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Weed control growing much more complex, new tools coming

The days of backing up to the tank, filling up with glyphosate and spraying it on every Roundup Ready acre for excellent weed control are officially behind us now. According to west Tennessee weed scientist Larry Steckel, a new era of weed control has begun.

Two things will be driving the change, Steckel says — glyphosate-resistant weed species and a number of new herbicide trait technologies coming on line to help producers manage those resistant weeds.

“We now have eight glyphosate-resistant weeds in the United States, most in the Mid-South, Southeast and Midwest,” Steckel told producers attending the annual Cotton Focus held at the West Tennessee Experiment Station. “The two biggest in the Mid-South are horseweed and Palmer pigweed (Palmer amaranth).”

Cotton Incorporated and the National Cotton Council helped finance Steckel’s research, which in 2007 tracked down two new locations of Palmer pigweed with above average tolerance to glyphosate in Lake County. Tennessee now has six confirmed glyphosate-resistant Palmer pigweed sites in three counties. Steckel also confirmed the presence of resistant giant ragweed in Ripley, Tenn., that survived one 40-ounce and two 88-ounce applications of Roundup.

Glyphosate-resistant Palmer pigweed was discovered in Georgia in 2005, and Steckel noted many cotton fields in south Georgia now have it. “One of the most successful weed management strategies, according to (University of Georgia weed scientist) Stanley Culpepper’s work, shows that resistant Palmer pigweed can be managed in irrigated cotton if producers can get some activation on residual herbicides like Prowl, Dual, Reflex and Valor. But in dryland cotton, where pre-emergence herbicides have not been activated, they’re losing the battle.

“They’re starting to think a little out of the box in Georgia,” Steckel said. “They’re putting down a cover crop just to get some kind of mulch on there to help keep Palmer pigweed down. Roundup alone just isn’t doing it.”

Glyphosate-resistant horseweed continues to be a big problem for Mid-South producers, especially those in west Tennessee. Currently, for burndown when glyphosate-resistant horseweed is present, Steckel recommends a tank mix of glyphosate and dicamba.

But in 2006-07, this tank mix wasn’t as consistent “because No. 1, we had a very dry spring. Dicamba wasn’t getting picked up through the roots and the shoots because it was so dry. No. 2, our horseweeds were a lot larger in general at the time of application. It’s a lot harder to kill a large weed.”

Steckel also reports more overall horseweed pressure in west Tennessee fields. “In 2004-05, a heavy horseweed population was 10 plants per square foot. In 2006-07, we developed populations that were 20 to 25 plants per square foot. We’re still getting 85 percent control, but 25 plants is a major issue.”

Steckel says the optimum burndown timing for resistant horseweed is at the end of February or the first part of March. “The idea is to control it while it’s small. That’s where we’re missing them. Catch them at the rosette stage and you’ll do a pretty good job. Last year, we saw horseweed still surviving in fields, particularly when Clarity was applied on bigger horseweed.

Control options inside the 21-day window for Clarity “do not work as well as Clarity,” noted Steckel, who recommends 48 ounces of Gramoxone, with either Caporal, Cotoran or Direx. “Ignite is also an option, but I’m a little hesitant because it has such environmental sensitivity. Put it on when it’s warm, and you’ll get an easier kill.”

According to a recent survey conducted by Steckel with area retailers, 90 percent of cotton acres in the region received some kind of pre-emergence product for horseweed. “In one case, a grower burned down with glyphosate and dicamba and got 100 percent control. Then he dropped the planter in and banded Prowl, which held back the second flush of horseweed. Most residuals do a good job.”

New technologies will change weed control again in a few years, noted Steckel. DuPont’s GAT Optimum traits with glyphosate and ALS tolerance is targeted for commercialization in 2009. “So you can mix Harmony or Express in the tank mix. It will be available for corn and soybeans in 2009, but could be a couple of years down the road on cotton.”

Bayer CropScience’s version of glyphosate tolerance technology, Glytol, will also be available, soon. “What this means with Optimum GAT and Glytol, Monsanto is going to have competition with its Roundup Ready trait for the first time.”

LibertyLink soybeans are expected to enter the market in 2009, “which will give us a post option for horseweed that we currently don’t have,” Steckel said.

Dow AgroSciences is expected to release a 2,4-D tolerance gene for cotton, corn and soybeans, according to Steckel. “This trait will also be linked to ACCase inhibitor tolerance, so corn is going to have tolerance to Fusilade and Assure. This technology is expected to hit the market in soybeans in 2010, followed a year later by cotton.”

Monsanto is developing a trait for dicamba tolerance which could be stacked with the Roundup Ready and LibertyLink traits and incorporated in cotton, soybeans and corn, Steckel said. “We’ve seen this technology in soybeans this past year, and it has very good tolerance. We’re looking at 2011 or 2012 on soybeans and cotton.

“Five or six years out, we’re going to see HPPD resistant traits (for pigment synthesis inhibitors) incorporated into a lot of our crops,” Steckel said. “This will provide resistance to a lot of the bleaching herbicides used in corn like Callisto. They’re excellent on small-seeded broadleaves. They have a very good environmental profile, and currently there is no known resistance to them.”

While these new tools are going to enhance the management of glyphosate resistant weeds, there can be concerns with drift and tank contamination. “The 2,4-D and dicamba traits are not interchangeable. If you spray your cotton that’s 2,4-D tolerant and move into another variety that’s dicamba tolerant, you’re going to have to rinse the tank out thoroughly. That’s a real concern. The best way to clean 2,4-D out of the tank is to put tank cleaner in there and let is sit for a day.”

Other issues include “controlling volunteer crops from the previous year when they have all these traits in them,” Steckel said. “And of course, anytime you throw one of these genes in there, it’s costing another $5 or $6 an acre. So costs are going up.”


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