Cotton Desiccation And Defoliation By Paraquat Influenced By Time Of Day

The voluntary removal of arsenic acid (H3AsO4) from the market as a cotton (Gossypium hirsutum L.) desiccant has led to increased use of paraquat [1,1'-dimethyl-4,4'-bipyridinium ion] for this purpose. Producers have reported differences in the activity of paraquat when applied under seemingly similar conditions. The objective of this study was to compare the efficacy of cotton desiccation and defoliation with paraquat when applied at three different times of the day. Experiments were conducted in 1996 and 1997 at the Texas Agricultural Experiment Station near College Station, TX. Treatments consisted of 280 g a.i. ha(-1) paraquat applied to cotton variety D&PL 50 in the morning (0600-0800h), noon (1200-1400h), and evening (1800-2000h). Evening applications of paraquat enhanced desiccation at 3 d after treatment (DAT) compared with morning and noon applications in both years. Delaying time of application did not consistently enhance defoliation at 3 or 14 DAT. Performance ratings (% desiccation + % defoliation) at 3 and 14 DAT were significantly greater for evening applications, compared with morning applications in both years. For desiccation at 3 DAT and performance at 3 and 14 DAT, there was a linear relationship with the accumulation of solar radiation in the 8-h period following application. Increased desiccation and performance ratings were associated with lower light intensities following application. These results suggest that applications later in the day will result in more optimal activity of paraquat as a cotton desiccant.