Repeated, Foliar Applications of Herbicides on Johnsongrass

Abstract

The responses of established, spaced-planted strains of johnsongrass (Sorghum halepense (L.) Pers.) to repeated, foliar applications of herbicides were studied under irrigated desert conditions. Monosodium methanearsonate (MSMA) and disodium methanearsonate (DSMA) were the most effective herbicides. Six to 10 applications with 3 to 6 lb aihg of either herbicide (totaling 24 to 60 1b/A) killed johnsongrass in most experiments. Treatments initiated in the fall were more effective than similar treatments started in the spring. Higher rates of organic arsenicals were required to destroy topgrowth of johnsongrass in the spring and fall than during the summer. DSMA applications at 4 and 6-week intervals were more effective than applications at 8-week intervals. Six 1b/A of DSMA in 100 gpa of water applied broadcast was as effective as 400 gpa containing 6 lb aihg of DSMA applied as a spot treatment. Removing topgrowth of johnsongrass 4 or 24 hr after treatment did not reduce the effectiveness of DSMA. Combining or alternating organic arsenicals with 2,2-dichloropropionic acid (dalapon) reduced the effectiveness of the arsenicals. Susceptible strains were destroyed by five or six applications of DSMA and MSMA while 9 or 10 applications were needed to kill more resistant strains.