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Chink in resistant pigweed’s armor?

Is there a chink in the armor of glyphosate-resistant Palmer amaranth? Weed scientists believe there could be. This season, a project in northeast Arkansas will determine if Mid-South farmers can take advantage of it.

According to studies conducted by University of Arkansas weed scientist Jason Norsworthy, the weed’s weakness is the short longevity of its seed in the soil. In fact, 80 percent of Palmer amaranth (pigweed) seed die within one year in the soil. After 4.5 years, virtually none of the seed, 99.9 percent, are viable.

Weed scientists believe if they can systematically reduce the seed bank, or the population of viable seed in the soil, they can slowly start to clean up heavily infested fields. The system can also be used to keep pigweed from reaching yield-robbing populations in the first place.

The tactic being field-tested is what University of Arkansas weed scientist Ken Smith calls zero tolerance — a method that combines hand hoeing and supplemental herbicide application to ensure that pigweed doesn’t reach maturity and produce seed.

Zero tolerance is being tested in two Arkansas cotton fields, one in Mississippi County and the other in Phillips County. Researchers selected the fields because they have a history of resistant pigweed infestation, aren’t flood-prone, and don’t receive runoff from infested fields.

On a recent sunny morning in late April, Smith inspected one of the fields in the zero tolerance study, confirmed as having over 100 million pigweed seed per acre, a consequence of an over reliance on Roundup for weed control, Smith said.

After last season, the field was rented to a new farmer, David Wildy of Manila, Ark., who agreed to put it in the zero tolerance study. Wildy will perform most of the zero tolerance tasks.

“Our objective is to not let one weed go to seed in that field this year,” Smith said. “Then we’ll come back and monitor it again. We think in three to four years, we’ll have this field manageable again.”

One reason why Smith contacted Wildy about cooperating in the zero tolerance study is that Wildy had already implemented most of the resistance management techniques the project will require.

For example, Wildy regularly hires hoe crews to walk his cotton even though he hasn’t seen a lot of resistant weeds on the farm. “If hoeing can buy us some time to fight resistance in the future, we can justify the costs,” Wildy said. “Whatever we have to spend, we have to do it. We have to stay ahead of the curve on resistance.”

“That’s a very aggressive philosophy,” Smith said. “David has done this for several years because he wants to keep the soil seed bank down. If he ever allows the pigweeds to get started, then he can’t afford the hoe crews.”

Smith says the first step in the zero tolerance program is a preplant herbicide. “The thought process is if you get it out preplant, you have a wide window there to get a rain and get it activated before you plant. That way, you’re not so dependent on that pre-emerge at planting — where you’re needing a rain quickly to make it work. If weeds germinate between the time you plant and when the herbicides get activated, we don’t have a lot of options.”

This spring, Wildy made a preplant surface application of Reflex on the zero tolerance field, which was subsequently activated by rainfall. Reflex, from the PPO herbicide class, was also applied on most of Wildy’s other cotton fields. The Reflex application has prevented pigweed from germinating on the zero tolerance field, although some small pigweeds that had emerged prior to the application might eventually recover.

Wildy is planning to make a Gramoxone application at planting to control any escaped pigweeds. But it’s going to be a bit tricky since oats planted in the middles to protect seedlings from sand blowing might not achieve enough growth by the time they’re killed with the Gramoxone application.

This won’t be the only challenge for Wildy. For example, Wildy doesn’t like to knock all his beds down at once, “because if it turns off hot and dry, we can’t get a stand of cotton. But with this approach, we have to knock all the beds down, get the Reflex out and get it activated before we plant.”

Preparing furrow irrigated fields can present another problem for Wildy. “We have to have our water furrows run two weeks before we plant cotton, which is when we’re trying to put this Reflex out. So we have to think way ahead. There are lot more logistics.”

Cultivation is not part of Wildy’s plan for managing resistant weeds. “With our system of planting a cover crop in the middle to stop sand blowing, we can’t plow anymore. It’s not an option. If we go back to plowing and we don’t have wheat in the middles, we’re in big trouble.”

The plans are for all Wildy’s cotton, including the zero tolerance field, to get an application of Roundup and Dual when cotton reaches the cotyledon stage. According to Wildy’s crop consultant, Dale Wells with Cotton Services, Inc., “Ten to 14 days later, we feel like we need to go with another shot of Roundup. If we don’t, morningglories are going to get away from us. The next residual will be Valor at layby.”

With resistant weeds steadily gaining a foothold in the Mid-South, Smith says Mid-South producers have to rethink how they use the Roundup Ready technology. “You can say we overused it or abused it. But in reality, it just ran its course. Now we’re going to change our techniques and capitalize on something else.”

But it won’t be easy. Without the flexibility that the Roundup Ready program once provided, growers will need to spend more time planning their weed control programs. “Weed resistance has caused a lot of gnashing of teeth,” Smith said. “It’s caused us to do some things we thought we’d never have to do again.”

While zero tolerance might be one of those techniques, it’s not a silver bullet. “I think zero tolerance has some value with the pigweed,” Wildy said. “But I don’t think we can fool ourselves into thinking that if we keep ourselves clean for a few years we won’t have a weed problem. If we’re going to farm, we’re going to be fighting weeds.”

“As weed scientists, we talk about how many weeds we can tolerate per row foot or per acre before we get a yield reduction,” Smith said. “But David thinks about what the situation will be like the following year. If we can get more farmers thinking that aggressively, it will go a long way toward helping us solve some of these problems.”


TAGS: Soybean
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