“But we are making some strides against herbicide-resistant Palmer amaranth pigweed,” he says.

“One of the most important things that has occurred since we discovered resistance in 2004 is that we’ve learned a lot about the biology of the plant. I think we already knew, but we had to re-learn some things.

“We’ve learned about the biology of the seed and how pollen moves, and how tillage and covers influence the emergence of that particular weed species. We’ve also learned a lot about herbicide efficacy.”

By tying all of these things together, Extension specialists can offer better control programs for producers, says Prostko.

The tools are available for managing herbicide-resistant Palmer amaranth, he says, “but it’s not easy, cheap or for the faint of heart. We got used to doing things a certain way, and this pest came along and disrupted that. Now we have to change, and nobody wants to change that much if you don’t have to.”

Challenges remain, says Prostko, and one of the biggest is irrigation and/or rainfall. “About 50 percent of our peanut crop is dryland. That will always be a challenge because we need moisture to make residual herbicides work. If you’ve got irrigation, you’re more likely to make herbicides work than with dryland.”

The other challenge, he says, is “farm-size/weed size.”

“We know how important it is to spray weeds when they’re small. We also know that pigweed can grow 1 to 3 inches in a day, and we only have a two to three-day window to be timely. So if you’re a large-acreage grower, that’s difficult to do, especially if you have bad weather conditions when you need to make those applications,” he says.

Growers should consider how many acres they can spray in a day, says Prostko. “How many acres can you spray in a day? What if there’s a lot of wind and rain? I’ve heard growers say they can spray 300 to 500 acres per day, but what if there are obstacles? Farm size is a challenge.”

Diligence also is important, he says. “Whenever I see a clean field, the grower tells me he is aware Palmer amaranth is a major problem, and he is committed to doing whatever is necessary to keep fields free of the weed. You need to start clean. If you go through a field where pigweed is already up, you’re planting peanuts, and the weeds are 6 to 8 inches tall, then you’re probably going to lose.”

Research has shown that if you bury pigweed deeper than 4 inches for three years, such as with a moldboard plow, it’s not going to come up, says Prostko. “So we can get control just by plowing. That doesn’t fit everyone’s system, but it can give you good control.”

Planting in twin rows also helps because peanuts in close rows produce more shade that helps in controlling pigweeds. In addition, irrigation helps control pigweed by activating residual herbicides like Prowl and Valor, and by helping the peanuts grow faster.