Volunteer glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] can be a problem in GR cotton (Gossypium hirsutum L.) grown in rotation with soybean, especially when hurricanes destroy the preceding soybean crop and leave unharvested beans in the field. No-till cotton planting and lack of fluometuron applied preemergence in GR cotton have intensified the problem. A field experiment was conducted to determine GR soybean response to pyrithiobac, trifloxysulfuron, MSMA, and combinations of pyrithiobac or trifloxysulfuron plus MSMA applied postemergence. Trifloxysulfuron at 2.6 and 5.2 g a.i. ha-1 controlled GR soybean 98 and 100%, respectively, compared with 60 and 77% control by pyrithiobac at 36 and 72 g a.i. ha-1, respectively. MSMA at 925 g a.i. ha-1 controlled soybean only 30%. Soybean control by pyrithiobac or trifloxysulfuron was reduced when either herbicide was mixed with MSMA. A second experiment evaluated control of soybean with traits for both glyphosate resistance and sulfonylurea herbicide tolerance (GR/ST). The GR/ST soybean was controlled 1, 13, and 36% by pyrithiobac at 36, 72, and 108 g ha-1, respectively, compared with 79, 98, and 100% control by trifloxysulfuron at 2.6, 5.3, and 7.9 g ha-1, respectively. These results demonstrate that volunteer GR or GR/ST soybean can be controlled in cotton by trifloxysulfuron applied postemergence at normal use rates.
