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Title: JOHNSONGRASS (SORGHUM HALEPENSE) CONTROL AND RAINFASTNESS WITH GLYPHOSATE AND ADJUVANTS

Author
item MILLER, DONNIE - LA AG EXP STATION
item GRIFFIN, JAMES - LA AG EXP STATION
item Richard Jr, Edward

Submitted to: Weed Technology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: In Louisiana, rhizome johnsongrass can reduce sugarcane yields by 31% or more. Complete control of rhizome johnsongrass can only be accomplished during the fallow period which occurs between sugarcane plantings every fourth year. To insure complete control of rhizome johnsongrass during the fallow period, growers rely on postemergence applications of glyphosate. Field studies were conducted to evaluate johnsongrass control with glyphosate as influenced by johnsongrass growth stage, adjuvant selection, and treatment to rainfall intervals. Simulated rainfall totaling 1.3 cm 15 or 60 min after glyphosate application reduced johnsongrass control regardless of growth stage or adjuvant used. Adjuvant selection appears to be most critical when glyphosate is applied to johnsongrass in the vegetative stage of development with a silicone-based adjuvant (Break-Thru) providing a higher level of control than a nonionic surfactant. Results emphasize the importance of adjuvant comparisons under field conditions, and the limited reliability of initial phytotoxicity for assessment of adjuvant utility for perennial weed control.

Technical Abstract: Regardless of adjuvant used, johnsongrass control 14 d after treatment at the reproductive stage with glyphosate at 2.1 kg ai/ha was at least 89% with no rainfall, but no more than 53 and 65% following 1.3 cm of simulated rainfall at 15 or 60 min, respectively. No consistent response among adjuvants in respect to improvement in rainfastness based on initial control was noted and johnsongrass regrowth did not occur. In contrast, when applied to johnsongrass in the vegetative stage, addition of the silicone-based adjuvant, Break-Thru, improved control over the nonionic surfactant, Induce, at both 15 and 60 min rainfall timings in one of two experiments. With no rainfall, addition of the nonionic surfactant, Kinetic HV, and Break-Thru increased johnsongrass control in only one experiment. For application at the vegetative stage, johnsongrass regrowth averaged across rainfall timings was no more than 10%. In other field experiments, glyphosate at 1.4 kg/ha plus nonionic surfactants, silicone surfactant, crop oil concentrate, methylated seed oil, or a blend of silicone surfactant and methylated seed oil were equally effective in reducing johnsongrass regrowth when applied after seedhead emergence. Antagonism associated with 2,4-D, glyphosate, and Induce combinations was inconsistent.