Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Daniel B. Reynolds

Second Advisor

James L. Griffin


Field studies conducted in 1996 through 1998 evaluated grass control with the graminicides clethodim, fluazifop-P, quizalofop-P, and sethoxydim alone and in combination with bromoxynil. Sequential application of bromoxynil at 1.7 kg ai/ha 7, 3, and 1 day (d) before or 1, 3, and 7 d after each graminicide reduced antagonism of rhizome johnsongrass [Sorghum halepense (L.) Pers.] control when compared with bromoxynil plus clethodim, fluazifop-P, or quizalofop-P. Antagonism of johnsongrass control associated with bromoxynil and graminicide mixtures reduced cotton yield 17%. A 3 d or greater interval between bromoxynil and graminicide application was necessary to maximize cotton yield. Bromoxynil at 1.7 kg/ha applied in mixture with increased rates of graminicides antagonized johnsongrass and barnyardgrass [Echinochloa crus-galli (L.) Beauv.] control with all graminicides regardless of rate except clethodim for johnsongrass. Bromoxynil antagonized broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash] control with only quizalofop-P, but antagonism was overcome with a doubled rate of quizalofop-P. Increasing the bromoxynil rate greater than 0.56 kg/ha increased antagonism of johnsongrass control by fluazifop-P, quizalofop-P, and sethoxydim. and broadleaf signalgrass control by quizalofop-P and sethoxydim. For barnyardgrass control, efficacy of all graminicides was reduced by bromoxynil, regardless of bromoxynil rate. Sensitivity of 14 cotton (Gossypium hirsutum L.) cultivars to pyrithiobac under early season ambient (no water stress) and water saturated (wet) field environments was evaluated. Greatest cotton injury was observed for the wet environment and for pyrithiobac postemergence (POST) at 140 g ai/ha. Injury was greatest for 'Stoneville BXN 57' 7 d after treatment (DAT), but negligible for all cultivars 28 DAT. Cotton yield was reduced no more than 5% when pyrithiobac was applied at 70 g/ha preemergence or POST. No adverse effect on cotton fiber characteristics was observed. In laboratory experiments, no differences were observed among cotton cultivars for pyrithiobac absorption or translocation, but differences in metabolism were observed. Stoneville BXN 57 and 'Stoneville BXN 58' metabolized pyrithiobac slower than 'Coker 315', 'Delta Pine 50', 'Stoneville 474', and 'Stoneville BXN 47'. Differential metabolism of pyrithiobac. between cotton cultivars may partially explain differences in cultivar sensitivity.