Growth Response of Cotton (Gossypium hirsutum L.) to Pendimethalin Formulation, Timing, and Method of Application.

Pendimethalin is applied to approximately 30% of Georgia cotton to control grasses and small-seeded dicot weed species and is often used in conjunction with glyphosate-tolerant cotton varieties.  Applications can be made preemergence (PRE) or preplant incorporated.  Selectivity in cotton to pendimethalin is thought to be through different metabolic rates and sequestration of pendimethalin in the lysigenous glands, while mode of action in susceptible species is the inhibition of mitotic cell division in the developing roots.  Crops either are planted below or grow through the treated soil zone while susceptible weed species are controlled.  Two liquid forms of pendimethalin are registered for use in cotton: one contains 37.4% pendimethalin (0.4 kg ai/L) formulated with aromatic naphtha as an emulsifiable concentrate (EC) (Prowl), and the other contains 38.7% pendimethalin (0.5 kg ai/L) formulated as a microencapsulated (ME) aqueous capsule suspension (Prowl H2O).
   Increased cases of glyphosate resistant weed species in Georgia has left farmers with minimal options for weed control in glyphosate-tolerant cotton systems.  Pendimethalin offers a reasonable alternative for weed control, but adequate information on the best method of application for the herbicide is not available.  Therefore, field studies were conducted in 2005, 2006, and 2007 to evaluate conventional tillage cotton response to pendimethalin applied PRE, at emergence, 3-leaf, and 5-leaf growth stages of development using either spray application or impregnated on 5-10-15 fertilizer.
     Herbicide treatments of pendimethalin included pendimethalin EC or pendimethalin ME both at 1.1 kg a.i./ha and in spray and impregnated forms.  Fertilizer rates for impregnated forms were applied at 283.8 kg/ha of 5-10-15 fertilizer.  Each treatment was applied PRE, at emergence, 3-leaf, and 5-leaf growth stages.  The experiment was conducted in a randomized complete block design with four replications and included a non-treated control for comparison. Plots were 1.8m wide by 7.6m long, and the sprayer was calibrated to deliver 141.9 L/ha.  Cotton height measurements were taken at 15, 30, 45, 60, and 75 days after planting (DAP).
    Cotton height at 15 DAP for all herbicide treatments and growth stage of application showed no significant differences.  Cotton growth trends remained constant through 60 DAP among treatments when compared to growth stage of application.  Exception lies with pendimethalin EC spray formulation which had a 40% reduction in cotton height at 45 DAP when applied at emergence and 3-leaf stage of development. At 75 DAP, difference in cotton height could be seen among treatments for applications made at emergence and at the 3-leaf stage.  For applications made at emergence, significant reduction in cotton height was seen for the spray form of pendimethalin EC, while impregnated fertilizer treatments of both herbicide forms allowed for greater cotton height.  Greatest cotton height among 3-leaf stage applications was seen with the non-treated control, while the spray form of pendimethalin EC at this stage was significantly lower.
    Applications made at PRE and at the 5-leaf stage showed consistently greater cotton heights at 75 DAP.  Greatest cotton height at 75 DAP observed (59 cm) was with PRE application of the impregnated form of pendimethalin EC, while the least (37 cm) was observed with a 3-leaf stage application of the spray form of pendimethalin EC.  These data indicate that safest application method for pendimethalin in a conventional tillage cotton system would be a PRE application of pendimethalin ME in impregnated fertilizer form.