Pigweed epidemic calls for strategic weed control in Louisiana soybeans, cotton

Linda Benedict, Schultz, Bruce  |  7/20/2012 11:54:58 PM

News Release Distributed 07/20/12

ALEXANDRIA, La. – A herbicide-resistant weed in soybeans and cotton has caused what an LSU AgCenter weed scientist calls an epidemic in Louisiana.

“This is the year of the pigweed,” Daniel Stephenson told farmers gathered for the field day on July 19 at the LSU AgCenter Dean Lee Research and Extension Station.

The hardy weed, also known as Palmer amaranth, which is native to the Southwest desert, was first documented as glyphosate-resistant in Louisiana in 2009 in Concordia Parish. Then, in 2010, the problem was identified in Tensas and Madison parishes, which are north of Concordia.

Though 2011 was quiet, the problem has exploded in 2012, Stephenson said, and now resistant pigweed can be found throughout the northeast region and is rapidly spreading to other parts of the state.

The remedy is a diversified weed control management program, which includes use of residual herbicides applied preplant, at planting and in-season, Stephenson said.

“Crop yield is determined early in the season,” Stephenson said, adding that farmers can’t allow weeds to outcompete the crop for sunlight, water, nutrients and space.

Herbicides should include products with different modes of action as a means to manage resistance development, which has occurred because of long-term use of glyphosate, or Roundup herbicide, Stephenson said.

Treat not only the field, but also turn rows and drainage ditches because the weeds that grow there produce seed as well, Stephenson said.

When pigweed plants are too tall for herbicides to kill, the only remedy is hand-pulling, Stephenson said.

To increase profits, LSU AgCenter weed scientist Jim Griffin encouraged sugarcane farmers to grow soybeans in their fallow fields. Once sugarcane is planted in August and September, a crop is harvested for four and even five years. After the final harvest, the sugarcane stubble is destroyed, and the field is fallowed until the crop is replanted.

Because farmers have to use a glyphosate product in fallowed fields anyway to hold down the weeds for the next sugarcane crop, they might as well get the potential value of another crop.

Soybeans are the perfect crop for these fields for several reasons. Predominant weeds in sugarcane are fairly easy to manage with current herbicides, and resistant weeds are not an issue, and planting on the raised beds used in sugarcane production helps with drainage. Soybeans also benefit the succeeding sugarcane crop by providing residual nitrogen.

Early-maturing soybean varieties can be harvested early enough to avoid interference with sugarcane planting. Another reason is soybeans are selling at record high prices, Griffin said.

“Even if the soybeans were planted as a cover crop and not harvested, this strategy would improve the soil without sacrificing weed control,” Griffin said.

The drawback is that sugarcane farmers need different equipment for soybeans. But Griffin said this is an investment that will pay off. Some sugarcane farmers in the Bunkie and Cheneyville area, for example, are already doing this, and they’re increasing their productivity, Griffin said.

Guillermo Scaglia, an animal scientist at the LSU AgCenter Iberia Research Station in Jeanerette, has expanded his beef nutrition project to include cattle at the Dean Lee Station, some of which he’ll move to the Iberia Station.

William “Buddy” Pitman, an agronomist at the LSU AgCenter Hill Farm Research Station in Homer, has a forage project at the Dean Lee Station testing the value of two legumes in summer pastures, cowpea and aeschynomene. Legumes provide the cows and calves needed protein and allow the calves to gain weight faster for fall marketing.

“There’s the potential to make a lot of money selling replacement heifers to Texas,” Pitman said. The Texas cattle industry has been devastated by drought.

The drawback to trying to grow legumes in pastures is the added management and expense. It also takes time for the animals to get used to eating legumes, Pitman said.

John Kruse, cotton and feed grain extension specialist at the Dean Lee Station, is developing management practices for sweet sorghum, a crop with potential for the growing biofuels industry. The crop is similar to grain sorghum but has more biomass that can be converted to fuel products. An advantage to sweet sorghum is that it can be grown on more marginal lands not as suitable for corn or other crops.

Josh Lofton, LSU AgCenter agronomist at the Macon Ridge Research Station in Winnsboro, has only been on the job a couple of months and is starting a research project on soil texture and how this can influence other aspects of crop production, including water and fertilizer use.

Lofton recommends that farmers have their fields analyzed for electrical conductivity, a measure of texture. They can contact their local parish extension agent for how to do this. With this information, combined with a soil test, they can potentially see greater yields with lower input costs, Lofton said.

Ronnie Levy, LSU AgCenter soybean specialist, said soybeans are still being planted to take advantage of high prices. Some farmers are even planting soybeans immediately behind their corn harvest, he said. Currently, Louisiana has 1.2 million acres in soybeans, he said.

Levy said he is working with researchers in Mississippi and Arkansas to study the best timing for irrigation to obtain optimum yield.

Donna Morgan of the Louisiana Master Farmer Program said changes have been made in the program that will streamline the process of becoming certified. She said the changes will allow the LSU AgCenter to review the conservation planning by farmers. Previously, the final phase was under the Natural Resources Conservation Service.

J. Cheston Stevens, LSU AgCenter soils specialist, advised farmers that years of ground work in fields can lead to soil compaction that can hinder a plant’s root growth. He said loaded combines can also add to soil compaction.

Julien Beuzelin, LSU AgCenter entomologist, said monitoring continues of the Mexican rice borer, a pest to rice and sugarcane. It has moved into western Louisiana from Texas. Beuzelin said its potential as a pest to corn and milo is being studied.

Jeff Davis, LSU AgCenter entomologist, said the kudzu bug, a threat to soybeans has been found on kudzu plants in Mississippi. He said the pest could move into Louisiana and feed on soybean plants, threatening an average of 18 percent of a crop’s yield.

Davis said the red-banded stinkbug has shown up earlier than usual this year, and already four to five generations of the insect have been found in clover. “That’s something I’ve never seen before.”

He said the relatively light insect pressure could be attributed to the mild winter, which allowed beneficial insects to thrive. They have fed on the harmful insects.

Boyd Padgett, LSU AgCenter pathologist, said continuous rains will promote aerial blight. “I’m starting to see some Cercospora blight in beans as well.”

Padgett said a study is being conducted by the LSU AgCenter at the Dean Lee Station and the Rice Research Station at Crowley to determine which varieties respond best to fungicides.

Linda Foster Benedict

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