Absorption of 2,4-D [(2,4-dichlorophenoxy)acetic acid], dicamba [3,6-dichloro-o-anisic acid], and the isopropylamine salt of glyphosate [N-(phosphonomethyl)glycine] by excised honeyvine milkweed [Cynanchum laeve (Michx.) Pers.] leaves was determined. Experimental variables included leaf position (terminal vs. basal), a surfactant, 4-isopropenyl-1-methylcyclohexane plus unspecified emulsifiers (SA-77), and leaf collection dates. Absorption of the three herbicides by terminal and basal leaves was increased by the addition of the surfactant. However, the surfactant increased absorption into basal leaves more than into terminal leaves. The surfactant reduced surface tension and increased drying time of water droplets on adaxial leaf surfaces by 50%. The pH of the herbicide solutions was reduced from about 5.8 to about 3.9 by SA-77. Absorption of all three herbicides was greater into terminal than into basal leaves when the surfactant was added. Generally, no differences were observed in the absorption of 2,4-D and dicamba. Glyphosate absorption was greater in terminal leaves collected after a period of adequate moisture than after a period of dry soil conditions.
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