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Glyphosate resistant pigweed arrives in VirginiaGlyphosate resistant pigweed arrives in Virginia

• With well over 100,000 acres of cotton and about a half million acres of soybeans planted this year in Virginia, concern is growing among producers as to the best tools available to manage Palmer amaranth and other herbicide tolerant weeds.

Roy Roberson 2

August 4, 2011

7 Min Read
<p> David Holshouser</p>

It’s here!  If there were any lingering hopes that glyphosate resistant pigweed would stop at the North Carolina line and not be an economic drain on cotton and soybean growers in Virginia — put those hopes to rest.

With well over 100,000 acres of cotton and about a half million acres of soybeans planted this year in Virginia, concern is growing among producers as to the best tools available to manage Palmer amaranth and other herbicide tolerant weeds.

At a recent meeting at the Tidewater Agricultural Research and Extension Center in Suffolk, Va., longtime Virginia Tech Researcher David Holshouser showed farmers the results of high rates of glyphosate applied to Palmer amaranth pigweed from several Virginia counties.

Needless to say, the picture wasn’t a good one for any of the growers in attendance. Some of the Pigweed treated with varying rates of glyposate, up to 56 ounces per acre, showed little or no negative effect from the herbicide.

Holshouser says soybean growers have a few more options than cotton growers in managing pigweed. Regardless of the crop, the key is to use different modes of action to manage these pests and to at least slow down the northward movement of glyphosate resistance problems.

He showed attendees to the recent field day results of a greenhouse study in which high rates of glyphosate were used to kill resistant pigweed. To get a 50 percent reduction in fresh weight in resistant weeds, it took 1-3 pounds of glyphosate, or about 24-56 ounces of Weathermax.

“In cotton or soybeans, you can talk about aphids, thrips, soybean rust, whatever — nothing compares with what growers are facing with resistant pigweeds,” the Virginia researcher says.

Pointing to a picture of a field in Greenville County, Va., Holshouser told grower attendees, “Last year we used a Roundup Ready program in that field, which has a high population of glyphosate resistant pigweed, and the yield was zero — it doesn’t get any worse than that,” he stresses.

Holshouser contends a big reason so many growers shifted to a total glyphosate program was convenience. “The material worked, it was easy to use and relatively inexpensive,” he says.

A good option

One of the options for growers to use, if they have glyphosate resistant weeds or want to reduce chances of having the problem is planting a LibertyLink cotton or soybean variety and using Ignite herbicide.

The system is similar to a Roundup Ready program in terms of being easy to use and for that reason alone Holshouser contends it will be a logical choice for growers who have depended on glyphosate for many years.

“LibertyLink will force growers to be better weed managers. It won’t kill weeds nearly as large as will glyphosate. Unlike glyphosate, LibertyLink and Ignite will not allow growers to do a salvage spray, like many growers did with glyphosate, he explains.

“I really think we need to focus on using LibertyLink soybeans, especially, in fields that have been in total, or near total, glyphosate weed management systems for three years or more. There are plenty of fields in the state in that category, Holshouser says.

Many growers have been reluctant to use genetically altered varieties in soybeans and cotton because of well documented yield drag on some varieties.

Now, Holshouser contends, LibertyLink-containing varieties are among the top yielding varieties in statewide variety testing, producing 3-5 bushels per acre more than average in state-wide testing.

Unfortunately for Virginia growers, glyphosate resistance isn’t the only problem they will face in the future.

In Southeast Virginia, veteran Crop Consultant Wendell Cooper says he checks some fields with pigweed, marestail and ragweed — all with varying degrees of resistance to glyphosate.

This year marestail came in much later than usual. In fields with glyphosate resistance, the combination of resistant pigweed and ragweed, plus marestail late was a tough one for growers to handle, he says.

One combination that Cooper says has looked good so far this year is LibertyLink, both in Widestrike cotton and in FiberMax and Stoneville varieties. “There isn’t a lot LibertyLink cotton in the area that I work, but what I’ve seen looks really promising,” Cooper says.

Glyphosate resistant ragweed isn’t widespread across Virginia, but in fields where it is resistant it can be a major problem. Ignite, the herbicide used on LibertyLink crops, is very good on ragweed, according to Cooper.

“It also works real well on marestail and will do a good job on pigweed, if the grower can spray these weeds before they get taller than 4-5 inches,” he adds.

Combination approach

“I have one grower who knew going into this season that he had some fields with glyphosate resistant pigweed. So, he planted LibertyLink cotton in these fields. He put on Reflex behind the planter and then came back with two applications of Ignite. As of mid-July those fields were very clean — the combination did a great job,” Cooper says.

The veteran Virginia consultant says one of the keys to using the LibertyLink system is to use a herbicide like Reflex to keep fields clean until they can be sprayed with Ignite.

Cooper stresses the importance of following the label, particularly to avoid spraying Ignite on cotton that is blooming. The herbicide can cause cotton blooms to abort and the result will be significantly lower yields, he explains.

Reflex is a post-emergence contact herbicide that is activated by exposure to sunlight to form products that destroy plant tissue by rupturing plant cell membranes. The destruction of cell membranes results in a rapid browning of plant tissue in target broadleaf weeds.

The primary active ingredient in Ignite is glufosinate. The way it kills weeds is similar to glyphosate, but its mode of action and chemistry are different. Both modes of action are different from Reflex.

Still, long-time North Carolina State University Weed Scientist Alan York, a pioneer in the study of glyphosate resistant pigweed, says growers need to remember the lesson learned from over-use of any herbicide.

Ignite, or glufosinate, is the best single option we have now to manage resistant pigweed, but over-use will likely put us in the same situation we found ourselves in back 2004-2005 with glyphosate resistant pigweed, York warns.

Increased cotton acreage in Virginia, with many growers having little experience growing the crop, increases the chances for over-use. It’s tempting to over-use a product that will kill pigweed, marestail, ragweed and volunteer soybeans from Roundup Ready soybeans grown the previous year.

Cooper says Ignite did a consistently good job of killing pigweed that were four inches tall or smaller. When weeds got six or more inches tall, it seemed to kill some weeds and didn’t kill others. “I just don’t feel comfortable killing pigweed more than four inches tall with Ignite,” Cooper says.

An advantage of using Ignite in LibertyLink cotton is that you can come back with a second, sometimes a third spray to manage bigger pigweed.

Three applications possible

“Our label allows for an Ignite season total maximum of 87 ounces per acre, says Bayer CropScience Representative Franklin Dowless. “To the grower, that means he can make up to three postemergence applications as long as none of the three applications exceed 29 ounces,” Dowless explains.  Bayer CropScience is the company that markets LibertyLink seed and Ignite herbicide.

In a perfect world, a grower would use a 3-way split application to keep more herbicide on target weeds for a longer period of time. However, in cases in which a grower is delayed in applying an initial spray or weather conditions produce a heavy, late flush of weeds, the grower can use up to 43 ounces per acres.

The bottom line Dowless says is the Ignite label gives growers a lot of flexibility on how and when they use the material in cotton.

Cotton growers also have some new flexibility in varieties that contain the LibertyLink technology. FiberMax varieties (FM 1735LLB2 and FM 1845LLB2) that have performed well for several years are still available to growers, but this year on a limited basis, growers had an option to use Stoneville’s ST4145 LLB2 variety, with a LibertyLink and Bollguard stack.

“Our FiberMax varieties are solid, but the Stoneville genetics are ideally suited for cotton production in Virginia. Stoneville 4145 has looked good this year and gives growers some good options, especially when they are dealing with resistance problems,” Dowless says.

Whether Ignite can be used in WideStrike cotton is an often-asked question among cotton growers dealing with glyphosate resistant weeds. Legally, it can be used, but growers may lose any leverage they had should they get crop damage from using Ignite on WideStrike cotton.

There are a number of herbicide resistance programs available to growers. Regardless of what system is used, the price for managing glyphosate resistant weeds is too high, to depend on one herbicide to do the job. The best strategy will involve multiple chemical families, combined with good crop rotation and a good scouting program to recognize resistant weeds when they first appear in a field.

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