Old weeds making new push in Louisiana

Stephen Crawford is increasingly worried over trends he’s seeing in Louisiana cotton fields. This season’s wet weather and subsequent weed flushes have only reinforced the weed scientist’s concerns. “We’re going to have to readdress weed control needs in the Mid-South,” says the consultant on some 15,000 acres of Tensas Parish cotton. “We need to bring some order back to what we’re doing.” Crawford, who worked with the LSU AgCenter until 1993, says northeast Louisiana producers got this year’s cotton crop off to an excellent start. The overwhelming majority of the acres he looks after — “over 95 percent” — were planted in Roundup Ready or stacked-gene varieties. “Of the 15,000 acres, I’d guess 70 percent are planted in Deltapine 555. That’s definitely the dominant variety around here.” Producers were able to spray properly early applications of glyphosate. In Crawford’s area, “we generally use two applications (of a pound of active ingredient). On a fair amount of the cotton we also made a post-direct application of a glyphosate-based treatment.” Rains that began to fall in early June shut growers out of fields for nearly a month. “We put out a lot of mepiquat chloride — Mepex, Pix, Ginout and others. It appears that where we depended on these products most heavily, we wound up with the best fruit set. Mepiquat chloride had more impact this year than normal. It was obvious from just looking at fields.”

Hot treatments Northeast Louisiana producers usually finish cotton weed control by June 25, says Crawford. This year, they didn’t get lay-by applications out until after July 4. In many fields, major catch-up work necessitated “very hot treatments.” There were a lot of glyphosate-based sprays put out at lay-by, says Crawford. “Growers felt almost compelled to use it. It appears that in many fields growers set sprayers too high or pressure up too much. Certain areas experienced tremendous amounts of fruit shed and poor pollination and fruit set. “If you read the label, glyphosate is very restrictive after the fourth-leaf stage of cotton. It should be applied with low pressure at the cotyledon-area node or below.” Crawford suspects a couple of things. Since glyphosate isn’t a contact herbicide later in the season, “growers can become lax. You know, ‘What you can’t see won’t hurt you.’ This may have been going on for a while. Last year, one of the best ever for cotton, we may have had some of the same type of damage. The cotton compensated for it, though, and we didn’t know there was any damage. This year, environmental interactions left cotton unable to compensate for glyphosate in the plant. Bottom line: I have no doubt many fields with bad fruit retention and poorly formed bolls have problems inter-related with weather and late-season glyphosate applications.” Crawford says fields most affected by the problem won’t make more than a third to a half of normal yield. One operation he’s familiar with normally yields 2 bales but will likely average half a bale this season. Even though the year was difficult on several fronts, Crawford believes the operation should have conservatively made 1.5 bales. “This isn’t uncommon around here. I don’t know if we can ever pin the culprit down absolutely. But other consultants say the same thing and circumstantially the case is strong. We agree we’re seeing glyphosate damage. From now on, I believe everyone will be much more careful with late-season directed applications of glyphosate.” If this season’s weather pattern repeats, what can be done differently? “Look, Roundup Ready is marvelous technology: it’s cheap, easy and fast. It’s wonderful, and I don’t want it to go away. But it’s so easy that growers have become excessively reliant on it. “That said, we must be more circumspect and responsible to not use any one product too much. One way is to depend on other herbicides with Roundup.” Crawford points to glyphosate-resistant weeds popping up across the nation. “It shocked the devil out of me, but I’ve found a few marestail plants around Tensas Parish that I suspect are glyphosate-resistant. They were thriving in Roundup Ready fields. They will be collected and tested. Resistance isn’t something to shrug off. Using other herbicides with glyphosate helps manage resistance.” Doing so also provides producers with a bit of insurance against desperate applications late in the season. “I think some of these glyphosate-related problems would have been less severe if we’d relied on other herbicides. I’m a big fan of residual herbicides at, or near, planting.” On the residual side of the Crawford’s early-season ledger: Cotoran, Direx, Caporal and Staple. A return to residuals at season’s start provides a foundation for a season-long weed control program. “Depending on only glyphosate gets us into a pattern of continuing flushes of weed. If weather prevents us from getting into the field in a timely fashion, the flush can grow large and growers rush to make applications.” Tank-mixes are also options. Staple can be tank-mixed with glyphosate in early, over-the-top applications. “That combination enhances contact-weed control, and provides residual control downstream. “A wonderful product introduced this year is Envoke from Syngenta, an over-the-top herbicide. I wasn’t prepared for how good it would be. It has outstanding activity on pigweeds, morning-glory, hemp sesbania and cocklebur. That’s another product to watch.”

Revisiting old foes Lately, Crawford has seen a return to weeds that were dominant 20 years ago. As producers have moved away from residual herbicides, the transition has happened incredibly fast, he says. “Many years ago, when I began working in weed science, we developed programs where every acre was tilled and Treflan was applied preplant incorporated to deal with pigweeds and annual grasses. Then we’d apply Karmex or Cotoran at planting. “But we’ve gotten away from those residual herbicides, and a shift has come. This year, we spent more money controlling annual grasses with foliar applications than any other time I can remember. Crabgrass, goosegrass, broadleaf signalgrass, barnyardgrass and sprangletop were weeds we had beaten into submission. They’d almost faded away. They’re back now in a big way now, though.” His concern is not overblown, Crawford insists. “Twenty years ago, teaweed (prickly sida) was the number one weed in cotton. Through the use of Zorial, teaweed had been beaten away. Now, it’s back making a strong showing. The point is, we’re seeing a return of old weeds. With excessive dependence on glyphosate, the old weeds have produced new problems. “We must recognize that any action we take results in nature’s own reaction — and sometimes we’re surprised by what she does. We need to be talking about the curveballs we’re being thrown on the weed front. And we definitely need to take some pressure off glyphosate.”

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