Donnie K. Miller, David C. Blouin, Robert G. Downer, Ralph Bagwell, Eugene Burris, Ernest L. Clawson, B. Rogers Leonard and Alexander M. "Sandy" Stewart
 
Predicted trends in the early 1990s indicated weed control would shift to genetically altered plants with high levels of tolerance to key herbicides. These predictions proved valid, and the vast majority of Louisiana cotton acreage today is devoted to glyphosate-resistant technology. Because the first commercially available glyphosate-resistant cotton did not allow over-the-top application of glyphosate beyond the four-leaf stage because of negative effects on growth and yield, a second generation of glyphosate-resistant cotton, Roundup Ready Flex cotton, was developed. This second generation glyphosate-resistant cotton allows topical applications of glyphosate throughout most of the season with no adverse effects.
 
Because glyphosate controls emerged weeds only and does not prevent emergence of new weed flushes, multiple applications are needed to maintain weed-free conditions season-long. Producers often include herbicides such as metolachlor (various formulations) or pyrithiobac (Staple LX), which have soil activity and prevent future weed emergence, with glyphosate applications over-the-top of cotton to extend weed control and perhaps reduce the amount of herbicide applications needed per sea son. These materials also offer a different mode of action than glyphosate, which may delay or prevent weed resistance to glyphosate.
 
Another possible herbicide that has the potential for this use and is being considered for such labeling is pendamethalin (Prowl H2O). Previous research at the LSU AgCenter shows good weed control and Roundup Ready Flex cotton tolerance to over-the-top co-application of glyphosate and Prowl H2O.
 
Cotton insect control and crop management often require multiple chemical applications throughout the growing season. Insecticide and plant growth regulator applications often coincide with applications of glyphosate. Producers may be able to combine pest and crop management strategies by co-applying glyphosate with Prowl H2O and insecticides or glyphosate with insecticides and plant growth regulators.
 
Such co-applications may result in fewer in-season applications and lower production costs, provided a negative response is not observed on crop safety. Therefore, research was conducted at the LSU AgCenter Northeast Research Station to evaluate tolerance of Roundup Ready Flex cotton to glyphosate (Roundup Weathermax) or glyphosate plus Prowl H2O alone or co-applied with nine insecticides.
 
Additional studies investigated the impact of glyphosate applied alone or plus mepiquat chloride (Mepex Gin Out), a plant growth regulator, with 20 different insecticides on second generation glyphosate-resistant cotton at two different cotton growth stages.

Glyphosate/Prowl H2O/Insecticide Co-application Research
 
Roundup Weathermax alone or plus Prowl H2O was applied to Roundup Ready Flex cotton at the four- to five-leaf growth stage with or without the following insecticide treatments: Orthene, Trimax, Centric, Karate Z, Vydate C-LV, Dimethoate, Bidrin, Baythroid and Mustang Max.
 
At seven and 14 days after treatment in the first year of the experiment (2006), injury in the form of leaf vein chlorosis (yellowing) and leaf curling ranged from 13 percent to 23 percent for herbicide/insecticide co-applications and Roundup Weathermax plus Prowl H2O alone, with no differences noted among these treatments. Injury from these treatments was different, however, from that observed with Roundup Weathermax applied alone, which resulted in no visual injury.
 
In 2007, visual injury was similar for all treatments and no greater than 4 percent. Visual crop injury did not result in height reduction at both seven and 21 days after treatment in both years of the experiment. In addition, differences in seedcotton yields were not noted among the treatments evaluated, indicating that early-season cotton injury did not result in yield reduction.

Glyphosate/PGR/Insecticide Co-application Research
 
Roundup Weathermax alone or co-applied with the plant growth regulator Mepex Gin Out was applied to Roundup Ready Flex cotton at the pinhead square or first bloom growth stage with the following insecticide treatments: no insecticide, Orthene, Trimax, Centric, Capture, Karate Z, Vydate, Curacron, Intruder, Dimethoate, Diamond, Bidrin, Baythroid, Prolex, Mustang Max, Steward, Lannate, Larvin, Tracer, Denim or Battery.
 
At seven days after treatment, co-application of insecticides Lannate or Curacron resulted in 5 percent and 9 percent injury, respectively. These were the only insecticide co-applications that resulted in greater injury compared with Roundup Weathermax or Roundup Weathermax + Mepex Gin Out alone. At 14 days after treatment, Lannate was the only insecticide co-application to result in injury greater than Roundup Weathermax or Roundup Weathermax + Mepex Gin Out alone. Injury for this treatment, however, was only 2 percent.
 
At seven and 28 days after treatment, addition of Mepex Gin Out resulted in a slight cotton height reduction. This result was not unexpected because the compound is a plant growth regulator used to manage the cotton crop by modifying the plant in many beneficial ways, including reducing height. Seed-cotton yield or percent first harvest (measure of cotton maturity) results did not differ among treatments, indicating that cotton injury and differences in height were not manifested in yield reductions or maturity delays.
 
These data indicate that producers may be able to combine multiple pest and crop management strategies to reduce application costs with minimal effect on the crop. Negative effects of co-applications evaluated in this study to Roundup Ready Flex cotton actively growing at the four- to five-leaf growth stage were limited to minor transient visual leaf vein chlorosis burn that lasted no longer than 21 days and did not result in reductions in crop height or yield.
 
Producers are cautioned to make insecticide co-applications only as dictated by insect threshold levels. Initiating unnecessary insecticide treatments may result in decreased populations of non-target insect pests. In addition, co-applications evaluated in this research applied to cotton in earlier growth stages, especially under less than optimal environmental conditions or to cotton under stress, may increase injury potential and result in greater injury than observed in this research.
 
Most insecticide applications are recommended for use with spray nozzles that produce fine droplets to maximize spray coverage. Caution must be exercised that use of such application technologies with glyphosate does not result in drift to nontarget plants. Product labels should always be consulted for allowable co-applications.