ANTAGONISM OF THE FOLIAR UPTAKE OF GLYPHOSATE INTO GRASSES BY ORGANOSILICONE SURFACTANTS .2. EFFECTS OF SURFACTANT STRUCTURE AND GLYCEROL ADDITION

A systematic investigation into the effects of a range of organosilico ne surfactants on the foliar uptake of radiolabelled glyphosate into w heat revealed significant antagonism associated with the chemical stru cture of these adjuvants. Rates of herbicide uptake were influenced by the ratio of the ethylene oxide (EO) hydrophile to siloxane hydrophob e, the mean EO chain length of the surfactant and the chemical nature of the terminal group on the EO chain. In general, antagonism was redu ced, or even eliminated, by increasing the polarity of the organosilic one surfactant molecule. Typically, these surfactants markedly enhance d the initial rate of glyphosate uptake, which then slowed abruptly to reach a maximum within 4 h, in contrast to the approximately linear u ptake response of the herbicide alone over 24 h following foliar appli cation. The addition of glycerol further increased the initial rapid r ate of glyphosate uptake in the presence of an organosilicone, but had no effect on the subsequent slowing down of foliar uptake. These find ings are discussed in relation to the possible mechanisms by which the se surfactants affect the cuticular penetration of glyphosate.