Individual droplets of 14C-2,4-D dimethylamine (DMA) were applied to oriental mustard seedlings to determine the effect of droplet size and herbicide concentration on absorption and translocation 1 or 3 d after application. Absorption of 14C-2,4-D was generally not affected by droplet size which ranged from 198 to 2760 μm. However, the percentage of absorbed 14C-2,4-D that was translocated away from the treated leaf increased as droplet size decreased. Absorption of 14C-2,4-D either increased slightly or was not affected by increased herbicide concentration, but translocation of absorbed 14C-2,4-D was reduced as herbicide concentration increased. Absorption and translocation of 14C-2,4-D DMA were reduced when applied after a drop of 21.3 mM commercially formulated 2,4-D had been applied at the same location on the leaf. These results indicate that large amounts of 2,4-D, either in the form of large droplets or a more concentrated herbicide solution, inhibit translocation of 2,4-D from oriental mustard leaves.

When herbicide concentration was constant, absorption of 14C-glyphosate increased with increasing droplet size (326 to 977 μm). Amount of 14C-glyphosate translocated away from the treated area, expressed as percent of absorbed, increased as droplet size decreased. Herbicide concentration of the droplet was more important than droplet number or droplet size in determining glyphosate absorption and translocation. Absorption and translocation increased with increasing herbicide concentration regardless of whether droplet size or number was altered in conjunction with herbicide concentration. This relationship explained why low spray volume (increased herbicide concentration) increased herbicide efficacy. The concentration gradient between droplet and leaf, rather than droplet coverage, was the primary mechanism responsible for the observed effect. Large droplets caused localized tissue injury, which may have caused decreased translocation.