Effects of Adjuvants and Environment on the Toxicity of Dalapon to Johnsongrass

Abstract

Johnsongrass [Sorghum halepense (L.) Pers.] regrowth was used as an index for penetration and translocation of dalapon (2,2-dichloropropionic acid) with and without adjuvants under different environmental conditions. Johnsongrass control with dalapon at 1.12 kg/ha without surfactant decreased as air temperature increased from 16 to 27 to 38 C. Plants at 16 and 27 C with 100% relative humidity (RH) were more susceptible to dalapon without surfactant than were plants at 35% RH. The presence of 1% surfactant in dalapon solutions increased johnsongrass control above that of dalapon alone at 27 and 38 C regardless of RH. Adding 1% surfactant and maintaining plants at 100% RH and 38 C resulted in significantly less johnsongrass control than that at 35% RH and 38 C. The addition of 0.1 M KH2PO4 to mixtures of dalapon + surfactant restored effectiveness at 100% RH and 38 C. Dalapon applied without surfactant was more injurious to johnsongrass grown with 40% soil moisture than with 25% soil moisture regardless of air temperature or RH. When plants were maintained at 27 or 38 C with 25% soil moisture, 0.1 M KH2PO4 + surfactant was more beneficial than surfactant alone in increasing dalapon toxicity to johnsongrass. At 40% soil moisture, johnsongrass control with dalapon + 1 % surfactant was not improved with the addition of KH2PO4 regardless of air temperature.