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New Options With Roundup Ready Cotton

Crop and Soil Environmental News, May 1997

Charles Hagedorn
Extension Specialist

As described above, several Roundup Ready cotton varieties will be available to growers for planting in 1997, although seed supplies are very limited. Monsanto has set a technology fee for varieties containing the Roundup Ready gene at $8 per acre for picker cotton varieties. Limited quantities of varieties containing both Roundup Ready and the Bollgard Bt gene will also be available with a technology fee of $40 per acre. Technology fees for varieties containing only the Bollgard gene will remain at $32 per acre, the same as last year. The technology fees are payable when the seed is purchased. Growers who lose their Roundup Ready cotton before July 25 will receive a full refund of their technology fee.

For many cotton growers the newly available Roundup Ready cotton offers early season, broad-spectrum control of annual and perennial weeds and grasses. However, university weed scientists across the Southeast caution growers that residual herbicides will still be necessary where certain weed species are present. They also urge growers using Roundup Ultra on Roundup Ready cotton to carefully follow label instructions, including the caution not to apply Roundup over-the-top of cotton larger than the four-leaf stage. Later applications in field tests in 1996 significantly reduced yields in some cases. While land-grant university scientists have worked with the new technology in field trials, none of the Roundup Ready cotton varieties have yet been placed in state Official Variety Trials for direct comparisons to other cotton varieties

Growers should understand that there is a big difference between Roundup Ready cotton and Roundup Ready soybeans. Growers need to keep soybeans weed-free for two to four weeks after planting. This can be done in narrow rows with one application of Roundup. If weeds come through in wider row soybeans, they can usually be controlled with one more application of Roundup over-the-top. There have not been yield reductions with even fairly high rates of Roundup applied over-top of Roundup Ready soybeans. That is not the case with Roundup Ready cotton. Cotton needs to be kept weed-free for six to ten weeks after planting. However, fairly substantial yield reductions have been reported where Roundup was applied over-the-top on cotton that was past the four leaf stage.

Growers should wait as long as possible to apply a pint and a half to two pints per acre of Roundup Ultra over-the-top of Roundup Ready cotton, but the application must be made before the plants are beyond the four leaf stage. That should replace the standard early post-directed herbicide application and get to the last post-directed, layby stage. To provide residual control at planting, growers should consider using Cotoran, or Cotoran and Treflan or Prowl or Dual. Also, roundup alone will not adequately control some specific weeds, including pitted morningglory, crabgrass, goosegrass, nutsedges, mayflower, hemp sesbania and Florida pusley. Palmer amaranth pigweed will require two applications of Roundup or Roundup plus MSMA.

When using Roundup Ready cotton planted no-till or strip tilled, standard at-planting herbicides like Prowl or Dual with Cotoran are not absolutely necessary. However, a burn-down herbicide prior to planting is still recommended. In most no-till situations, growers should consider a standard pre-emergence herbicide application followed by Roundup Ultra early post over-the-top, followed by Roundup Ultra and a residual like Bladex late post-directed at layby.

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