Application Timing for Spot Treating Johnsongrass

H.R. Hurst and J.R. Williford


'DES 119' cotton was planted on a silt loam soil in April, 1988-89 in a RCB design with split plots. Main plots were prior treatment with 0.75 lb/A trifluralin (Treflan®) during 1985-87 or not. The entire area was treated in March, 1988-89, with 0.75 lb/A Treflan to land previously subsoiled and incorporated to a 3-inch depth with a tandem disk harrow followed by hipping, both within 1 hour after application. Sub-plots were in-season applications of clethodim (Selecte® with Agri-Dex® at 0.5% v/v) made on two dates for johnsongrass control and 4 methods of application. The dates were June and/or June and July of 1988-89. Methods of in-season applications and rates of Select were: (1) no herbicide, (2) boom application (i nozzle over each row) at 0.125 lb/A in 10 gal/A broadcast volume, (3) spot-spray treatment at 0.24% v/v with a hand-held boom, and (4) an electrically-operated tractor-mounted spot-spray device applying 0.125 lb/A in 10 gal/A broadcast volume when activated. Other production practices were those recommended for optimum non-irrigated cotton production including preemergence and directed postemergence herbicides and row cultivation.

Seedling johnsongrass control was determined by counting an area 6 inches to each side of the row by 40 or 9 feet long in May, 1988 or 1989, respectively. Rhizome johnsongrass control was evaluated in a similar manner, except counts were made before and after treatment. Visual estimates of control were made in July, 1988 and in July and August, 1989. Cotton response was evaluated by determining the stand and yield.

Prior use of trifluralin (1985-87) resulted in 53 and 57% fewer seedling johnsongrass plants in 1988 and 1989, respectively. The average number of seedling johnsongrass plants per unit area was 6.9 to 7.5 times less in 1988, probably due to the very dry soil condition in spring, 1988. Even with fewer seedling johnsongrass plants, visual ratings of control was not affected by prior use of Treflan. This may be a reflection of the rhizome johnsongrass population already present in the old Treflan plots.

In-season treatments during l988 resulting in high levels (85-97%) of visual rhizome johnsongrass control in early July were the hand-applied and the tractor-mounted spot-spray treatments. These same treatments gave high levels (86-98%) of control in early July, 1989. In early August, 1989, excellent (91-97%) visual johnsongrass control was obtained with the tractor-mounted spot-spray treatments applied 1 or 2 times and with the hand-applied spot treatments applied 2 times.

Reprinted from 1990 Proceedings: Beltwide Cotton Production Research Conferences pp. 355 - 356
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998