Unrelenting rains cause farmers major weed problems in 2015

Frances Gould, McClure, Olivia J.  |  8/8/2015 12:03:24 AM

Without herbicide treatment, Palmer amaranth weeds growlarge quickly and cause major problems for farmers. LSU AgCenter weed scientist Daniel Stephenson is holding Palmer plants – one treated with herbicide and one not treated. Photo by Olivia McClure

Photo By: OLIVIA MCCLURE

Unrelenting rains that delayed planting of many Louisiana crops may have set the stage for a tougher-than-usual battle with weeds this year.

Occasional dry spells in the spring saw farmers hurrying to get crops in the ground, only to be greeted by weed infestations thriving because of the wet weather. The herbicides that farmers applied a few months before planting to "burn down" winter weed populations failed in many cases.

"When it rains a lot, herbicides leach through the soil profile, and the weeds begin to regrow," said Daniel Stephenson, LSU AgCenter weed scientist at the Dean Lee Research and Extension Center in Alexandria. "Farmers are just having a difficult time. Any time they would spray a herbicide, it would rain."

If farmers don’t plant "clean," or free of weeds, their crops will face competition from weeds that may not be controlled easily post-emergence. Worse, some crops this year are sensitive to foliar herbicides because the constant rain caused fast growth, leaving plants with thin cuticles, Stephenson said. "

If you start early and use a good burn down so you plant clean, then apply a soil-applied residual herbicide behind the planter and follow with an early post-herbicide with another residual, then you’re protecting those beans so you maximize yield. And you’re controlling all those weed species," Stephenson said.

Little of that actually happened this year. Weeds such as nutsedge – a big problem this year – can still be controlled, but only with multiple sprayings.

That costs growers more, but some weeds must be approached with a zero-tolerance policy to avoid disaster down the road, Stephenson said. A single Palmer amaranth plant, a glyphosate-resistant weed that has spread to all but a few row-crop parishes, can often produce about 1 million seeds.

"It could disperse via seed, birds, wind or pollen and get on your neighbor’s farm, so we have to combat that," Stephenson said.

Another resistant weed on the move in Louisiana is johnsongrass, which is nearly impossible to control in grain sorghum. Johnsongrass spreads by rhizomes in the soil, so keeping equipment clean is critical.

Options are limited when it comes to weeds resistant to glyphosate – the active ingredient in Roundup, which has long been a go-to herbicide because it can safely be sprayed over emerged crops and controls a host of weeds.

Two new technologies the AgCenter has been evaluating may help fill that gap. The Enlist Weed Control System allows application of 2,4-D on soybeans, while Roundup Ready Xtend is a similar system that uses dicamba.

"2,4-D and dicamba are not new herbicide chemistries, but the formulations being marketed with these technologies are new to help mitigate off-target movement issues," said AgCenter weed scientist Donnie Miller. "They’ve been around a long while, but we did not have the option to apply them over the top of soybeans. We’ve not had the gene in crops to tolerate them."

Detailed product label use restrictions will accompany each herbicide, he said. Both new systems have been deregulated by the U.S. Department of Agriculture and should be commercially available for the 2016 growing season.

Glyphosate remains a key component in each system through premix herbicides or allowable tank mixtures, offering growers another tool to use against weeds. However, Miller said, it’s important to remember to use the entire toolbox, not just one tool.

"Dicamba and 2,4-D offer an option in soybeans to include a different mode of action," he said. "To help mitigate weed resistance, the main thing you want to do is alternate modes of action."

In addition, use of residual herbicides within each system is highly encouraged, he said.

Olivia McClure

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