Cotton is a major cash crop in America. It is a semi-tropical, slow-growing, annual plant that is highly susceptible to weed damage, yet only a small amount of herbicides are available to help control weeds. Controlling cotton weeds with herbicides is a difficult matter.
According to the University of Missouri Extension, the fluometuron herbicide is used to control a majority of weeds in cotton. It works with both broadleaf and grassy weeds. Cotton has a good resistance toward fluometuron, so more herbicide may be used. If the pre-emergent herbicide treatment is not effective and weeds grow taller than the cotton, post-emergent treatment will not be effective.
Pre-emergent herbicides will work for two to four weeks after cotton begins to grow. After that, the herbicides break down and weeds begin to sprout. Directed herbicides are then used, sprayed directly onto the weeds. Post-emergent herbicides are applied at a 3 to 8 inch height or at a 8 to 14 inch height. Most cotton post-emergent herbicides are contact herbicides that kill the area of the weed touched by the spray, so thorough coverage is necessary.
Selective Grass Control
Nonselective post-emergent herbicides may not be effective against grassy weeds in cotton. This requires a selective, post-emergent herbicide for grassy weeds. MSMA and DSMA herbicides are effective, although Kentucky bluegrass is resistant.
When weeds are sprayed with the same herbicide for four to 10 years, they become resistant to the herbicide. This is a particular danger in cotton because of the small amount of herbicides available for cotton weed control. The Herbicide Resistance Action Committee catalogs all resistant weed varieties in the wild (see References).
Diversifying crop production practices, says Cotton Inc., is one of the ways to reduce resistance. Rotating crops, tilling and changing herbicide mode of action on the weed plant will reduce resistant strains from popping up. Using the correct herbicide rates also reduces the occurrence of weed resistance.