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Production Papers

Effect of Planting Date, Mepiquat Chloride, and Glyphosate Application to Glyphosate-Resistant Cotton

^a USDA-ARS-NPRL, 1011 Forrester Dr. SE, Dawson, GA 39842
^b USDA-ARS-SRRC, 5883 USDA Rd., Houma, LA 70360
^c Soil Science Dep., Campus Box 7619, North Carolina State Univ., Raleigh, NC 27695
^d Dep. Crop Science, Campus Box 7620, North Carolina State Univ., Raleigh, NC 27695

^* Corresponding author (rnuti{at}nprl.usda.gov)

Received for publication December 29, 2005. Management decisions and common misapplication of glyphosate may impact fruiting of glyphosate-resistant (GR) cotton (Gossypium hirsutum L.). Experiments were conducted to determine if planting date affected the ability of GR cotton to compensate for fruit loss after misapplication of glyphosate and to evaluate mepiquat chloride's (MC) contribution to fruiting. Field studies were conducted in Rocky Mount, North Carolina, from 2001 to 2003. Treatments included optimum and late planting and a series of five glyphosate, 0.84 kg a.e. (acid equivalent) ha^–1, treatments representing recommended and common misapplication timings including a control. The 10 planting date and glyphosate combinations were factored across treatments of MC and no-MC as needed according to growing conditions in 2001 and 2002. All plots were treated with MC in 2003. Optimal-planted cotton produced more than late-planted cotton. Yield was reduced in optimal-planted cotton in 2001 and late-planted cotton in 2001 and 2002 when glyphosate contacted plants after the four-leaf stage. Misapplication of gyphosate did not affect yield in 2003. Yield was improved with MC by 11% in 2001. Bolls were at higher nodes in late-planted cotton and cotton not treated with MC. Glyphosate contact after the four-leaf stage in 2001 and 2002 shifted the fruitload above Node 10. Late planting of cotton decreases opportunities for fruiting compensation when glyphosate reduced early boll retention. Results support previous research showing application methods that allow glyphosate contact to GR cotton plants after the four-leaf stage increase risk of yield reduction regardless of planting date.

Abbreviations: a.e., acid equivalent • DD15.5, growing degree day • 4OT, over-the-top at four-leaf stage • 8 non-Prec PD, nonprecision postdirected at eight-leaf stage • 8 Prec PD, precision postdirected at the eight-leaf stage • 8OT, over-the-top at eight-leaf stage • GR, glyphosate-resistant • MC, mepiquat chloride • OT, over-the-top • PD, postdirected • PGR, plant growth regulator