In the 1950s, cockle bur and velvetleaf were a big deal, because there weren't good chemical controls yet. Some people today may not know what a cockle bur looks like. In the 1980s it was johnsongrass and giant ragweed. Both are still around, and giant ragweed is making a comeback with resistance to glyphosate in some areas, but in general, they can be more easily controlled than three decades ago. Marestail is tough – showing up when no-till soybeans came into play. It takes the right chemicals applied at the right timed, especially if the marestail is resistant.
Despite all that, experts say Palmer amaranth is a weed like no one in the Midwest has seen before. Farmers in the south have been fighting it for years, and losing the battle as it became resistant to glyphosate. They've resorted to hiring crews to chop it out, and in some cases load it on a wagon, haul it out of the field and burn it.
"We've found out that if you just throw it down between the crop rows, it can send out roots and regrow," says Bill Johnson, Purdue University weed control specialist. "This is a tough one that we don't want to get started here."
Palmer amaranth was identified in six northwestern Indiana counties last year, and is known to exist as resistant to glyphosate in southwestern Indiana. Samples from the northern counties are being tested in the lab for resistance this winter.
Mark Lawson, a farmer and regional rep with Syngenta, says the best policy is to assume that you have Palmer amaranth, and that it is resistant to glyphosate and ALS herbicides. Then plan your weed control program accordingly. It will likely include one or more soil-applied residual products so that you no longer count just on postemergence control to stop the weed.