Systems of johnsongrass control in soybeans

The objective of this study was to determine an effective combination of herbicides for controlling johnsongrass while growing soybeans. Experiments were conducted at four locations in Tennessee in 1972. Observations were made on the control of seedling and rhizomatous johnsongrass, soybean injury, and the broadleaf weed control. Soybean seed yield from each plot was measured. A split-block experimental design was used. Main blocks consisted of two preplant foliar treatments for the control of rhizome johnsongrass, viz., dalapon (2,2-dichloropropionic acid) and glyphosate tN-Cphosphonomethyl)glycine]. Subblocks consisted of four herbicides to control seedling johnsongrass, i.e., alachlor [2-chloro-2',6'-diethyl- N-(methoxymethyl)acetanilide], nitralin [4-Cniethylsulfonyl)-2,6-dinitro- N,N-dipropylanilide], trifluralin Ca,a,a-trifluoro-2,6-dinitro-N,Ndipropyl- p-toluidine), and vernolate (S-propyL dipropylthiocarbamate), plus a weed-free check and a weedy check included in each main treatment of each replication. Johnsongrass rhizome control was good to excellent at all locations with both dalapon and glyphosate treatments. Significant differences between these two herbicides existed only at the Middle Tennessee Experiment Station. No significant differences among herbicides used for seedling johnsongrass control were found at any location. All treatments gave good to excellent control of most broadleaf species present. All herbicide treatments caused some soybean injury with vernolate causing the most severe injury. No significant differences existed between soybean seed yields at Ames Plantation. At Mascot and Pulaski, yields from glyphosate treated plots were significantly higher than were those from dalapon treated plots. Dalapon treated plots yielded significantly more soybeans at the Middle Tennessee Experiment Station, probably due to the fact that the glyphosate treated plots were turned after only four days.