The keys for control of glyphosate-resistant Palmer pigweed in cotton sound simple enough – start clean, overlap residuals and manage escapes.

But this approach may demand a little more luck, better eyesight and a few more calluses than previous methods of pigweed control, according to Ken Smith, Extension weed scientist at the University of Arkansas.

“In 2004, we made three applications of Roundup, put out a layby on 25 percent of our acres, and the crop was relatively clean. But there was a field out there (in 2004) that was not clean. We were selecting for these resistant biotypes even back then. We probably started selecting for the resistant population as early as 2000,” said Smith, a speaker at the National Conservation Systems Cotton and Rice Conference in Tunica, Miss.

Smith believes that even if producers had been using residual herbicides with the Roundup Ready system, resistance to the herbicide was likely inevitable. “We could have delayed it, and we could have lessened the impact. But we could not have prevented it.”

Since glyphosate-resistant pigweed gained a foothold in the Mid-South in 2005, it has progressively covered almost every county in Arkansas, noted Smith. “It has caused us a lot of consternation and caused us to change our cultural practices in a way that we weren’t quite ready for.”

Smith referred to studies conducted by University of Tennessee weed scientist Larry Steckel that indicate that one pigweed per 60 feet of row can reduce a yield of 1,340 pounds per acre by 390 pounds, or 38 percent. Four pigweed every 60 feet of row can reduce yield by 600 pounds or 45 percent, while 8 or more pigweed per 60 feet of row essentially took the crop.

Smith says both the strength and weakness of pigweed can be found in its seed. “Cocklebur seed will live for a very long time in the soil. That’s its method of survival. Pigweed seed do not last long. You’ll lose 80 percent of them the first year. But pigweed win with sheer numbers. They flood the soil every year with seed.”

Smith’s assistants counted over 1.8 million seed produced by a single plant, nicknamed Elvira. “Even if you lose 90 percent of them the first year, you still have a lot of seed to contend with.”

Producers should follow three steps for success in managing glyphosate pigweed — start clean, overlap residual herbicides and manage escapes. But just because a field looks clean doesn’t mean it is. Producers should check closely for small pigweed.

Can be risky approach

Relying on a pre-emergence residual herbicide only can be risky, according to Smith. “Cotoran or Direx behind the planter will work well if you get some rain. The worst scenario is you come in, you have good moisture, you knock the top of the bed off for good soil moisture, plant into it, put down your Cotoran, then it just doesn’t rain. For seven days, the pigweed comes up with the cotton. That is a no-win situation. There is not anything you can do at that point that is not a salvage situation. So don’t let them get a head start on you. If they do, you’ll never catch up.”

An application of a pre-plant herbicide in a Roundup Ready program can alleviate some of that risk, according to Smith.

“If we put out Reflex pre-plant, it’s already activated when we get there with the planter. You’re clean.”

Smith noted that producers who prefer to knock the top of the bed off prior to planting should have beds ready to plant ahead of time. “Put out Reflex, wait for a rain, come in and plant.”

While many residuals may last as long as three weeks, Smith recommends putting out a new residual every two weeks. “We can’t allow the residual to break. We get a week’s overlap, hoping to get a rain to get one activated before that other one plays out. It’s critical we overlap residual herbicides.”

Smith says there may be a little more flexibility with a LibertyLink herbicide program. “It can include either a pre-plant or pre-emergence herbicide. “If the pigweed is coming up with the cotton, we have Liberty herbicide to take out the escapes.”

During the season, producers should make chopping pigweed escapes a priority. While it is not likely that this will eliminate the pigweed problem, it can make it more manageable.

“One farmer reduced pigweed to below detectable levels by going in and taking out every pigweed in the field. In that field, we moved from 110 hours of chopping in 2010 to five hours in 2011.

“In another field, we reduced the seed in the field by 65 percent in the first year and we got to undetectable levels in year 2. Some of our farmers have gone into area wide zero tolerance, where each farmer in a geographical area works to keep his fields clean, to keep the problem from spreading.”

The bottom line for Smith is to control the number of pigweed seed in the soil. “If our soil seed bank continues to increase, this is not sustainable.

“It’s hard for us to comprehend, but when we’re dealing with pigweed, Liberty (formerly Ignite), Roundup, Staple and all those over-the-top herbicides are not our base programs anymore. Our base program today is a soil residual herbicide. We use the over-the-top herbicides to clean up escapes.

 “It’s a little hard to conceptualize this as many years as we’ve sprayed Roundup and for as many years as it’s worked for us. Roundup is no longer a pigweed material.”

Smith added that lower glyphosate prices and rebates for the use of residual herbicides have helped lower the costs of a residual-based control program. 

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