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.