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Wet wrench in weed control

Many producers in the north Delta properly laid the groundwork for getting control of resistant weeds this spring, but excessive rainfall threw a monkey wrench into many plans.

In some cases, weeds got away from growers, forcing them to plow up planted fields.

According to weed scientists in the region, the problematic weeds include the usual suspects — horseweed and Palmer amaranth, or pigweed — but Italian ryegrass was also an early-season concern.

Tennessee weed scientist Larry Steckel says growers had “checkered” success burning down resistant horseweed this spring and in late May were moving to take out resistant Palmer pigweed. “We’ve had an almost perfect storm for pigweed to develop with all this rain.

“The preplant and pre-emergence herbicides are wearing out much more rapidly than what we would typically see. We can usually get five weeks out of Valor. We’re getting closer to three weeks with all the wet weather. We’re only getting 10 days out of Caporal and Cotoran. On top of that, we had people who couldn’t get those herbicides (Caporal and Cotoran) because they ran out.

“Growers were really in a Catch 22. They were trying to get Dual on quickly over the top. They had rain, couldn’t it get on and now they have a flush of pigweed.”

Steckel said farmers will end up starting all over on some fields. “It makes you kind of sick. I think most all farmers had a good game plan. But between all the rain and the pre-emergence and preplant herbicides not lasting as long and with the shortage issues in places, it’s been a perfect storm.”

Steckel says planting a cotton variety which will allow Ignite to be sprayed over-the-top is one option for growers who’ve taken out stands and want to go back to cotton. “But that’s a little easier said than done, because from what I’m hearing, LibertyLink cotton has long been sold out and PhytoGen 375 WRF (a WideStrike variety that is tolerant to Ignite) is hard to come by. If you’re going to go back to Roundup Ready, the question is whether you can get a residual herbicide on there and get it activated. Murphy’s Law could pop its head up real quick on that strategy.”

According to Mississippi weed specialist Jason Bond, the first sign of weed problems for the state’s growers surfaced with difficulties controlling Italian ryegrass. “The worst of it was in all crops 40 miles to 50 miles north and south of Hwy. 82, from the Mississippi River to the Hills.

“Corn was hit pretty hard because the burndown window is so compressed. After producers planted, they realized that the ryegrass wasn’t dying. After the corn is coming up, there’s nothing they can do.

“From October to three weeks ago, all my calls were about ryegrass. It’s getting worse, and we don’t have any answers. We’re starting to advocate residual herbicides in the fall, but we got so much rain. Those residuals washed out, and we got a huge flush of grass in the spring. A lot of clethodim went out this year.”

After that, continued wet weather and/or high winds delayed herbicide applications directed at glyphosate-resistant Palmer pigweed. “We have cotton emerged in foot-tall pigweed,” Bond said. “There’s really not much you can do in those situations except to plow it up and start over if you can. Even if it was LibertyLink cotton, you’re not going to kill a foot-tall resistant pigweed with multiple applications of Ignite.”

Resistance pigweed also popped up in rice, according to Bond. “That has been the No. 1 call I’ve had over the last three weeks. They come up as a purple carpet. Most of the time, once you flood, you’ll drown them out. The problem is the competition loss between the time you plant and when you flood. You also have to control them on the levees, or it’s going to get worse on your soybean crop.”

Bond pointed to several cases where pigweed resistance has appeared in fields without a history of the problem. “They made an application of Roundup and got no control. They didn’t realize it until 10 days after the Roundup application, and now there’s nothing they can do about it. It’s continuing to spread south on our side of the river. It’s going to continue to catch people off guard.”

Arkansas weed scientist Ken Smith said that generally the state’s farmers have done a good job with their weed control programs this spring. But it hasn’t been easy. “I talked to one farmer who had really clean fields and I asked how he did it. He said he hadn’t missed an application, but he had to really hump it. He’s in north central Arkansas, and they’ve had some rain there, too. It’s not like he was spared all the bad weather.”

Smith said some cotton fields with resistant pigweed had to be plowed up. “On those fields, the farmers wanted to get back to the field to spray, but the wind was blowing and they couldn’t. It got away from them. The weeds came up with the cotton.”

Smith added that where rainfall has been heavy, “we lost some herbicide off the top of the beds and it went into the middles. We got good control in the middles, but we’re losing some control up on the bed. But overall, the disasters have been relatively small.”


TAGS: Management
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