Efficacy of five different formulations of clopyralid on Cirsium arvense (L.) Scop, and Polygonum convolvulus L

Abstract

Summary:Experiments were conducted in a controlled environment growth room to compare the phytotoxicity of five formulations of clopyralid and to determine the effect of post‐treatment simulated rainfall applications on the efficacy of these formulations. Clopyralid, formulated as the acid, monoethanolamine salt, potassium salt, 2‐ethylhexyl ester or 1‐decyl ester was applied to Cirsium arvense or Polygonum convolvulus at the 8–10 and 4–6 leaf stage of development, respectively.Based on the shoot and root dry weight data from both species, the 2‐ethylhexyl ester was consistently less phytotoxic than the other four formulations. With respect to Cirsium arvense shoot and root dry weight, there was no difference in efficacy between the acid, monoethanolamine salt, potassium salt and 1‐decyl ester. Root dry weight changes of Polygonum convolvulus did not correlate well with an increase in dose of the five formulations. Maximum increase in stem diameter of Polygonum convolvulus occurred after application of 100 g a.e. ha−1 of the acid, monoethanolamine salt, potassium salt and 1‐decyl ester whereas with the 2‐ethylhexyl ester, the maximum increase occurred after application of 200 g a.e. ha−1. Simulated rainfall was applied 3, 6, 9 or 12 h after application of 200 g a.e. ha−1 of the five clopyralid formulations to both species. Rainfall reduced the efficacy of clopyralid, regardless of the formulation or species. The 2‐ethylhexyl ester was less efficacious than the other four formulations, irrespective of the timing of simulated rainfall.