Trifloxysulfuron (EnvokeŽ, Syngenta Crop Protection, Greensboro N.C.) is an ALS-inhibitor herbicide applied postemergence to cotton with five leaves or larger to control broadleaf weeds and sedges. Trifloxysulfuron (TFS) typically causes shortened internodes on cotton, leading to questions on mepiquat chloride (MC) rate when the growth regulator is used in conjunction with TFS.

A field experiment was conducted at Rocky Mount, NC in 2005 and 2006 and at Clayton, NC in 2006 to investigate potential interactions between TFS and MC. Treatments consisted of a factorial arrangement of two TFS rates {none and 5.25 g a.i./ha at 7 days after pinhead square (DAPHS)} and the following five MC regimes: 1) no MC; 2) MC at 25 g a.i./ha 21 DAPHS; 3) MC at 49 g/ha 21 DAPHS; 4) MC at 25 g/ha 21 DAPHS and 38 g/ha 31 DAPHS; and 5) MC at 18, 25, and 38 g/ha at 14, 21, and 31 DAPHS, respectively. Two additional treatments included TFS at 5.25 g/ha 14 DAPHS with and without MC at 18, 25, and 38 g/ha 14, 21, and 31 DAPHS, respectively.

No TFS by MC interactions were noted. All MC regimes reduced plant height, but regime 2 was less effective than other regimes (18% vs. 24% in mid-September). Greater fruit retention (48 vs. 51 to 53%), fewer total nodes (one less), and a lower height-to-node ratio (5.3 vs. 4.2 to 4.5) was noted with all MC regimes. Lint yield was unaffected by MC at any location.

Trifloxysulfuron reduced plant height at all locations 13 and 6% 36 and 48 days after application, respectively, and 5% in mid-September. It reduced height-to-node ratio 10% but increased total nodes 3%. Sympodial boll retention was reduced 4% overall. Boll production was reduced 7% on nodes 8 to 10 but increased 24% on nodes 14 to 16, indicating a delay in crop maturity. Trifloxysulfuron decreased lint yield 7% averaged over years at Rocky Mount but had no effect on yield at Clayton.

A separate factorial analysis was conducted for the two additional treatments to determine the effect of TFS application timing. Factors included three TFS rates (none, TFS at 5.25 g/ha 7 DAPHS and TFS at 5.25 g/ha 14 DAPHS) and two MC rates (none, and MC at 18, 25, and 38 g/ha 14, 21, and 31 DAPHS, respectively). Trifloxysulfuron increased total nodes and reduced plant height and height-to-node ratio, but differences between application timings were not observed. Averaged over years and locations, TFS application timing had no effect on boll production, boll distribution, or lint yield.

The results indicate MC and TFS behave independently, as evidenced by the lack of MC by TFS interaction in statistical analyses. Both chemicals have similar effects on cotton growth and, some evidence indicates that MC rates should be reduced in the presence of TFS to avoid excessive suppression of plant growth and possible adverse effects on yield. The results also indicate that timing of TFS application (7 or 14 DAPHS) did not affect growth or yield of cotton..