Effect of Moisture Stress on Canada Thistle (Cirsium arvense) Control

Abstract

Laboratory experiments were conducted to determine the effect of moisture stress on the absorption and translocation of14C-labeled picloram (4-amino-3,5,6-trichloropicolinic acid), dicamba (3,6-dichloro-o-anisic acid), and glyphosate [N-(phosphonomethyl)glycine] within the Canada thistle [Cirsium arvense(L.) Scop. # CIRAR] plants. The absorption and translocation of picloram and dicamba were unaffected by moisture stress. Absorption and translocation of glyphosate to the roots and apical meristem of Canada thistle was reduced by increasing moisture stress. Weekly differential irrigation of Canada thistle field plots during the summers of 1980 and 1981 established three soil moisture regimes averaging −6.6, −11.3, and −15.0 bars at the time of herbicide treatment. When Canada thistle control was evaluated 1 year after application of glyphosate, dicamba, and picloram at 2.5, 1.1, and 0.6 kg/ha, respectively, no differences in Canada thistle shoot control were found between moisture stress treatments.