Adjuvant influence on glyphosate efficacy in the presence of CA2+

Calcium ion in the spray water diminishes the efficacy of glyphosate. Ammonium sulphate (AS) is long shown to be the most efficacious remedy to this antagonism. However, it is sometimes claimed that acidified lecithins or terpenic alcohols can also be used to this aim, and even ethoxylated amines (EA) provided that calcium concentrations are not higher than 5 mM. We investigated these claims by examining dose-response curves of barley plants treated with glyphosate at various calcium concentrations in the presence of these adjuvants. Calcium ion increased the slope of the curves and ED50 values, indicating a decrease in efficacy. Acidified lecithins and terpenic alcohols did not modify the response of barley to glyphosate observed in the presence of calcium ion. AS restored the ED50 and slope values observed in the absence of calcium ion. It was concluded that the inhibition of glyphosate efficacy by calcium ion was not relieved by terpenic alcohols and acidified lecithins but was completely reverted by AS. EA decreased ED50 and ED90 values of the response curves established in the presence of calcium ion without influencing the slopes (which remained high). As a consequence, at calcium concentrations up to 5 mM the response curves obtained with AS and EA intersected. It followed that in the presence of EA, ED50 values were higher than AS, whereas ED90 values were lower. By contrast, at 10 mM calcium the response curves did not intersect and consequently, in the presence of EA, ED50 and ED90 values were both higher than AS. Since ED90 relates to efficacies sought in the field, our experiments explain why at low calcium concentrations EA may appear as a better remedy than AS. Glyphosate formulations containing ammonium sulphate or a higher concentration in surfactants than usual glyphosate formulations were also studied. In the presence of calcium ion they behaved in a way which was in agreement with the above observations.