RELATIONSHIPS BETWEEN THE HERBICIDAL ACTIVITY AND FOLIAR UPTAKE OF SURFACTANT-CONTAINING SOLUTIONS OF GLYPHOSATE APPLIED TO FOLIAGE OF OATSAND FIELD BEANS

The enhancement effects of a C-13/C-14 series of aliphatic alcohol (A) surfactants on the herbicidal activity of glyphosate-(monoisopropylam monium) were examined using a model formulation system that enabled re lative performance to be assessed at equivalent spray deposition rates . Surfactants with mean molar ethylene oxide (E) contents of 11, 15 an d 20 were more effective in increasing glyphosate activity against oat s (Avena sativa L.) than AE6, when the herbicide was sprayed at 54 or 108 g acid equivalent (a.e.) ha-1. At the lower herbicide dose, these surfactants, when added at 1 and 5 g l-1, caused approximately 70% mor e reduction in foliage weight than the herbicide alone, compared with a approximately 50% greater reduction at 0.2 g l-1. However, there was no significant concentration effect on enhancement with the same thre e surfactants at the higher herbicide dose. On field bean (Vicia faba L.), surfactant E content had little influence on herbicide enhancemen t compared with surfactant concentration. This effect was noticeable o nly with glyphosate at 54 g a.e. ha-1, with the highest surfactant con centration (5 g l-1) inducing a approximately 35% greater reduction in foliage weight than herbicide without surfactant. At the higher glyph osate rate there were only modest improvements in efficacy on this spe cies after surfactant addition. Results obtained from experiments usin g 0.2 mul droplet applications of formulations containing [C-14]glypho sate at herbicide (0.35 and 0.7 g l-1) and surfactant concentrations e quivalent to those of the spray solutions, showed that the correspondi ng enhancements in uptake and translocation of the radiolabel correlat ed well with the observed improvements of herbicidal activity on oats but not on field bean. On both species, AE6 was antagonistic to uptake of radiolabelled herbicide. Increased herbicide absorption or biologi cal activity did not appear to be related to the spreading properties of the surfactant-containing formulations on the two target surfaces. The implications of these findings arc discussed with reference to the problems of predicting and optimizing the performance of agrochemical formulations with surfactant adjuvants.