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Residuals, mixes enhance burndown

Cotton farms come in all shapes and sizes, and there may be as many strategies for burning down winter weeds in reduced-tillage cotton as there are reduced-tillage cotton farmers.

Steve Crawford, a retired university weed scientist who now consults on weed control for farmers in the Louisiana Delta, has taken one of the more systematic looks at some of the ways growers can control winter vegetation in stale seedbed or reduced-tillage cotton planting systems.

Crawford, whose consulting firm, Crawford AG, operates in Tensas Parish near St. Joseph, La., provided weed control consulting services on about 12,000 acres of Roundup Ready cotton in 2001.

“Essentially all of this cotton was produced on mixed to clay soils in stale seedbed systems where beds were formed in the previous fall and planting was accomplished in the spring with minimal soil disturbance,” says Crawford. “Cotton acreage per farm unit ranged from 1,000 to near 3,000 acres.”

In a paper he presented at the National Conservation Tillage Conference in Tunica, Miss., Crawford said the weed control on the farms varied primarily in the residual herbicide used between early preplant burndown until cotton emergence.

“Many weed species were present on these farms, including morningglory species, hemp sesbania, pigweed species, spotted spurge, prickly sida, wild okra, wild poinsettia, annual grasses and johnsongrass,” he noted. “The dominant weeds, and those which most frequently escaped control, were pitted morningglory and hemp sesbania.”

The weed control programs used on the farms fell into one of the following categories:

No residual: Glyphosate at 1 pound of active ingredient per acre plus 2,4-D LVE at .33 pound of active ingredient per acre early preplant or EPP. Glyphosate at .5 to 1 pound ai/acre at planting or AP.

Residual early preplant: Glyphosate plus 2,4-D LVE plus diuron (Direx 4L, Karmex) at .5 to 1 pound ai/acre EPP. Glyphosate AP.

Residual pre-emergence tank-mix: Glyphosate plus 2,4-D EPP. Diuron plus paraquat (Gramoxone Max) at .33 to .5 pound ai/acre on a band at planting.

Residual post plant tank-mix: Glyphosate and 2,4-D EPP. Broadcast application of diuron and paraquat two to four days after planting.

Most of the cotton received two over-the-top applications of glyphosate within the one- to four-leaf cotton range. Farmers made post-directed applications of diuron or prometryn (Cotton-Pro or Caparol) plus MSMA at the seventh to ninth nodes and lay-by applications of diuron or diuron plus linuron.

The glyphosate/no residual approach proved to be a challenge for some growers, he noted.

“The largest farm in the group, with ample labor, equipment and excellent management, coupled with benevolent weather, accomplished the highest level of overall weed control of any of the farms,” said Crawford. “But a similar-sized farm with fewer resources failed miserably on several of its units due to its inability to achieve proper timing of the first postemergence-directed application.”

The best approach for the majority of farms was the last, the glyphosate and 2,4-D for burndown and diuron and paraquat broadcast two to four days after planting.

“This strategy had the advantages of allowing maximum weed emergence prior to crop emergence, divergent chemistry for control of glyphosate-tolerant weed species and optimum timing of application of diuron for extended residual weed control,” he said.

“Beyond the requirement of scheduling and executing this treatment in sequence with planting, there were no negative issues. Farms which utilized this method of diuron delivery received the greatest benefits for their investment.”

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