Mepiquat chloride (MC), a plant growth regulator, is commonly used to manipulate growth and maturity of cotton. Most MC is applied during the pre-bloom period, at first bloom, or soon thereafter. However, there have been claims that late-season or cutout MC applications can increase yields and improve defoliation possibly through enhancing leaf maturity and reducing regrowth. Experiments were conducted during 2007 and 2008 in North Carolina and in two locations in Georgia during 2010 to investigate the effects of MC applied at early bloom versus cutout (nodes above white bloom = 3 to 4) on growth, defoliation, regrowth potential, and lint yield of cotton. Treatments consisted of a factorial arrangement of three MC rates (0, 0.05, and 0.1 kg ai ha-1) applied at early bloom and two MC rates {0 and 0.05 kg ha-1} applied at physiological cutout. Mepiquat chloride applied at early bloom reduced plant height in most environments and had variable effects on other growth characteristics. In some environments, MC applied at early bloom increased terminal regrowth and reduced basal regrowth in one year but had no effect on basal regrowth-to-height ratio. At most locations, cutout applications of MC had little or no practical effect on plant height, nearly all growth characteristics, crop maturity, defoliation, regrowth potential, yield, or fiber quality. Results from this experiment suggest that plant modifications resulting from MC occur when applications are made earlier in the season and that MC applied at cutout offers little or no advantage in plant management, lint yield, or fiber quality.
