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 simil ar conditions. The objective of this study was to compare the efficacy of c otton desiccation and defoliation with paraquat when applied at three diffe rent times of the day. Experiments were conducted in 1996 and 1997 at the T exas Agricultural Experiment Station near College Station, TX. Treatments c onsisted 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). Eve ning applications of paraquat enhanced desiccation at 3 d after treatment ( DAT) compared with morning and noon applications in both years. Delaying ti me of application did not consistently enhance defoliation at 3 or 14 DAT. Performance ratings (% desiccation + % defoliation) at 3 and 14 DAT were si gnificantly greater for evening applications, compared with morning applica tions in both years. For desiccation at 3 DAT and performance at 3 and 14 D AT, there was a linear relationship with the accumulation of solar radiatio n in the 8-h period following application. Increased desiccation and perfor mance ratings were associated with lower light intensities following applic ation. These results suggest that applications later in the day will result in more optimal activity of paraquat as a cotton desiccant.