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Published: 2004

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Agronomic and cultural practices have been developed to manipulate the indeterminate growth habit and perennial nature of cotton (Gossypium hirsutum L.) to facilitate management and production as an annual crop. The regulation of excessive vegetative growth and redirection of photoassimilates toward reproductive growth has been most successful with the plant growth regulator mepiquat chloride (MC). Mepiquat chloride decreases growth; thereby creating a more compact plant by controlling plant height and main-stem node development. Weed management in cotton has historically been implemented by the use of preemergence, post-directed, and layby herbicides. Postemergence over-the-top (POST) broadleaf herbicides have not been an option for weed management in cotton until the recent registrations of pyrithiobac and CGA 362622. Both compounds are members of herbicide families that inhibit acetolactate synthase in susceptible plants. These herbicides control a range of troublesome weeds in cotton, while cotton tolerance to POST treatments has been observed. Pyrithiobac and CGA 362622 are registered for use on nontransgenic and transgenic cotton including glyphosate-resistant cotton. Current glyphosate POST applications are prohibited after the 4-leaf stage in glyphosate-resistant cotton, whereas pyrithiobac and CGA 362622 POST are allowed after the 4-leaf stage in nontransgenic and transgenic cotton. The application of these herbicides in a tank mixture with MC could be beneficial for cotton producers. However, research has not been conducted to evaluate cotton response to tank mixtures of MC with pyrithiobac and CGA 362622. Field studies were conducted in 2002 and 2003 near Goldsboro and Rocky Mount, NC. Treatments were applied to 9- to 10-leaf cotton and arranged in a randomized complete block design with four replications. Treatments consisted of MC rates of 0, 11.8 and 23.6 g ai ha-1 in a factorial arrangement with herbicide treatments of untreated, pyrithiobac at 75 g a.