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1 October 2003 Glyphosate Response to Calcium, Ethoxylated Amine Surfactant, and Ammonium Sulfate
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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 did not affect the rate of uptake. AMS but not EA restored foliar uptake to values observed without calcium ion. EA increased spray retention, and this probably accounted for the increased glyphosate efficacy at low calcium concentration.

Nomenclature: Glyphosate; barley, Hordeum vulgare L.

Additional index words: Ammonium sulfate, bioassay, dose–response curve, ethoxylated tallow amine, foliar uptake, spray retention.

Abbreviations: AMS, ammonium sulfate; EA, ethoxylated tallow amine; SE, standard error.

CHRISTIAN GAUVRIT "Glyphosate Response to Calcium, Ethoxylated Amine Surfactant, and Ammonium Sulfate," Weed Technology 17(4), 799-804, (1 October 2003).[0799:GRTCEA]2.0.CO;2
Published: 1 October 2003

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