Donnie K. Miller and Alexander M. “Sandy” Stewart
Since the release of the first generation of Roundup Ready cotton in 1995, U.S. cotton producers have shown a willingness to adopt this new technology. Benefits to this Roundup Ready technology in cotton include the potential to reduce or eliminate chemical applications and tillage operations, control of a broad spectrum of both grass and broadleaf weeds, and rotational flexibility to plant other crops because of lack of residual soil activity with glyphosate. The primary drawback to this technology was the label restriction for over-the-top application of glyphosate to occur before the fifth-leaf stage of development. Over-the-top application of glyphosate to Roundup Ready cotton beyond this restriction has the potential to result in misshapen and sterile pollen and no or poor fruit set, which can lead to maturity delays or yield loss. Subsequent glyphosate applications beyond the label restriction must be directed underneath the crop, minimizing herbicide-to-plant contact.
In 2006, the second generation of Roundup Ready cotton, termed Roundup Ready Flex cotton, was made commercially available. This new version allows cotton producers to make glyphosate applications over-the-top regardless of crop growth stage. This places less reliance on specialized spray equipment intended to reduce herbicide-plant contact and allows the use of larger, faster-moving equipment. In addition, this affords the ability to reduce the number of application trips through the field by co-applying insecticides, plant growth regulators or micronutrient fertilizers with glyphosate in over-the-top applications.
Research conducted with Roundup Ready Flex cotton by LSU AgCenter weed scientists has focused on weed efficacy, crop tolerance and co-application effects. Efficacy trials indicate that excellent control of a number of grass and broadleaf weeds in cotton can be achieved season-long with glyphosate applications to small (1-3 inches tall) weed infestations. Glyphosate applications should be made as early as possible once weeds emerge to eliminate weed competition. Delaying initial glyphosate application from the two-leaf cotton growth stage to the five-leaf stage has resulted in a 20 percent yield reduction, even though excellent weed control was observed with glyphosate applied at this timing, indicating the effect of early-season weed competition. In addition, no visual injury or negative growth effects with over-the-top glyphosate applications beyond the current restriction on Roundup Ready cotton indicate excellent tolerance to glyphosate in Roundup Ready Flex cotton.
Research has also shown that including additional herbicides with residual soil activity at planting can buy producers time in making the initial glyphosate application. This would limit weed competition that can occur when the initial glyphosate application is delayed because of environmental conditions and weeds become larger. Co-applying herbicides with residual soil activity with glyphosate over-the-top can prevent weeds from emerging later in the season and negatively affecting the crop.
Perhaps the main benefit to including other herbicides with glyphosate in Roundup Ready Flex cotton is to introduce different modes of action to prevent weed resistance associated with the continuous use of glyphosate. The need for additional herbicides in the Flex system will be a decision made by the individual producer. Decisions will depend on the ability of a producer to cover his acreage in a timely fashion to eliminate early season weed competition, environmental factors that affect timeliness of application, and increased technology costs associated with the new technology and the cost efficiency of using additional inputs.
Research was conducted on the effects of glyphosate co-applied with most insecticides commonly used in cotton production and a commonly used plant growth regulator. It indicated that no negative effects were observed on control of most of the common weed problems in cotton in Louisiana. In addition, negative effects of these co-applications to cotton at the pinhead-square or first-bloom growth stages were limited to minor, temporary leaf burn lasting no longer than 14 days with no reduction in crop maturity or yield. When applied according to the herbicide labeling on Roundup Ready Flex cotton, glyphosate co-applications can offer producers the ability to integrate pest and crop management strategies and reduce application costs with minimal effect on the crop.
Although increased costs will be associated with this new technology, it offers the potential to reduce costs and time associated with applications over-the-top throughout the growing season. Producers may be able to use larger, faster-moving equipment and to reduce the number of application trips through the field by co-applying insecticides and plant growth regulators with glyphosate. A careful evaluation of the cotton varieties in which Roundup Ready Flex will be available will be critical to the system’s profitability. Early research with Roundup Ready Flex varieties in Louisiana has indicated that their yield may not always be competitive with the best currently available varieties. With new variety development taking place each year, this situation could change. Roundup Ready Flex varieties will require yields that are competitive with existing varieties to offset the increase in technology fees and contribute to overall profitability.
(This article was published in the summer 2006 issue of Louisiana Agriculture.)