Herbicide resistance looms as problem for Louisiana soybean farmers

Linda F. Benedict, Stephenson, Daniel O., Schultz, Bruce

LSU AgCenter agronomist Brooks Blanche is trying to develop a variety of corn resistant to aflatoxin. (Photo by Bruce Schultz)

LSU AgCenter entomologist Jeff Davis said soybean loopers, like the one on his finger, are causing problems in soybean fields across the state this year. (Photo by Bruce Schultz)

LSU AgCenter cotton and feed grains specialist John Kruse is studying the plant growth regulator Pentia for use on cotton. (Photo by Bruce Schultz)

LSU AgCenter weed scientist Daniel Stephenson holds up one of the problem weeds in Louisiana soybeans, the pigweed. (Photo by Bruce Schultz)

News Release Distributed 08/12/10

ALEXANDRIA, La. – Herbicide-resistant weeds have been causing havoc in soybean fields across the South, and they appear to be “just an eyelash away” from being confirmed as a problem in Louisiana, said Daniel Stephenson, an LSU AgCenter weed scientist.

The three weeds Louisiana soybean farmers are starting to have trouble controlling are Johnson grass, waterhemp and palmer amaranth, also known as pigweed – all vigorous growers and prodigious pollen producers.

These weeds have been found resistant to glyphosate herbicide in Arkansas, Georgia, Mississippi and Tennessee. Glyphosate herbicide is safe to use on Roundup Ready soybean varieties, which are grown on more than 95 percent of the soybean acreage across the country and on nearly all of the million acres of soybeans planted in Louisiana.

Alternative weed control programs – should weed resistance occur here – are under investigation in tests being conducted by LSU AgCenter scientists.

“We don’t have a lot of control options right now, but we’re working on that,” Stephenson told farmers at an Aug. 4 field day at the LSU AgCenter Dean Lee Research Station in Alexandria.

He outlined for them some steps to take to prevent glyphosate from becoming ineffective, which would be a severe blow to the soybean industry here. One of his recommendations was to go back to applying pre-emergence herbicides with soil residual activity.

“This allows you to expose weeds to another chemistry,” Stephenson said.

Before Roundup Ready soybeans became ubiquitous, farmers used pre-emergence herbicides to control weeds in soybean fields.

“We’re recommending going back to using soil residual herbicides,” Stephenson said.

These herbicides have a different mode of action compared with glyphosate. The ones being tested in the Dean Lee research plots include Authority MTZ, Boundary, Enlight, Prefix, Fierce, Sharpen and Valor.

Farmers will still have to apply glyphosate to weeds that emerge, but the pre-emergence herbicides will have slowed down weed growth and bought time for the soybean plants to grow taller and outcompete the weeds for water and light.

Another technology that would extend glyphosate effectiveness is to plant Liberty Link soybeans, which are resistant to glufosinate, the active ingredient in Ignite herbicide. The only problem is Liberty Link beans are just not available in large quantities.

“Mississippi soybean producers are trying to buy every bag of Liberty Link seed they can find,” Stephenson said.

Stephenson also told the farmers to try to destroy these troublesome weeds wherever they find them to prevent seed production. He told the story of finding pigweed prevalent in a Louisiana cotton field this summer. He recommended the farmer destroy the field before the pigweed went to seed.

“Pigweed pollen can travel 600 yards,” Stephenson said, and this can extend the weed resistance characteristic to plants in other fields.

Roger Leonard, LSU AgCenter entomologist, said the red-banded stinkbug has not been a problem in soybeans this year because of the unusually cold winter, but he said a brown stinkbug has caused problems.

The morningglory stinkbug, identified by a white dot in the middle of its back, is not a soybean pest, he said, and it feeds on morningglory plants.

Jeff Davis, LSU AgCenter entomologist, said the soybean looper is a problem this year, and it can be controlled with the chemical Intrepid.

Besides using herbicides on fields before plants emerge to help prevent herbicide resistance in weeds, Brooks Blanche, LSU AgCenter agronomist, recommends crop rotation and narrowing the row space in fields.

“You get maximum light interception and maximum photosynthesis per area of ground,” he said. “Because of the shading and because of the increased competitiveness of the crop, you reduce weed competition.”

Blanche is conducting research on twin-row soybeans in which two rows of seed are planted instead of one row.

“You get a good canopy, which is what you want with late-planted beans or on heavy clay soils,” he said.

Blanche is also conducting research on aflatoxin in corn. He’s trying to breed new varieties with resistance to this disease. He’s done 35 crosses so far this year.

“The best way to attack aflatoxin is with breeding and genetics,” he said.

Donna Morgan, coordinator for the LSU AgCenter Master Farmer program in north Louisiana, explained her research project at the station in which she’s trying to determine the level of contaminants in runoff from cotton and soybean fields that have been planted with winter wheat. She has 12 water monitors in her 1.5-acre research plot.

“We’re looking at the effects of wheat stubble,” Morgan said.

Sid Derouen, LSU AgCenter cattle researcher, said cattle genetics have changed drastically in the past few decades. He said the LSU AgCenter is part of a study to determine what breeds and combinations of breeds work best in the South.

He said diseases are a problem for cattle at feedlots, especially bovine respiratory disease. Derouen said the mortality rate for calves at feedlots can range from 10 percent to 30 percent, and research is being done to find the best vaccines for BRD, he said.

Matt Garcia, LSU AgCenter geneticist, said a DNA repository is being developed for all cattle with the LSU AgCenter as part of a multi-state study. Blood and ear notches will be taken from each animal, he said, and detailed individual records will be kept, from birth weight to carcass condition.

Garcia said DNA marker-assisted selection can be used to cull calves with more desirable traits, instead of waiting until maturity, and the repository will help identify genes with important traits.

Wayne Wyatt, LSU AgCenter cattle researcher, said cattle breeds’ coats are being studied to determine which breeds perform best on the Gulf Coast in the summer.

He said black cattle bring the highest prices, but they have difficulty in the Gulf South heat. Buyers prefer cattle with less than 25 percent Brahman, although they perform best in the heat, he said.

Wyatt said cattle breeds that typically have short, slick hair can stay cooler, and that means the cattle can endure the summer heat with less effect.

He said and University of Mississippi study has concluded that cows that shed all of their winter coats by May will have calves weighing 25 percent more at weaning.

John Kruse, LSU AgCenter cotton and feed grains specialist, discussed a study of the plant growth regulator Pentia on cotton. He said the first application of the PGR showed no significant difference, but the second one about a month later increased plants’ growth by 8-12 inches.

Boyd Padgett, LSU AgCenter plant pathologist, said Cercospera blight is showing up in later-planted soybeans. “Our fungicides are OK at best at managing this disease.”

Downy mildew has been found, and it is easily confused with Asian soybean rust, he said, but rust is not a problem so far this year. Aerial blight is showing up in some fields, he warned.

Padgett advised farmers to apply fungicides between the R3 and R5 growth stages.

After the field tour, David Boethel, LSU AgCenter vice chancellor for research, said looming budget cuts of 23 percent threaten the AgCenter’s core mission of helping farmers. Boethel said 200 positions could be cut. “Most of this would be layoffs.”

Paul Coreil, LSU AgCenter vice chancellor for extension, warned the cuts would be drastic, and they would include elimination of 19 4-H agents.

“There are people in your parishes that won’t be here a year from today,” Coreil said. “We could be giving notices for layoffs before the end of the year.”

Linda Foster Benedict and Bruce Schultz

1/4/2011 1:08:22 AM
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