Some factors influencing glyphosate control of Panicum repens (torpedograss) in up-country tea fields in Sri Lanka

Abstract

Glasshouse and field studies were conducted to investigate the effects of formulation and application methods on the activity of glyphosate [N-(phosphonomethyl) glycine], against the perennial, rhizomatous grass-weed Panicum repens L. (torpedograss). Glasshouse studies were carried out with an application of 0.125 kg a.i/ha, a sub-lethal rate of glyphosate on 11-12 week old, well-established P. repens plants. Phytotoxicity evaluations were based on visual symptoms and an evaluation of inhibition of regrowth potential of the weed. Among the ammonium salts added to spray solutions, (NH4)2SO4 and urea, each at 0.5 or 2.0 kg/ha, enhanced the visual phytotoxicity caused by glyphosate, while NH4SCN did not. However, synergism between ammonium salts and glyphosate inhibition of regrowth was not clearly seen. Glyphosate was less phytotoxic when applied in the presence of 100 mM solution of CaCl2 or ZnSO4, while the addition of MgSO4 or NaHCO3 did not alter activity. The addition of non-ionic surfactants Agral-90 or Surfactant-N, at 0.1 % or 1.0 % (v/v), significantly increased the efficacy of glyphosate. However, petroleum oils (car engine oil or petrol) or vegetable oils (coconut or soybean oil) in the spray solutions at 1.0 % (v/v), did not significantly alter glyphosate activity. Glyphosate applications exhibited the highest phytotoxicity against P. repens at pH 6.0. The field performance of glyphosate at 1.0 or 2.0 kg/ha was significantly enhanced when de-ionized water was used instead of ground water, as a carrier. Reduction of the carrier-volume from 600 l/ha to 300 l/ha did not significantly alter activity. The performance of a high rate of glyphosate (4.0 kg/ha) was not affected by water quality or carrier-volume.