Linda F. Benedict, Sanders, Dearl E., Vidrine, Paul R., Millhollon, Eddie P., Miller, Donnie K.
Donnie K. Miller, P. Roy Vidrine, Eddie P. Millhollon, Dearl E. Sanders, Daniel B. Reynolds, David L. Jordan and Christopher B. Corkern
Before 1996, farmers controlled weeds following cotton emergence almost exclusively by directing herbicides underneath the canopy to minimize injury to the cotton plant. For these postemergence, directed applications to be effective, there had to be sufficient height differences between the crop and the weeds and adequate rainfall or mechanical incorporation to activate residual components. Herbicides applied over the top of cotton often resulted in only fair weed control, delays in crop maturity and yield reductions.
In 1996, the commercial release of the herbicide pyrithiobac, sold under the trade name Staple, for postemergence application in cotton provided a means of obtaining excellent postemergence control of many broadleaf weeds while not injuring the crop. In addition, Staple provides preemergence, residual weed control through soil activity and has since received a label for application after planting.
Recent advances in plant biotechnology, allowing transfer of genes for herbicide tolerance to plants, have led to the development of herbicide-tolerant crops. Cotton varieties tolerant to herbicides glyphosate and bromoxynil, sold under the trade names Roundup Ultra and Buctril, respectively, are commercially available. Both provide postemergence control of a number of weeds in cotton. Questions about the utility of Staple, Roundup Ultra and Buctril compared with the traditional weed management systems have arisen. An overview of research results to date is presented.
Staple provides optimum control when applied to small, actively growing weeds or when adequate rainfall is received for activation following soil application. Postemergence activity is slow, with chlorotic, or yellowing, symptoms visual within seven to 10 days. Susceptible weeds not killed are stunted, reducing competition with cotton and establishing required height differential for subsequent postemergence, directed applications.
Redroot pigweed, smooth pigweed and smartweed are extremely sensitive to Staple. Control of pitted, entireleaf and ivyleaf morningglory is good to excellent, with the latter two species being slightly more sensitive. Hemp sesbania control is good, with weed susceptibility increasing as growth progresses beyond the cotyledon to first true leaf stage. Staple provides good control of common cocklebur with optimum activity when applied before the two-leaf stage. Prickly sida control is fair to good if applications are made when weeds are no taller than 1 to 2 inches. Sicklepod control with Staple is fair to poor regardless of application timing. Staple provides suppression of small seedling grasses and nutsedge and little control of smellmellon, wild okra and copperleaf.
When applied before emergence, Staple provides good control of prickly sida and fair to poor control of morningglory species, hemp sesbania and common cocklebur. Addition of fluometuron is recommended for increased control of less susceptible species.
Staple programs can reduce the number of herbicide applications while maintaining optimum cotton yield. In trials comparing weed control systems, a program including postemergence application of Staple followed by one postemergence, directed treatment provided comparable control (more than 96 percent) of pigweed, pitted and entireleaf morningglory, common cocklebur, hemp sesbania and prickly sida and yield to a standard program which included three postemergence, directed applications. Because of limited activity on grass species and lower preemergence activity on problem broadleaf weeds, use of soil-applied residual herbicides was still needed to maximize weed control and yield in a Staple program.
Note that addition of postemergence grass herbicides to Staple has resulted in reduced johnsongrass control when compared to grass herbicides alone. A reduction in control was observed with combinations of Staple and Poast Plus, Fusilade 2000 or Select in studies conducted in Alexandria when adequate rainfall was received before and after application. In St. Joseph, however, reduced control was observed with addition of Assure II, Bugle, Fusilade 2000, Poast Plus or Select under low rainfall conditions.
Following Staple application, cotton plants occasionally have exhibited chlorosis, or yellowing, in terminal areas and a ragged appearance on leaf margins. These symptoms seem to be transient and have not caused serious yield reductions.
Roundup Ready system
Roundup Ultra has been extremely effective in controlling small, actively growing weeds. Like Staple, activity of Roundup Ultra is slow, with visual symptoms of damage not appearing until seven to 10 days after application. Roundup Ultra, at a single application rate of 1.5 pints per acre, exhibits good to excellent activity on barnyardgrass, crabgrass, goosegrass, broadleaf signalgrass, fall panicum, rhizome and seedling johnsongrass, smooth and redroot pigweed, sicklepod, spurge, common cocklebur, copperleaf, wild okra and wild poinsettia.
Fair to poor control of pitted morningglory, entireleaf and ivyleaf morningglory, hemp sesbania, prickly sida, velvetleaf and spurred anoda can be expected with a single application. Higher initial rates or sequential applications improve control.
Research has shown that, because of lack of residual activity, sequential applications generally provide the most consistent weed control. The first application should be made when the initial flush of weeds reaches 1 to 3 inches in height. Sequential applications should follow 10 to 14 days later, preferably when weeds are not stressed.
The postemergence application window with Roundup Ultra is narrow because applications cannot be made over-the-top to cotton beyond the fourleaf stage, after which applications must be directed. Delaying the initial Roundup Ultra application, whether in a single or sequential program, beyond the 1- to 3-inch stage can result in longer competition among weeds and the crop and necessitate a higher rate of herbicide and possible interference with spray coverage.
Results from research evaluating the necessity of residual herbicides in a Roundup Ready program have been varied. In general, success of a total postemergence weed control program with Roundup Ultra is enhanced under drier conditions, which usually results in fewer weed flushes and allows more timely applications. Under these situations, sequential applications have provided weed control and yields comparable to programs that included a preemergence herbicide.
Hemp sesbania, morningglory, prickly sida and nutsedge have been shown to be less susceptible to Roundup Ultra, especially beyond the three- to four-leaf stage. Preemergence herbicides that control these weeds would be beneficial in increasing the overall performance of the Roundup Ready program and act as insurance in case adverse weather prevents timely postemergence application. Additionally, use of soil-applied herbicides may eliminate the need for a second Roundup application. Research to date has shown that use of residual herbicides may still be needed to produce maximum yields over a range of environmental conditions, especially on less susceptible weeds.
In a Roundup Ready weed control systems test in 1997, postemergence application of Roundup Ultra at 1.5 pints per acre (as needed but limited to two applications) following Prowl plus Cotoran preemergence at labeled rates produced 1,876 pounds per acre of seed cotton. Yield dropped to 1,210 pounds per acre when Cotoran was removed from the preemergence program. When Prowl was removed from the preemergence program, seed cotton yield was 1,703 pounds per acre. Plots receiving no preemergence treatment, Roundup Ultra postemergence application and Bladex late-directed application produced 1,842 pounds per acre of seed cotton.
In a conventional cotton production system test, plots receiving a preemergence application of Treflan pre-plant incorporated and Cotoran preemergence followed by Roundup Ultra outyielded plots receiving only Roundup Ultra by 853 pounds per acre. In stale seedbed and wheat cover crop production systems, plots receiving Roundup Ultra and no residual herbicide, although exhibiting good weed control 37 days after treatment, were heavily infested with weeds late in the season and were unharvestable.
The weakness of Roundup Ultra on several weed species, including hemp sesbania and morningglory, may be overcome by tank mixtures with other herbicides. Limited research has shown that Staple, which has good morningglory and sesbania activity, may be an appropriate tank mix partner with Roundup Ultra.
Buctril works quickly on susceptible weeds, with symptoms visible as soon as one day after application. Like the previous herbicides, maximum control is observed on actively growing weeds. Buctril provides good to excellent control of morningglory species, common cocklebur, hemp sesbania, Pennsylvania smartweed, wild okra and wild poinsettia. It provides fair control of small pigweed and prickly sida and little or no activity on sicklepod or grasses.
As with the Staple and Roundup Ready programs, use of the BXN system can reduce the number of postemergence, directed herbicide applications while maximizing yields. When evaluating weed control systems, a program consisting of Buctril applied postemergence followed by Bladex plus MSMA postemergence, directed resulted in weed control and seed cotton yield equivalent to programs including two postemergence, directed applications. Tank mixtures of Buctril and grass herbicides Assure II, Fusilade DX or Select reduced johnsongrass control when compared with the grass herbicides applied alone. The greatest antagonism was noted with Assure II and the least with Select. Buctril application one day before application of grass herbicides was generally more antagonistic than when applied one day after. In most cases, a three-day interval was sufficient to eliminate antagonism. Residual herbicides are needed to maximize weed control and cotton yield with the BXN system. Addition of Staple to Buctril applied postemergence or preceding Buctril as a preemergence treatment has been effective in increasing pigweed control in the BXN system.
Staple, Roundup Ready and BXN weed control systems offer advantages for control of certain problem weeds in Louisiana cotton. Staple offers freedom of variety selection, residual weed control and a wide application window. Disadvantages include higher herbicide necessitating banding (application to a certain percentage of the row) to reduce costs, limited grass activity and carryover potential to rotational crops. Roundup Ready offers broad spectrum control of both grasses and broadleaves, low herbicide cost (allowing broadcast applications) and lack of carryover potential to rotational crops. Weaknesses include lack of residual weed control, restriction to varieties containing the Roundup Ready gene, higher seed cost and technology fee, and a narrow window for postemergence application. The BXN system offers rapid activity on susceptible weeds, relatively low herbicide cost and minimal carryover concerns. Disadvantages include lack of residual weed control, no grass activity and restriction to varieties containing the BXN gene.
Added technology assessment fees and added cost in seed purchase for transgenic varieties, cost of herbicide programs, efficacy, agronomic characteristics and yield potential of varieties are all factors that should be considered in the decision to adapt these new weed control systems.
Acknowledgment: The authors express appreciation to Cotton Incorporated, DuPont, Monsanto and Rhone-Poulenc for providing funds to help support this research.
Roundup Ready and Roundup Ultra are registered trademarks of Monsanto; BXN and Buctril are registered trademarks of Rhone-Poulenc; Staple is a registered trademark of DuPont.
Donnie K. Miller, Assistant Professor, Northeast Research Station, St. Joseph, La; P. Roy Vidrine, Professor, Dean Lee Research Station, Alexandria, La; Eddie P. Millhollon, Associate Professor, Red River Research Station, Bossier City, La; Dearl E. Sanders, Extension Weed Specialist, LSU Agricultural Center, Baton Rouge, La.; Daniel B. Reynolds and David L. Jordan, both formerly at the Northeast Research Station; and Christopher B. Corkern, Graduate Research Assistant, Department of Plant Pathology and Crop Physiology, LSU Agricultural Center, Baton Rouge, La
(This article was published in the winter 1999 issue of Louisiana Agriculture.)