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Glyphosate Resistant Weeds

20:34PM Sep 05, 2008

This article is a supplement to "Glyphosate Resistant Weeds on the Move,” which ran in the October 2005 issue of Farm Journal

Continuous Glyphosate-Tolerant Crops

As more farmers adopt Roundup Ready Corn 2 and Agrisure GT hybrids, it's apparent that more fields that rotate between corn and soybeans and even corn on corn are in a continuous Roundup Ready weed management program.

Monsanto recommends adding other herbicides and even cultivation to this program to help with tough weeds, timeliness of post applications and potential resistance.

"Our general recommendations to farmers are to evaluate the weed spectrum you have and include other weed control methods such as cultivation and other herbicides,” Says David Heering, Roundup technical manager for Monsanto. In continuous rotations of Roundup Ready corn and soybeans, he says farmers should consider using residual products.

This is especially helpful for farmers managing a large number of acres, where tough weeds such as waterhemp and lambsquarters may get too big to control before they can be sprayed.

In Roundup Ready Corn 2, Heering suggests using Harness Extra or Degree Extra followed by Roundup. In Roundup Ready soybeans, he says Intrro, Prowl and Valor followed by Roundup can be a good combination.

Additionally, Heering stresses the need to use glyphosate at the right rate and time. "Make sure you use a full rate and treat when the weeds are growing,” he says. "If weeds get large before you spray them, use the higher rate to control them.”

Resistant Weeds

For farmers facing glyphosate-resistant horseweed in the eastern Corn Belt , the basic program is a pre-plant burndown that includes 2,4-D when the weeds are small. "The base recommendation is to control the weeds before you plant,” says Heering. "You need to kill them before they are 6” tall.”

If late flushes of horseweed are expected, a residual may also be indicated. Heering says farmers can take an extra shot at the resistant winter annual with a fall application of 2,4-D, which is also a handy strategy for cleaning up other weeds, such as dandelions, in no-till fields.

In no-till cotton, Heering says it's possible to control glyphosate resistant horseweed. He says adding Clarity to glyphosate for a pre-plant burndown is effective. Valor also helps control if no horseweed has emerged, but if the weeds are up, there can be some antagonism and loss of control when Valor is added to Clarity and glyphosate. For later flushes, Heering says farmers should use residual products at planting and layby. "About 70% of cotton farmers already use a residual at layby,” he says.

University of Tennessee weed scientist Larry Steckel the best results he has seen come from a program that uses a burndown of Gramoxone, glyphosate or Ignite—if the weather is warm—combined with 8 oz. of Clarity 21 days before planting. "The problem is being timely when weather and breakdowns can cause delays,” he says.

And that's just the beginning. "We have horseweed emergence 10 months out of the year,” Steckel says. Farmers have learned to put down residual at planting—so much so that Direx, Cotoran and Caporal were in short supply at ag retailers last spring, he adds. "But after these products go down, you are in no man's land for resistant horseweed control until lay by.”

If horseweed does break out before lay by, Steckel says Envoke will stunt it enough to keep the height differential between the weeds and the cotton enough so that hooded applications of products such as Suprend, Direx or MSMA can still control it later.

Steckel says the extra cost of such a program can run about $20 an acre. Despite the need for this extra management to control one resistant weed, he says the anticipated introduction of Roundup Ready Flex cotton varieties will offer a lot of extra value for controlling summer weeds.

New Arrivals

In 2004, University of Missouri scientist Reid Smeda confirmed the first glyhposate-resistant summer annual—a common ragweed biotype. So far it hasn't been found to stray far from the 20-acre central Missouri field where it was discovered. The weed is much shorter in stature than susceptible common ragweeds, making it less competitive in tall corn, but a bigger problem in soybeans.

The field where glyphosate-resistant common ragweed was found had been planted with Roundup Ready soybeans continuously for several years. Smeda says different modes of action than glyphosate, such as growth regulators, acid amides and atrazine, can control the resistant ragweed. He's evaluating different strategies in the field where it was found.

Heering says Monsanto is working with Smeda to establish control strategies using a rotation of Roundup Ready Corn 2 and Roundup Ready soybeans. In both crops, a pre-emergence product with residual activity is used to help control the resistant ragweeds.

At press time, University of Georgia weed scientist Stanley Culpepper confirmed Palmer amaranth to be resistant in Georgia.

"The population we are is on less than 500 acres,” says Culpepper. "We are sampling about 100 fields around it to see if it has spread.”

Culpepper is trying a variety of weed control strategies with the suspected population, but won't be sure what the best recommendation is until after cotton harvest. Based on available data, Monsanto recommends farmers with glyphosate-resistant Palmer amaranth growing Roundup Ready cotton or Roundup Ready Flex cotton for 2006 use a pre-emergence residual herbicide such as Prowl with Roundup. At early post, use Roundup with metolachlor and Roundup plus diuron at lay-by. Farmers without confirmed resistance also are advised to use a pre-emergence residual herbicide.

Other Suspects

Though only five weed species have confirmed resistance to glyphosate, a few more or suspected.

At North Carolina State University, weed scientist Alan York is beginning to collect Palmer amaranth seed samples from the coastal plain to test them for resistance.

"We have suspicions, but we don't want to jump the gun and say we have it,” York says. Confirmation will require demonstrating that resistance to glyphosate is inherited.

York's concern is that if resistance is a problem, there will be few good tools in Roundup Ready soybeans. The loss of Canopy XL removes one possible tool, he says. Other herbicides are limited by the sandy soil texture in many parts of the state as well as pockets of Palmer amaranth that resist ALS inhibitors.

"Most fields are no-till, so we can't use soil-incorporated Treflan. That leaves Prowl, Lasso and Dual then post applications of Blazer and Reflex.”

In cotton, the situation is similar, but there is no good post control option except Ignite in glufosinate-resistant cotton, explains York. That program is limited by the number of varieties available and requires very timely application before weeds are more than a few inches tall. He says a more likely program is to use residuals at planting to hold the weeds down until lay by, then apply products such as Valor and Suprend, which have residual activity.

Other weed suspects being evaluated include giant ragweed in Ohio and Indiana. Common lambsquarters is also proving tough to control with glyphosate in some areas, though resistance has not been confirmed.

Ohio State University weed scientist Jeff Stachler suggests farmers in his state use a residual herbicide to control common lambsquarters. "You can use about anything you want, except alachlor and metolachlor,” he says. "Most products will work with the exception of Sencor when weeds have triazine resistance.”

Stachler says there are not as many choices for giant ragweed. Pre-emergence options for this weed include FirstRate, Gangster and Scepter—and any herbicide with chlorimuron. However, the wide-spread presence of ALS inhibitor resistant giant ragweed will reduce the effectiveness of these pre-emergence options. He also tells farmers to use 1.1 to 1.5 lb. acid equivalent per acre of glyphosate if weeds are more than four inches tall. That's the equivalent of 33 oz. to 44 oz. of Roundup WeatherMax.