The Influence of Inorganic Cations on Glyphosate Activity-Review and Perspectives

Abstract

The influence of calcium, iron, magnesium, other cations, and mixtures of cations on the activity of glyphosate and the interaction with pH is reviewed, discussed, and demonstrated with laboratory data. The strategy to overcome antagonistic effects of the cations via use of adjuvants is discussed and illustrated with data. The influence of calcium concentration on glyphosate efficacy was investigated on black nightshade (Solanum nigrum L.) by monitoring glyphosate activity via fluorescence technology and inhibition of growth. Giant foxtail (Setaria faberi Herrm) was also used for biological evaluation. Calcium concentrations equal to the glyphosate concentrations were most antagonistic; higher concentrations of calcium were less antagonistic. Different adjuvants were tested to overcome the antagonistic effect of calcium. Results are discussed in relation to the physical chemical properties of the adjuvants. With a more appropriate scientific base, remedial measures may protect field performance of glyphosate and reduce farm costs.

Interactions of Calcium Ions with Weakly Acidic Active Ingredients Slow Cuticular Penetration: A Case Study with Glyphosate

“…With Ca(OH) 2 at pH 4.0, one Ca 2+ ion neutralized two carboxyl groups; that is, 2 mol of glyphosate reacted with 1 mol of Ca 2+ . Hence, the molar ratio of calcium to glyphosate was 1:2, which is clearly in conflict with the conclusion drawn by Smith and Raymond (4), Sundaram and Sundaram (5), Gauvrit et al (16), and de Ruiter et al (17), who reported a 1:1 relationship. The molecular weight of the Ca-glyphosate species that penetrated the cuticles can be estimated as the size selectivity of poplar CMs is known (19).…”

“…Even though the solution was not saturated, this is much higher than the solubility product of 4.8 × 10 -6 measured by Smith and Raymond (4). Again, this is good evidence that the Ca salt of glyphosate formed by titrating glyphosate acid with Ca(OH) 2 up to pH 4.0 differed in structure from the complex formed at pH 7-8 when the phosphonate groups are also ionized and interact with Ca 2+ ions (4,5,16,17). These complexes clearly have a lower solubility as seen from the fact that precipitates formed when titration was continued beyond pH 4.0 (Figure 1).…”

Effect of the Time of Herbicide Application and the Properties of the Spray Solution on the Efficacy of Weed Control in Maize (Zea mays L.) Cultivation

“…In the experiment, the increased content of iron ions contributed to the decrease in the efficacy of the herbicides used. The decrease in the herbicidal efficacy of plant protection products due to the increased content of iron ions and other metal ions has been described for various herbicides [80,81]. The addition of citric acid made it possible to eliminate the negative influence of the additional substance on the efficacy of weed control.…”

Glyphosate Response to Calcium, Ethoxylated Amine Surfactant, and Ammonium Sulfate1

“…This complex or salt is poorly absorbed by plants (Nilsson 1985); in addition, suggested that glyphosate is entrapped in a solid residue on the leaf surface. The stoichiometry of the glyphosatecalcium association was shown to be 1:1 (de Ruiter et al 2002;Gauvrit et al 2000Gauvrit et al , 2001, as suggested by Nalewaja et al (1996). Under conditions in France calcium concentration is rarely higher than 5 mM (Blondlot et al 1999).…”