How to Manage Pests

UC Pest Management Guidelines


Transgenic Cotton

(Reviewed 5/13, updated 5/13)

In this Guideline:


Herbicide-tolerant cotton varieties provide growers a weed management option that can both reduce weed control costs and provide effective control of hard-to-control weeds such as nutsedge, nightshades and annual morningglory. The two types of herbicide-tolerant cotton available are Roundup Ready and Liberty Link varieties. Roundup Ready varieties comprise about 50% of the Acala and Upland cotton acreage, whereas Liberty Link cotton is planted on limited acres. Approximately 80 percent of Pima acreage is glyphosate herbicide-tolerant.

There are several glyphosate-tolerant varieties available, all genetically engineered to tolerate over-the-top applications (from the time the cotyledons appear to the emergence of the first true leaves) of glyphosate. Glyphosate provides excellent control of the many winter and summer annual weeds associated with cotton as well as suppressing or controlling many perennials. With Roundup Ready Flex systems, glyphosate can be applied postemergence until seven days before harvest, which provides increased crop safety and enhances flexibility. Either over-the-top or postdirected (post-crop emergence, directed at weeds) applications can be made, depending on the type of coverage needed for best weed control. There are no restrictions on timing of sequential applications, and insecticides and mepiquat can be tank mixed, if needed, with over-the-top applications.

The Liberty Link system uses glufosinate (Rely), which has a different mode of action than glyphosate—an important consideration in preventing the development of herbicide resistance. Rely provides broad-spectrum weed control and has no growth restrictions for over-the-top applications or rotational restrictions. Glufosinate can also be applied postdirected with hooded sprayers to non-Liberty Link cotton. At this time there are few varieties available for California with this trait.


Potential for weeds to develop resistance to specific herbicides is always a concern with herbicide programs, but with transgenic herbicide-tolerant cotton, weed resistance is of greater concern.

Glyphosate is an herbicide that controls many weeds, including hard-to-control species. Because the potential to reduce weed control costs by using glyphosate exclusively is so great, a real potential exists for the rapid development of weed resistance. Researchers in California have identified glyphosate-resistant ryegrass species (Lolium rigidum) as well as horseweed and hairy fleabane (Conyza spp.). Also, while not confirmed as having resistance to glyphosate, lambsquarters (Chenopodium album) and pigweeds (Amaranthus spp.) are becoming more difficult to control with glyphosate.

No matter which type of production system is used, conventional versus no-till or standard versus herbicide-tolerant varieties, a well-balanced, long-term weed management approach will incorporate resistance management strategies. Management strategies should include crop rotation, rotation of herbicides that have different modes of action, herbicide combinations, and control of escaped weeds by tillage or hand removal in order to delay or prevent development of resistant weeds.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cotton
UC ANR Publication 3444

General Information

  • L. D. Godfrey, Entomology, UC Davis
  • P. B. Goodell, UC IPM Program and Kearney Agricultural Research and Extension Center, Parlier
  • E. T. Natwick, UC Cooperative Extension - Desert Research and Extension Center, Imperial County
  • D.R. Haviland, UC Cooperative Extension, Kern County and UC IPM Program
  • V. M. Barlow, UC Cooperative Extension, Riverside County and UC IPM Program

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