Weed control in ultra narrow row cotton in Arizona

In two 1999 Arizona studies, a preplant incorporated (PPI) application of Prowl (pendimethalin) (2.4 pt/A) or Treflan (trifluralin) (0.75 lb a.i./A) followed by Roundup Ultra (glyphosate) (1 qt/A) at the 3 to 4 true leaf cotton growth stage provided good to excellent weed control. At the University of Arizona Maricopa Agricultural Center site that had low weed pressure, a postermergence topical Staple (pyrithiobac) (1.8 oz/A) application also provided acceptable weed control but was more expensive. At the Buckeye, Arizona study site, a PPI application of Prowl at a reduced rate (1.2 pt/A) was as effective as the full rate (2.4 pt/A) but a preemergence application of Prowl (2.4 pt/A) was not as effective as either of the PPI Prowl rates or PPI Treflan. A sequential postemergence topical Staple application (1.8 oz/A) following the Roundup Ultra application did not significantly improve weed control. Preliminary conclusions after one field season of experimentation and observation in Arizona UNR cotton suggest that in fields with low to moderate weed populations, a PPI Prowl or Treflan application followed by a postemergence topical Roundup Ultra application will provide acceptable weed control in most fields. However, the presence of nutsedge or other difficult to control weeds may require two postemergence topical Roundup Ultra application prior to the four leaf growth stage and possibly the additional use of other herbicides. More research is needed to further explore weed control options in Arizona UNR cotton production systems.