If you had fields that looked like this in late August or early September, it’s time to ask yourself a key question, Bill Johnson says: Why are tall waterhemp plants escaping control in your soybeans?
“There are several possible reasons,” says Johnson, a Purdue University Extension weed specialist. “Perhaps whoever is making herbicide applications is doing a sloppy job and shaving rates or not applying at the right time,” he begins. “Or maybe you’re not applying soil-applied, residual herbicides that are robust enough to provide long-lasting control during the season.
“Pigweed species like waterhemp and Palmer amaranth are late-emergers. Some emerge early, but flushes will continue to germinate later than most weeds. If the soybean canopy is delayed in closing, it gives them more opportunities to emerge and compete.”
The other complicating factor is that some waterhemp populations are resistant to glyphosate. Some are also resistant to PPO herbicides, such as Flexstar, Cobra and Blazer.
The best bet is starting with a strong, soil-applied residual herbicide program, Johnson says. There are several options, including Authority or Valor herbicide products. There’s also Sencor or Sharpen if you add a Group 15 herbicide, Johnson says.
“You can’t shave rates and expect to control waterhemp throughout the season,” Johnson emphasizes. “You really need a robust residual program, and that means applying them at full labeled rates.”
If you spot waterhemp escapes, the secret is applying post herbicides in time, he says. “Pull the trigger when the largest waterhemp plants are 4 inches tall,” he advises. “You have several options for post control, but you need to get them while they’re small. Consider including a strong residual herbicide again in the postemergence pass.”
Liberty will kill waterhemp, if your soybeans are Xtendflex or LibertyLink with the glufosinate-tolerant trait. However, Johnson says Liberty works best when waterhemp are small and actively growing.
You can help yourself on waterhemp in corn-soybean rotations by applying full-rate soil-applied residual herbicides with the maximum rate of atrazine in corn. Control them postemergence in corn with dicamba, Callisto, Laudis or Impact.