Evaluation of Contact Herbicides as a Follow-up Defoliation Treatment in Upland Cotton

Studies were conducted in 2002, 2004, and 2005 at the University of Arizona Maricopa Agricultural Center located near Maricopa, Arizona. Objectives of the study were to evaluate the efficacy of contact herbicides for defoliation as a second application and evaluate any potential improvement in lint return (gin turnout), fiber quality and overall lint value. In 2002, Delta and Pine cultivar 458 BR was planted on 17 April. In 2004 and 2005 Delta and Pine cultivar 449 BR was planted on 13 May and 12 April respectively. In all years, plots were 13.33 ft wide and 25 ft long and treatments were arranged in a randomized complete block with four replications. Irrigation was terminated at cutout (NAWF=5) and defoliation treatments were initiated 22 days later. All plots received an initial defoliation treatment of thidiazuron plus diuron (Ginstar) at 8 oz/A. In 2002 and 2004, follow-up applications occurred seven days after initial defoliation, and included the following treatments: paraquat (Gramoxone Max) at 10.7 oz/A plus nonionic surfactant at 0.25%v/v, flumiclorac (Resource) at 8 oz/A plus crop oil concentrate (COC) at 1% v/v, pyraflufen (ET) at 1.5 oz/A plus COC at 1% v/v or carfentrazone (Aim) at 1.0 oz/A plus COC at 1% v/v. In 2005, follow-up application occurred eight days after initial defoliation and included the same treatments as 2002 and 2004 with the addition of fluthiacetmethyl (Blizzard) at 0.5 oz/A plus COC at 1% v/v and an adjustment of the Gramoxone Max rate to 20 oz/A. For comparison purposes, in all years, one treatment did not receive a follow-up application. In 2002 and 2004, percent defoliation was significantly higher in treatments that received an application of Ginstar followed by Aim, Resource, ET or Gramoxone Max compared with a single application of Ginstar. However, follow-up treatment with Gramoxone Max at 10.7 oz/A provided significantly less defoliation than Aim or ET treatments. In 2005, defoliation was significantly increased by follow-up application with all of the contact herbicides evaluated, No significant differences were observed in gin turnout, lint yield or loan value in the years these variables were evaluated