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Resistant weeds in Louisiana?

Through mid-July, Louisiana’s growing season has seen patchy rains, pest worries and increasing fears of resistant weeds.

Based at the Dean Lee Research in Alexandria, La. – which will host a field day on August 5 (see sidebar for more information) – Daniel Stephenson, LSU AgCenter weed specialist, spoke with Delta Farm Press on July 19. Among his comments:

Current conditions in central Louisiana?

“About three weeks ago we caught some significant rainfall. That really helped the soybean and cotton crops on the (Dean Lee) station. Prior to that rain, soybean and cotton in central Louisiana were really drought-stressed. Unfortunately, we turned off dry again until the rain that’s falling today.

“The rain has come in pockets, unfortunately, not across the whole state. For example, the research station at St. Joe hasn’t been catching the rains.”

Cropping season so far?

“I work with corn, cotton, grain sorghum and soybean weed control in the state.

“Where corn has gotten rain, there’s good yield potential. I’ve heard reports on irrigated ground of 150 to 175-plus bushels. On dryland acreage that hasn’t had enough rain, there’s very low yield potential.

“Early this season, this station was parched. In April, we only had a quarter-inch of rain. If my research corn, planted in mid-April, cuts 120 bushels, I’ll dance in the street. It’s been so, so dry— including when the corn began tasseling and pollinating, which is when it needs good moisture.

“To give you an idea, I wasn’t able to evaluate soil-applied residual herbicides in corn effectively. It’s just been too dry.

“However, where there’s been rain or irrigation, cotton and soybeans look great.”

What about weeds?

“Growers have done a fairly good job of controlling weeds. One of the things we’re seeing this year – in soybeans and cotton, in particular – is large flushes of grass. We’re seeing brown top millet, broadleaf signalgrass and, in the northwest, Texas millet. The rows haven’t lapped and that’s allowed light to reach emerging weeds and help them grow.

“At the same time, there’s been a huge influx of fall armyworms and yellow-striped armyworms feeding on those grasses.

“Roger Leonard, (LSU AgCenter entomologist), has been all over this. He points out that if you spray glyphosate and kill that grass, guess where the worms are going? Straight to the crop.

“So, we’ve been recommending that growers soil-apply a pyrethroid with glyphosate when they’re targeting grass. Or, kill the worms and then kill the grass. That issue is throughout the state.”

Update on potential resistance in weeds?

“In Louisiana, too many cotton and soybean producers have been applying two rounds of glyphosate with little use of a residual herbicide. That’s an unfortunate standard for Louisiana — unlike in Arkansas, Mississippi and Tennessee where glyphosate-resistance is common.

“However, over the last couple of weeks I’ve received a call, or two, every day from producers saying ‘Man, I think I’ve got resistant weeds.’

“They’re seeing small patches of pigweed or johnsongrass that have emerged through the canopy. Everything else is dead.

“At this point, (LSU AgCenter colleagues) Donnie Miller, Bill Williams, or I – along with a county agent from the parish with the issue – will visit the problem field.

First off, resistance is not assumed. We’re trying to find out if they had a ‘miss’ when spraying.

Second, was the weed too big when it was sprayed? That’s important to know because if a pigweed is 8 or 10 inches tall with your first application, it may not be controlled. If a weed is too big, a second application won’t control it, either.

“I’ve seen producers allow pigweeds to reach 8 to 12 inches in height before they treat with glyphosate. Then, they re-treat with glyphosate when those same pigweeds are 18 to 24 inches tall. (Even after the second treatment) they don’t die.

“Are those weeds resistant to glyphosate? It’s hard to tell since they were large at the first application. In those situations I’m advising producers to act like the weeds are resistant and hand remove the weeds in that area. “Unfortunately, many fields I’ve visited were treated correctly (application at 2 to 4 inches in height), and the pigweeds or johnsongrass isn’t dead.

“I tell growers that if we continue on this path, we’re just following the footsteps of Arkansas and Tennessee, where resistance has become a major issue. We’ve got to change how we approach this or we’ll be right with them in terms of resistance in about three years.”

On greenhouse trials and suspected resistance…

“We’re working with greenhouse trials to substantiate glyphosate resistance in johnsongrass. I’d bet my paycheck that central and south Louisiana has resistant johnsongrass. And reports are popping up in the northeast.

“I have some Palmer amaranth from fields near the Mississippi River in east Louisiana. It’s showing resistance in the greenhouse. I’ll verify that with more study.

“We also suspect that Louisiana has resistant waterhemp. I know Mississippi State University weed scientists confirmed glyphosate-resistant waterhemp earlier this year.

“Two weeks ago, I visited a soybean field in north Louisiana. It hurt my heart but I recommended a grower destroy a 90-acre soybean field. He was unable to control waterhemp in the field. He’d put out a 1X rate for the first shot, a 2X rate for the second shot and a 1.54X rate – plus one pint of fomesafen – for the third. Even after that, the waterhemp was as tall as my chin.

“So, we’ve got resistance issues rising up here in Louisiana. It’s scary and I hope our growers will learn from their farming brethren to the north and east.”


TAGS: Soybean
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