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

Abstract

Calcium ion in the spray water can reduce glyphosate efficacy. Ammonium sulfate (AMS) is commonly added to the spray tank to overcome the reduced efficacy. However, it is sometimes claimed that ethoxylated tallow amine surfactant (EA) is also efficacious, provided that calcium concentration is moderate (= 5 mM, 200 ppm). On response curves of ‘Plaisant’ barley treated with glyphosate, the presence of calcium ion increased the glyphosate dose needed to obtain 50% (ED50) barley growth reduction. The addition of AMS to the spray tank overcame the antagonistic effect of the calcium ion and restored glyphosate efficacy. EA was less effective than AMS at 5 or 10 mM calcium ion concentration as measured by ED50. However, at 90% growth reduction (ED90), EA was more effective than AMS at the 5 mM calcium ion concentration but less effective at the 10 mM concentration. Hence, at a moderate (= 5 mM) calcium concentration, EA would be an effective adjuvant. Calcium ion decreased the foliar uptake of glyphosate but di...