee, Arkansas and Mississippi. Last year's heavy spring rains in several states delayed soybean planting and gave weeds a head start, says Koger. “That made pigweed a real challenge because it grows so fast and emerges over such a wide window of time.”

A 40 percent increase in soybean acres in Mississippi between 2007 and 2008, and a 25 percent soybean acreage bump predicted for 2009 could help improve control options for glyphosate-resistant Palmer amaranth, he notes. “This weed is a bit easier to control in soybeans than in crops such as cotton. The key is to attack early.”

A new product that offers some real advantages in dealing with pigweed, Envive uses three active ingredients and two modes of action. “Envive is an excellent tool for controlling glyphosate-resistant pigweed,” says Koger. “It has long residual activity, with good efficacy on both broadleaves and grasses.”

Envive also is a good control option when Palmer amaranth is resistant or tolerant to ALS-inhibitor herbicides, he adds. “ALS resistance was a problem years ago, then Roundup Ready crops came along and reduced the use of ALS herbicides. Now we're starting to hear about some ALS resistance again,” he says. “By using a product with another mode of action, growers can help reduce the chance that either type of weed resistance will develop in their soybean fields.”

Field trials conducted at the University of Tennessee in 2008 demonstrate the value of incorporating a pre-emergence herbicide into a weed control plan, especially when pigweed is a problem, says Angela Thompson McClure, corn and soybean specialist.

A very wet spring resulted in tremendous weed pressures early in the season. “While most of the pre-emergence products gave out early, we got three to four weeks of good control with DuPont Canopy and Envive. Where we used only burndown treatments, the weeds came on strong and caused serious competition for the young crop.”

She says last year's field conditions helped make the case for including a pre-emergence treatment in a Roundup Ready program. “Even the early glyphosate treatment couldn't keep up with the weed pressure we saw.”

Despite having talked with growers for the past four years about the potential for glyphosate-resistant Palmer amaranth, most haven't taken a proactive approach, she notes. “Until it shows up in their field, or their neighbor's field, most don't do much to prevent it. After the sizeable increase in confirmed glyphosate-resistant Palmer amaranth cases we saw last season, I hope more growers will be motivated to broaden their weed-control strategies.”