- To manage glyphosate-resistant Palmer amaranth, weed scientists suggest that producers start clean, overlap residuals and manage escapes.
- A cover crop could also help manage resistance by blocking sunlight needed to germinate pigweed seed.
- Mid-South fields appear cleaner this year as producers continue to fine tune their weed control program. But Palmer amaranth is far from under control.
Many Mid-South crop fields appear cleaner this summer compared to years past, which means one thing to weed scientists Ken Smith and Larry Steckel. Farmers are getting the message about managing glyphosate-resistant Palmer amaranth.
While there are still plenty of fields with ugly infestations of Palmer amaranth to go around, many producers have found it’s best to manage the weed, rather than try and spray it out of their fields, Smith and Steckel said at the 14th annual Union City Field Day at Monsanto’s research facility in Union City, Tenn.
“We’re doing a better job than we have in the past,” said Smith, Extension weed scientist at the University of Arkansas. “Even though the weather has not been good for weed control, the commitment to reduce that soil seed bank (of Palmer amaranth) has been encouraging.”
“Going into this year I was very apprehensive,” said Steckel, Extension weed scientist at the University of Tennessee. “I thought we were going to have some major issues with Palmer pigweed because it was so dry, and we struggled to get out a lot of our pre-emergence herbicides. But as it turned out, we are actually a little better off this year in our weed control than we were this time last year.”
Steckel says three factors are helping growers manage pigweed. No. 1, more growers are rotating to corn, where they can do a better job of managing fields with intense soil seed banks of Palmer amaranth.
“No. 2, we’re using a lot more LibertyLink in soybeans and cotton and getting some new modes of action in those crops. No. 3, growers have been really on the ball on timing in all our crops. I’m pretty proud of where we are this year compared to where we were last year. I don’t know that we’ve turned the corner, but I don’t think it’s getting any worse.”
Smith noted that farmers continue to fine tune their weed control arsenal, and are showing interest in participating in area wide management and zero tolerance. These are great concepts, Smith said. “Get together with farmers in your community, sit down and figure out how all of you can manage the seed bank. You’ll find the next year, it will be a lot easier.”
An example, Smith said, was a highly infested 60-acre field in a zero tolerance program, (where the goal is to not allow a single plant to go to seed). The first year in zero tolerance, it required 100 hours of hand-chopping to clean the field. The next year, the field required only five hours of hand chopping. “We’re never going to eradicate Palmer pigweed, Smith said. “But we can get it down to a manageable level.”
Smith says growers should start clean, overlap residual herbicides and manage escapes. “If you manage escapes, you’re going to contribute a lot to managing that soil seed bank. A Palmer pigweed seed lasts only 3 to 4 years in the soil. If there is a weakness in Palmer amaranth, it’s the fact that the seed do not live a long time in the soil. Let’s exploit that weakness.”
Smith urged producers to plan for a two-week overlap between residuals, even though many residuals may last longer. “If you do that, you’re more likely to get it done on time. It’s not rocket science. Get in there early enough so that the residual has enough time to start working before the other one runs out.”
Steckel says research on cover crops as a resistance management tool “is promising.”
“If we can get the ground shaded with a cover crop early on, we can really curtail how many weeds we may have to fight with our herbicides later on,” Steckel said. “Pigweeds need three things to germinate, heat, water and sunlight.”
Steckel said research on vetch, wheat and crimson clover saved about $15 per acre in herbicide cost due to the elimination of one burndown and did a good job of suppressing horseweed, and pigweed early in the season. The effect of the cover crop “starts to play out in mid-June, and we had to go to post applications.”
Steckel said legume-based cover crops like vetch can put 50 pounds to 60 pounds of nitrogen into the soil.
Smith added that new technologies coming down the road like dicamba and 2,4-D tolerant crops “are not panaceas. None of them are as good as we thought Roundup Ready was when it was first introduced. Residual herbicides will always be a component of our weed control programs.”