How to Manage Pests

UC Pest Management Guidelines

Transgenic, Herbicide-Tolerant Cotton
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 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 with the Liberty Link acreages limited to a very small number of acres. The first herbicide-tolerant Pima varieties are now available on limited acreage.

There are several glyphosate-tolerant varieties available, all genetically engineered to tolerate over-the-top applications of glyphosate. Glyphosate provides excellent control of the many winter and summer annual weeds associated with cotton as well as suppresses or controls many perennials. Some varieties have the older technology and some varieties have the new "Flex" system. With the older Roundup-Ready technology, over-the-top applications are limited to cotton before it reaches the 5^th true leaf stage. Over-the-top applications beyond the 5^th leaf stage have resulted in deformed and lost bolls and ultimately yield reductions. Later applications, at the 8- to 12-leaf stage, need to be applied as a post-directed treatment to prevent injury and yield loss.

With Roundup Ready Flex systems, glyphosate can be applied through layby (14 node) and beyond, which provides increased crop safety and enhances flexibility. Either over-the-top or post-directed 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 (Ignite), which has a different mode of action than glyphosate—an important consideration in preventing the development of herbicide resistance. Ignite 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.

Weed Resistance Concerns
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 weeds that are resistant to it. Researchers in California have identified and confirmed a glyphosate-resistant ryegrass species (Lolium rigidum) as well as horseweed (Conyza spp.); also, while not confirmed as having resistance to glyphosate, lambsquarters (Chenopodium album) is becoming more difficult to control with it.

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

PUBLICATION

UC IPM Pest Management Guidelines: Cotton
UC ANR Publication 3444
General Information
L. D. Godfrey, Entomology, UC Davis
P. B. Goodell, UC IPM Program, Kearney Agricultural Center, Parlier
E. T. Natwick, UC Cooperative Extension, Imperial Co.
D. R. Haviland, UC IPM Program, Kern Co.

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