The ester of 2,4-D [(2,4-dichlorophenoxy) acetic acid] at 1.1 kg active ingredient (a.i.)/ha failed to control tansy ragwort (Senecio jacobaea L.) in the late-rosette, bud, and flowering stages of development. Sprays of 2,4-D ester at 2.2 kg/ha were more effective than when applied at 1.1 kg/ha, but repeated treatments were necessary for optimum tansy ragwort control. Comparative dosage levels of the esters of 2,4-D and of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] showed 2,4-D giving better control of tansy ragwort when applied at equivalent rates. Control of relatively immature tansy ragwort plants with 2,4-D was not significantly different at the 1.1, 2.2, and 4.5 kg/ha levels, but injury to grasses and white clover (Trifolium repens L.) increased at the higher rates. The ester of 2,4-DB at 2.2 and 4.5 kg/ha effectively controlled seedling and early-rosette tansy ragwort plants only. The quantity of water applied as a spray was critical since the frond-type leaves which fold over one another prevented thorough wetting of the entire foliage with spray volumes under 275 liters/ha. Data obtained over a 4-yr period showed that CMPP (±) 2-[(4-chloro-1-tolyl) oxy) propionic acid], CP 1815 (chlorinated benzoic and cresoxy-acetic acids), 2,3,6-TBA (dimethylamine salt of 2,3,6-trichlorobenzoic acid), 2,4-D acid [weedone 638, emulsifiable concentrate-(2,4-dichlorophenoxy) acetic acid], and fenoprop [2-(2,4,5-trichlorophenoxy) propionic acid] were as effective as the ester of 2,4-D for tansy ragwort control but caused more injury to components of the sward.
