Surfactants foliage-applied herbicides

Activator adjuvants are often added to spray mixtures of foliage-applied herbicides to enhance the level of weed control. Such adjuvants increase herbicidal activity by increasing cuticle retention, penetration, absorption, and possibly translocation. Activator adjuvants include surfactants, wetting agents, penetrants, and oils. Oils used as additives have been found to influence wetting properties, evaporation, leaching, and foliar uptake and translocation of herbicides while surfactants increase their solubility, spreading, and penetration. " ... 

Low phytotoxicity of herbicide and surfactants will be of greatest importance in regard to the efficient target site delivery of foliage-applied systemic herbicides such as asulam or glyphosate. These polar herbicides are useful in the control of perennial rhizomatous weeds such as bracken [Pteridium aquilinum (L.) Khun.] in which the target site exists in the buds associated with the underground rhizome system. ... 

Six imidazolinones are commercially available. These herbicides have extremely low toxicity or are non-toxic to mammals, birds, invertebrates and fish [36]. The crops on which these herbicides are registered and whether or not they are applied to foliage or to the soil is determined by the structure of the chemical (Table 2.3.2). When applied to the foliage of plants, a non-ionic surfactant or oil adjuvant is required for maximum activity. The addition of either urea or another form of nitrogen can also increase herbicidal activity. 

Temple and Hilton [253] determined the solubilities of ametryne, diuron, and atrazine in a wide range of 0.5 % surfactant solutions. Weight for weight the solubility is greatest in the cationic Nalquat and Aliquat series. The Carbowaxes and dimethylformamide have no effect on the solubility of these herbicides (Table 10.28). These authors point out that when no surfactant is present, saturated solutions applied to the foliage will precipitate as the spray evaporates. When a solubilized preparation is used, two effects should be considered firstly the increased amount of surfactant in the initial spray, and secondly, the solubility of the herbicide in the remaining surfactant after the water has evaporated. It has been found that the solubility (4 to 15%) of ametryne, diuron, and atrazine in some pure surfactants is sufficient to keep all of the herbicide present in a saturated spray solution in solution on the leaf surface but it is possible that solubilized preparations are more readily lost from leaf surfaces in rainwater. 

The physicochemical properties of compounds affect not only the transport of compounds in plants but also their penetration through the leaf cuticle. This topic is discussed in Chapter 8, and here only brief comment is made on the implications of penetration for subsequent transport. The influence of physicochemical properties on the movement of compounds across cuticles seems to be rather similar to that observed for membranes, in that compounds of intermediate lipophilicity (log Ko values of 1 to 3) appear to penetrate the most rapidly. Uptake of more polar and more lipophilic compounds can, however, be greatly increased by the use of appropriate surfactants. Acidic compounds are taken up relatively slowly, presumably because of poor penetration of the anions in consequence, they are often applied as esters which enter the plant rapidly and are then metabolized to the active acid. Thus, herbicides of a wide range of structures and physical properties are applied to plant foliage the use of surfactants and adjuvants increases the uptake of chemical into the leaf and hence its availability for transport via xylem and phloem but does not otherwise influence transport patterns. 

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