Antagonism of the foliar uptake of glyphosate into grasses by organosilicone surfactants. Part 2: Effects of surfactant structure and glycerol addition

Abstract

A systematic investigation into the effects of a range of organosilicone surfactants on the foliar uptake of radiolabelled glyphosate into wheat revealed significant antagonism associated with the chemical structure of these adjuvants. Rates of herbicide uptake were influenced by the ratio of the ethylene oxide (EO) hydrophile to siloxane hydrophobe, the mean EO chain length of the surfactant and the chemical nature of the terminal group on the EO chain. In general, antagonism was reduced, or even eliminated, by increasing the polarity of the organosilicone surfactant molecule. Typically, these surfactants markedly enhanced the initial rate of glyphosate uptake, which then slowed abruptly to reach a maximum within 4 h, in contrast to the approximately linear uptake response of the herbicide alone over 24 h following foliar application. The addition of glycerol further increased the initial rapid rate of glyphosate uptake in the presence of an organosilicone, but had no effect on the subsequent slowing down of foliar uptake. These findings are discussed in relation to the possible mechanisms by which these surfactants affect the cuticular penetration of glyphosate.

Adjuvant Effects on Imazethapyr, 2,4-D and Picloram Absorption by Leafy Spurge (Euphorbia esula)

“…7 X-77 non-ionic surfactant, United Agri-Products, Greeley, CO 80632. 8 OSi Specialties, Inc., 777 Old Saw Mill River Road, Tarrytown, NY 10591-6728. 9 Dow Corning Corp., Midland, MI 48640.…”

“…The surfactants may have been absorbed into the cuticle (33), developing hydrophilic channels (39). The hydrophilic head group of the surfactants then behaved as a selective pore for the herbicide, with absorption dependent upon the physicochemical properties of the herbicide (8). Absorption of polar herbicides such as difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) (11) and glyphosate (N-(phosphonomethyl)glycine) (8) would be greatest with higher EO contents, while more hydrophobic compounds such as imazethapyr and 2,4-D would require shorter EO chains.…”

Hygroscopic salts support the stomatal penetration of glyphosate and influence its biological efficacy

“…The reduced foliar uptake of a commercial formulation (Roundup) due to a higher surfactant concentration was reported previously (Field et al . ; Gaskin & Stevens ). In many cases, (NH 4 ) 2 SO 4 is used to enhance Gly bioefficacy by stabilizing the calcium (Ca +2 ) of hard water to CaSO 4 , while the remaining ammonium ion combines with Gly to form the Gly–ammonium salt, thus enhancing the herbicidal bioefficacy (Thelen et al .…”

Influence of sugars on the foliar uptake of bentazone and glyphosate

“…It has been reported that glycerol, a well-known humectant, improved aminotriazole penetration into bean leaves (Cook et al 1977) and glyphosate uptake into wheat leaves (Gaskin & Stevens 1993). Sugars are generally not considered to have hygroscopic properties and their effect on pesticide uptake has not been tested previously.…”