Johnsongrass Control in Soybeans with Dalapon and Preemergence Herbicides

Treatments were applied to heavily infested johnsongrass [ Sorghum halepense (L.) Pers.] in the field for 3 years. Half of each treated plot was cultivated 5 weeks after planting soybeans [ Glycine max (L.) Merr.]. Dalapon (2,2-dichloropropionic acid) applied to johnsongrass foliage 7 to 10 days before plowing plus either of the following three preplant incorporated herbicides, trifluralin (α,α,α-trifluoro-2,6-dinitro- N,N -dipropyl- p -toluidine), nitralin [4-(methylsulfonyl)-2,6-dinitro- N,N -dipropylaniline], or vernolate ( S -propyl dipropylthiocarbamate), gave 85, 68, and 82% johnsongrass control, respectively, at the end of a 3 year study. The preemergence incorporated herbicides applied following plowing and two diskings resulted in poor (35 to 50%) johnsongrass control with significant yield reductions in 2 of the 3 years. When the preemergence incorporated herbicides were applied following two diskings and eight rototillings, johnsongrass control was 62% after 3 years which was significantly less than the 79% control resulting from dalapon plus preemergence herbicides. Cultivation 5 weeks after planting soybeans increased johnsongrass control 22, 35, and 36% and soybean yields 439, 370, and 320 kg/ha in 1969, 1970, and 1971, respectively, over comparable plots without cultivation.