Tolerance of grazed and ungrazed Phalaris aquatica to glyphosate, tetrapion and 2,2-DPA

Abstract

In 3 experiments carried out near Orange, N.S.W., we assessed the tolerance of grazed and ungrazed Phalaris aquatica to glyphosate, tetrapion and 2,2-DPA each applied at 3 rates. P. aquatica, when maturing, or when dormant during summer, or at any time of the year after heavy grazing had removed almost all green leaves, was tolerant to glyphosate (0.54, 0.72, 1.08 kg/ha a.i.). At any time of the year, ungrazed P. aquatica tolerated low rates of tetrapion (1.5 and 2.25 kg/ha a.i.) or 2,2-DPA (4.4 and 11.1 kg/ha a.i.). It was most tolerant to the high rate of tetrapion (3.0 kg/ha a.i.) in winter and the high rate of 2,2-DPA (22.2 kg/ha a.i.) in spring, summer or autumn. Grazing before or after spraying did not improve the tolerance of P. aquatica to 2,2-DPA but grazing after spraying improved its tolerance to tetrapion. Maximum selective effect, on taking into account the optimum time for killing weeds and retaining pasture (P. aquatica and Trifolium subterraneum), could be achieved by applying: tetrapion to Nassella trichotoma, Eragrostis curvula or Sporobolus africanus in late spring or summer; 2,2-DPA to N. trichotoma in summer; and glyphosate to Poa labillardieri and heavily grazed pasture in summer, provided the weed was actively growing.