Irrigation improves the consistency of cotton (Gossypium hirsutum L.) yield during seasons with inadequate or poor rainfall distribution, but comparisons of irrigation methods, including overhead sprinkle (OSI) and sub-surface drip (SSD), are limited. Irrigation may affect the response of cotton to mepiquat chloride and the response of glyphosate resistant cotton to glyphosate. The objectives of this study were to compare the response of cotton with OSI and SSD irrigation and to determine if any responses to glyphosate and mepiquat chloride were different between irrigation systems. Field trials were conducted from 2001 through 2003 at the Peanut Belt Research Station in North Carolina to evaluate eight treatment combinations of glyphosate application method, mepiquat chloride application, and irrigation method. Glyphosate isopropylamine salt at 0.84 kg acid equivalent (a.e.) ha-1 was applied over-the-top at the four-leaf stage or non-precision post-directed at the eight-leaf stage. Mepiquat chloride was applied according to North Carolina Extension Service recommendations. Lint yield, which averaged 1400 and 1470 kg ha-1 under OSI and SSD, respectively, was not affected by mepiquat chloride application. Non-precision post-directed glyphosate reduced lint yield by 160 kg ha-1 in 2002, but did not affect yield in 2001 or 2003. Compared with untreated cotton, cotton treated with mepiquat chloride was 31 cm shorter with 2 fewer nodes in 2001 and 2003, and averaged 0.6 fewer first position bolls and 0.2 mm longer fiber. The two irrigation systems produced similar yields, and non-precision glyphosate applications reduced yield. In this study, irrigated cotton did not exhibit sufficient vegetative growth to benefit from the recommended applications of mepiquat chloride.
