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In cotton: Weed control options many, varied

Cotton weed control programs using the Roundup Ready system, Liberty Link system, the BXN system and standard conventional weed control systems provide ample opportunity for excellent postemergence weed control prior to layby.

However, each system has limitations and careful planning is necessary to alleviate any potential for weed outbreaks for any given system. Also, each of these systems perform better with a well-planned pre-emergence program and usually require a well-planned residual layby program to extend weed control into the harvest season.

For weed control in conventional non-transgenic systems, Staple herbicide for pre-emergence or postemergence use in cotton provides growers a more convenient way to control troublesome weeds such as spurge, pigweed, morningglory and prickly sida (teaweed) in conventional non-transgenic cotton.

A program approach is necessary for these systems and use of residual soil-applied herbicides is recommended for optimum control of especially troublesome weeds such as morningglory and common cocklebur. Typically, Staple has good soil residual control of troublesome small-seeded broadleaf weeds and good postemergence control of key large seeded weeds.

Staple applied postemergence when cotton is about 3 inches high or when weeds are less than 2 inches high provides control of pigweed, morningglory and common cocklebur.

Roundup Ready cotton allows for the use of glyphosate (Roundup, WeatherMax, Touchdown, others) for weed control in cotton. Glyphosate cannot be applied postemergence in conventional cotton unless it is directed with the use of a hooded sprayer.

Do not apply glyphosate postemergence over the top of Roundup Ready cotton if it is beyond the four-leaf stage of growth. Applications subsequent to this stage can be made as a post-directed spray provided contact to the plant is minimized. Glyphosate provides excellent control of many annual broadleaf and grass weeds and provides control or suppression of many perennial weeds.

Control of morningglory and hemp sesbania is probably glyphosate's weakest link. Multiple applications of glyphosate or glyphosate in combination with Staple are necessary to provide acceptable postemergence control of these two weeds. Roundup does not have residual activity. Season-long weed control usually requires a residual herbicide, especially at layby.

The new kid on the block is Liberty Link cotton, which was developed to be resistant to Ignite herbicide. This provides a new weed control system for consideration in 2004. Ignite is similar in activity to glyphosate, but works faster. It does not translocate to the extent that glyphosate does, so is not as good on perennial weeds.

Liberty Link cotton is very tolerant of Ignite and postemergence applications can be made anytime up to 70 days prior to harvest. It is somewhat weaker on grasses than glyphosate, but works faster on key broadleaf weeds such as morningglory and hemp sesbania. Like glyphosate, Ignite does not have any residual activity, so residual herbicides are very helpful in fulfilling the potential for this system.

Another option is the BXN system, which is the use of Buctril in genetically modified BXN cotton. This system provides excellent postemergence control of some very troublesome broadleaf weeds. Buctril does not have residual activity. Season-long weed control usually requires a residual herbicide, especially at layby.

Let me stress, however, that growers should always use a grass herbicide in the BXN system. Buctril does not control annual or perennial grass species, nor does it control nutsedge. Buctril provides excellent control of morningglory and common cocklebur, but is somewhat weak on large pigweed and teaweed. If either are larger than 1 inch, consider including Staple at 0.6 to 0.8 ounce per acre along with Buctril to control these problem weeds.

Charles E. Snipes is Northwest District cotton specialist with the Mississippi State University Extension Service.

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