Cotton's Response to Combinations of Mepiquat Chloride, Pyrithiobac, and CGA 362622

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Title: Cotton's Response to Combinations of Mepiquat Chloride, Pyrithiobac, and CGA 362622
Author: Casteel, Shaun Nathan
Advisors: Keith L. Edmisten, Committee Co-Chair
Jim Burton, Committee Member
Jan Spears, Committee Member
John W. Wilcut, Committee Member
Randy Wells, Committee Co-Chair
Abstract: 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 ai ha-1, CGA 362622 at 4.04 g ai ha-1, and a tank mixture of pyrithiobac at 37.5 g ha-1 plus CGA 362622 at 4.04 g ha-1 for a total of twelve treatments. Mepiquat chloride reduced plant height, nodes, and height to node ratio at 7, 14 and 21 days after treatment (DAT); canopy closure at 14 and 21 DAT; and increased single leaf photosynthesis at 5 and 12 DAT. Mepiquat chloride increased boll retention and enhanced maturity, whereas lint yield and fiber quality results were not consistent. CGA 362622 reduced plant height and delayed canopy closure shortly after application. Herbicide effect on node development and discoloration was transient, where CGA 362622 exhibited the greatest discoloration (< 7%). Herbicides applied alone caused greater discoloration than herbicides applied with MC. Herbicides did not influence lint yield in 2002, but tank mixes including CGA 362622 were the lowest in 2003. The tank mixture of MC and pyrithiobac seem to be a feasible option given the proper situations, while injury caused by CGA 362622 deters such an application. CGA 362622 tank mixed with any other chemical is currently prohibited by label. This data suggest that the application of MC in a tank mixture with pyrithiobac or applied alone was safe when there was ample fertility and moisture.
Date: 2004-07-04
Degree: MS
Discipline: Crop Science

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