Skip navigation.
New Mexico State University

Weed Management in Cotton Under the Roundup Ready® System

Guide A-224

Richard D. Lee, Extension Weed Specialist
Shane T. Ball, Extension Agronomy Specialist

College of Agriculture and Home Economics New Mexico State University

This Publication is scheduled to be updated and reissued 7/03.

Weed management in cotton has taken a new twist with the introduction of transgenic cotton. Some cotton plants have been genetically altered to make them tolerant to a particular herbicide.

One cotton-herbicide system is built around Roundup Ultra® (glyphosate)-the Roundup Ready System®. Research has demonstrated that this system is an effective tool for managing perennial weeds, particularly bermudagrass (Cynodon dactylon), Johnsongrass (Sorghum halepense), silverleaf nightshade (Solanum elaeagnifolium), yellow and purple nutsedge (Cyperus esculentus and C. rotundus), weeds common to many New Mexico cotton fields.

Under this system, Roundup Ultra® can be applied at different growth stages.

Over-the-Top

  • Broadcast applications can be made from cotton emergence through the 4-leaf stage (4 nodes) of growth. Apply before the 5th leave is the size of a quarter.
  • Sequential applications can be made for the management of perennial weeds. The second application should be made after 2 nodes of incremental growth and a minimum of 10 days following initial application.
  • Application rates should not exceed 2 pint/acre (32 fl oz/acre).

Post-directed

  • Two (2) directed applications can be made from 5th-leaf through layby.
  • Application rates of should not exceed 2 pint/acre (32 fl oz/acre).
  • If sequential applications are made, allow a minimum of 10 days between a applications (2 nodes of incremental growth).

Preharvest

  • Broadcast application may be made for preharvest after 20% of the bolls have cracked.
  • Allow a minimum of 7 days between application and harvest.
  • Apply 2-4 pint/acre (32-64 fl oz/acre).

For the Roundup Ready System to perform its best, follow these guidelines:

  • Identify weeds in the early seedling stage.
  • Make applications to actively growing weeds. Stressed weeds won't absorb the herbicide as effectively as non-stressed weeds.
  • Total spray solution for ground applications needs to be near 10 gallons per acre.
  • Flat fan nozzle tips are recommended.
  • Add 17 lb of sprayable-grade ammonium sulfate per 100 gallons of total spray solution.
  • When using Roundup Ultra® there is no need to add any additional surfactant.
  • Application made to leaves covered with dust result in reduced activity, so cultivation is restricted at application time.

New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.

Printed July 1998
Electronic Distribution July 1998