Farmers preparing to do battle with Palmer amaranth or pigweed in 2016 need to be getting their ducks lined up in a row, so to speak, as they wait for fields to dry enough to begin putting seed in the ground.
And, in the case of soybeans, their weed control approach may be centered around the LibertyLink technology, according to Larry Steckel, Extension weed scientist with the West Tennessee Research and Education Center in Jackson.
“Currently, in soybeans, it really is going to pivot on LibertyLink technology or LibertyLink soybeans because they’re the only sure thing we have for controlling pigweed once they’ve emerged,” says Steckel. “And, particularly for the counties where PPO resistance has been confirmed, a lot of growers are going that way.”
Judging from what he’s heard during the winter meeting circuit, Steckel said, “There’s going to be a lot of LibertyLink planted, particularly in those areas but also in some others where it has been confirmed.”
Multiple applications of glufosinate or Liberty herbicide will put more pressure on the compound to control Palmer amaranth, which already has developed resistance to a number of herbicide modes of action.
That’s why Steckel, who has been fighting herbicide-resistant weeds for most of his career, and other weed scientists are urging growers to put more reliance on pre-emergence or residual herbicides in their efforts to control pigweed.
Start clean with burndown
“The best program is to burn down and start clean, use a pre with at least two effective modes of action on pigweed,” he said. “The Duals, Zidua, those Group 15 herbicides and metribuzin need to be considered to be in the mix or some pre-mix of those. And then coming back over the top with Liberty and a Prefix-type herbicide.”
Dr. Steckel also recommends growers make another spray of Dual and Fomesafen or Reflex over the top to help with resistance management. His comments came after he spoke to the Memphis, Tenn., Ag Club about herbicide resistance and its impact on agriculture.
“That’s because we can’t afford to lose Liberty,” he said. “So putting a PPO (inhibitor herbicide) like Flexstar or Reflex that’s in Prefix can help us manage resistance that’s starting to form on Liberty herbicide with pigweed.”
Steckel was asked about the confirmation of resistance in Palmer amaranth to postemergence applications of the PPO inhibitors.
“In Roundup Ready or conventional beans, the best strategy is to never let the pigweed come up,” he said. “So, again, use effective pre-emergence herbicides like a Dual with a metribuzin or even some of the ones that have PPO herbicides in there like Authority MTZ, Verdict.
More residuals applied
“But put a metribuzin in with them and then come back early postemergence – really within 10 to 14 days – and get another residual out there like a Dual and, hopefully, never let them come up. That’s going to be the best bet.”
Growers should also consider planting in narrow rows to help the soybeans canopy quicker and shade out any later-emerging weeds. “You need narrow-row populations, not 30s and 38s, but 7.5s will help a lot,” he noted. “Try to get one or two residuals over the top and hope you get an activating rainfall. That’s always the biggest drawback to any of these.”
Cover crops are also helping provide weed suppression, particularly when growers allow them to continue growing closer to planting the commercial crop. Studies at UT are showing weeds can be kept at bay for up to 28 days with cover crops.
Steckel acknowledged more growers are planting into green cover crops, but he urged farmers to use caution in adapting the practice to avoid losing a stand.
“We’re seeing more of that,” he noted. “It’s something that you need to have a little bit of experience before you try to do the whole farm wall-to-wall because it’s a little tricky setting the planter.
“We’ve learned some things the hard way. You basically want to get the row cleaners up and out of the way. You just want to run with the coulter and do your best to get good seed-soil contact. With corn and soybeans it’s not such a problem because you punch them in a little deeper. Cotton has been more of a struggle because you plant it so shallow it can be difficult to get good seed-to-soil contact.”
For more on herbicide-resistant Palmer amaranth, visit http://takeactiononweeds.com/wp-content/uploads/2014/01/palmer-amaranth-management-in-soybeans.pdf