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Glyphosate as a Postemergence Treatment for Johnsongrass Control in Cotton and Soybeans¹

Johnsongrass (Sorghum halepense (L.) Pers.) is a troublesome weed on cropland in the southern U. S. It is difficult to control due to its ability to reproduce from both rhizomes and seed. Glyphosate (N-(phosphonomethyl)glycine) has demonstrated phytotoxicity to Johnsongrass, so experiments were conducted to evaluate its use in cotton and soybeans for Johnsongrass control. Glyphosate was applied postemergence to ‘Lankert LX 571’ cotton (Gossypium hirsutum L.) and (‘Lee 68’ or ‘Bragg’ soybeans (Glycine max L.) in the field. Applications were made either over-the-top of the crop plants (topical) or directed to the base of the stems. Glyphosate controlled Johnsongrass best when treatments of at least 3.4 kg/ha were applied to plants 60 cm or greater in height. Discing the Johnsongrass 4 or 8 days after treatment had no effect upon control. Chlorflurenol (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate), when used as a tank mix with glyphosate, did not influence the degree of Johnsongrass control. Glyphosate at 0.8 kg/ha used postemergence topically on cotton and soybeans caused significant visual injury and crop yield reduction. When directed to the basal 2.5 cm of the crop stem, glyphosate at 3.4 kg/ha provided season-long control of Johnsongrass with the least amount of crop injury or yield reduction. The crops were least affected by the directed treatments at the taller crop heights. Thus, selective control of this perennial weed in cultivated crops was demonstrated with directed applications applied to the basal 2.5 cm of the crop stem.

Key Words: Herbicides • Weed control

² Research assistant and regents professor, Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74074.

Received for publication May 29, 1976.