Interactive effects of diluent pH and calcium content on glyphosate activity on Panicum repens L. (torpedograss)

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Abstract
Summary. Solutions of glyphosate containing 0–5 mM calcium, both with buffering (pH 4, 6 or 8) and without, were sprayed onto Panicum repens L. (torpedograss) foliage. Growth inhibition of the treated foliage was poorly correlated with inhibition of shoot regrowth from rhizomes. Therefore, when evaluating long term control of perennial grasses regrowth data should be utilized. Spray solution pH, calcium and glyphosate rate influenced herbicidal activity on torpedograss. Calcium affected glyphosate activity on torpedograss except at the highest herbicide rate (4·48 kg ha−1). Optimum herbicidal activity on torpedograss was achieved with soft water (i.e. no calcium), however, when calcium was present in the spray solution a pH of 6 reduced its antagonistic effects.