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Weed control could be circle of truths

Controlling weeds in Mississippi's row crops is definitely more scientifically approached now than even 10 years ago. Transgenic technology and the resulting concept and application of herbicide resistant varieties changed the lineup when it comes to problem weeds. According to the state's top weed control researchers and educators, the spectrum will continue to change.

When asked what growers should expect in weed control in the next five to 10 years, participants of the 12th annual Mississippi Weed Science Roundtable in Orange Beach, Ala., wouldn't offer specific predictions but all seemed in agreement that “resistant” was fast becoming a word growers would tire of hearing. The meeting was held just prior to the opening of the concurrent summer meetings of the Mississippi Agricultural Industry Council and the Mississippi Seedmen's Association.


A variety resistant to damage from a certain herbicide is a good definition of “resistant.” A weed resistant to that same herbicide is a bad definition of “resistant.”

Marestail, also called horseweed, which first developed resistance in Tennessee, but is now in other Mid-South states, including Mississippi, is the most notable “resistant” weed. While over-the-top applications of glyphosate are suppressing and killing weeds such as cocklebur and teaweed that once could only be controlled with multiple herbicide applications and cultivation, other weeds such as horseweed are emerging and thriving despite direct contact with glyphosate.

Horseweed isn't widespread, but for growers unlucky enough to get it, it means additional herbicides and probably some plowing are now part of their weed control program. For decades the plow and the hoe kept Mississippi fields clean. While those tools may have been pushed to the back of the shed, the experts agree plowing may be the only defense for weeds that develop resistance to available herbicides.

If so, it will be the completion of a full circle in search of the best weed management. However, within the circle will be the huge advancements of herbicide-resistant varieties, variable rate applications, species-specific herbicides and precise application methods.

Charles Snipes, MSU cotton specialist for the Delta, says before growers had herbicide-resistant cotton, their four main weeds of concern were morningglory, hemp sesbania, teaweed and cocklebur. Today, after several years of experience with herbicide-resistant cotton and having observed the millions of acres of it planted in the past five years, the priorities for control have shifted.

“Post transgenic, or today, our top four weeds of concern are morningglories, hemp sesbania, pigweed and annual grasses,” says Snipes.

Discussions about pigweed were directed at concerns that it might be headed to glyphosate-resistant status.

Mark Kurtz, a weed control researcher at the Delta Research and Extension Center in Leland, Miss., says rice growers know the threat they have is of outcrossing of red rice. Stewardship to keep red rice out of fields is perhaps the most effective weed control method left. There are places where growers and their workers are walking fields and manually pulling up red rice to keep it out of the fields.

Dan Reynolds, professor of weed science at MSU, says the future will bring more technology and more stacked traits varieties.

“If anything scares me, it's discovering that herbicides designed for the various transgenic varieties do leave room for other weeds to thrive.

“We have old chemicals now that we know will kill our weeds, but companies can't keep them on the shelves forever just in case we might need a specific chemical for a specific weed.

“When a weed gets resistant to glyphosate, I'm worried we will not have the chemical we need. Companies are dropping chemicals, and they are putting money into research for specific weeds. They now bring forth products that will work on a number of weeds at one time or will work in a herbicide-resistant program.”

David Shaw, weed scientist and director of the GeoResources Institute at MSU, says, “One of the big threats is if we have a big blowup with a weed that just won't go away, we may not have access to alternative chemistry. If that happens, we will see years of conservation tillage abandoned because growers will have to put plows back into fields to control weeds.”

Weed specialists say farmers don't want to get the plows back out, but that could the only option on some weeds. Snipes said this year is a good example of when plowing could have made a big difference.

“Our crop suffocated, first from water and then from the crusted soil — a good old plow did a world of good in the fields I saw cultivated,” he says.

Snipes says it would be difficult for producers to go back to tillage and make any money with 50-cent cotton.

“We can still produce a crop without transgenic cotton, but not at the same costs. Herbicide-resistant varieties have rapidly been adopted and used, and growers have in some cases sold their cultivation equipment and laid off their tractor drivers.

Alan Blaine, MSU Extension soybean specialist, says even though growers rapidly adopted Roundup Ready soybeans, he personally believes “given a dry spring and half an opportunity that many producers would go back to growing soybeans the old way if for no other reason than as a statement.

“We've been talking about resistance management for a number of years,” says MSU weed scientist John Byrd. “We are concerned because we are losing so many of our conventional products and getting very few new products. If we have a breakout of resistant weeds, it may be a bigger problem to deal with than we realize.

“I read an old book this morning that was written in 1939. The author said kudzu was not going to be a problem to control. I don't think any of us would make that statement about any weed. We see areas where we are concerned that resistance will become a problem, but those problems may never materialize. We have to keep a close watch on which weeds survive beyond a farmer's weed management program,” says Byrd.


Reynolds was not surprised the first cases of resistant horseweed were in Tennessee because of that area's widespread adoption of conservation tillage. Glyphosate didn't kill it; producers weren't plowing; and at first, they weren't using a residual or herbicide combination that suppresses horseweed.

“That started resistance in the horseweed and from there it's gone through its own selection process,” says Reynolds. “We are concerned other weeds will go through a similar selection process, and our list of resistant weeds will grow.”

Representatives of industry attending the session admitted research dollars were not heavily allocated for the development of new herbicides, but had been shifted to other areas such as fungicides, insecticides and resistant variety development.

“Many of these concerns about resistant weeds are realistic,” says Eric Palmer with Syngenta. “But with good product stewardship, we will have the products it takes to control these weeds. The question will be whether the grower is willing to spend $20 to $25 an acre for that control.”

Eva Ann Dorris is a freelance writer based in Pontotoc, Miss. e-mail:

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