“Our county agent in Crockett County sprayed some 8 to 10-inch Palmer pigweeds with glyphosate and nothing happened. He sprayed them with Ignite, which will control pigweeds if they’re less than 6 inches tall. Ignite burned them, but they came back. He sent in a chopping crew — but if you leave any root stump at all, it’ll send out auxiliary buds that will re-grow. If you pull them up and stack them at the end of the row or the edge of the field, more often than not they’ll re-root. It’s an unbelievably resilient plant.”Resistant pigweed is “on the move,” he notes. “We’re seeing basically a three-year progression from just a few plants to major infestation. Three years ago, you could hardly find one anywhere in middle Tennessee. In 2009, we found them in a few fields. Last year, we had them a big way.

“A single 5-foot plant can produce 500,000 or more seeds, which are easily spread by wind, animals, equipment. You may have just a few plants in a field this year, and if you run a combine or a picker through them, you’re spreading seeds everywhere, and next year you’ll have pigweeds galore.

“In Tennessee this past year, growers generally did a better job of managing Palmer pigweed in cotton than in soybeans. Of course, we had a lot more soybean acres, and it’s harder to get around timely with postemerge applications in soybeans. Also, with 38-inch rows, you can run hooded sprayers through the field, it’s easier to chop out weeds in cotton than in soybeans, and in cotton many growers have moved to an Ignite system.

“With Flexstar in soybeans, the cutoff for pigweeds is 3 to 4 inches, whereas for Ignite in cotton its 6 inches, 7 if you’re lucky.”

A key component in controlling pigweed, Steckel says, is residual herbicides, including Authority, Fierce (a new experimental material), Prefix, Optill, Prowl H20, Valor XLT, KIH485 (“which we were hoping would get labeled this year, but didn’t, so we’re looking for it in 2012”), and Valor SX.

The amount of control is important, he notes. “If you get less than 90 percent control, pigweed is going to get away from you. It can grow from 0.21 to 0.18 centimeters per growing degree day. If you go out in a middle Tennessee field mid-June, when daytime temperatures will hit the mid-90s and the nighttime low is about 70, if you have 2-inch Palmer pigweeds early morning, they can grow 2.5 inches that day.

“Twenty-four hours later, they can be 4.5 inches tall, too big for Flexstar to control. In 36 hours, can be too big for Ignite to control. At that point, you don’t have enough postemerge spray power to control these weeds. Two days can make all the difference in the world.