Perennial sowthistle control with sequential herbicide treatments applied under minimum and zero tillage systems

Perennial sowthistle (Sonchus arvensis L.) control under minimum and zero tillage systems, and following several sequential in-crop herbicide treatments applied in a crop rotation of canola (Brassica rapa L.) followed by 3 yr of barley (Hordeum vulgare L.), was evaluated in two field experiments near Grande Prairie, Alberta. Under zero tillage, in-crop applications of clopyralid at 0.1 to 0.3 kg active ingredient (a.i.) ha−1 to canola, followed by annual in-crop applications of clopyralid at 0.15 kg a.i. ha−1 plus MCPA at 0.42 kg a.i ha−1 to barley in the next 2 yr, reduced perennial sowthistle from a mean initial density of 3.9 shoots m−2 to ≤0.5 shoots m−2 in the fourth year. Glyphosate at 0.45 kg acid equivalent (a.e.) ha−1 plus dicamba at 0.14 kg a.i. ha−1 was applied annually before seeding barley in these plots. In the second experiment, the same herbicide treatments, except for an additional preseeding application of glyphosate at 0.9 kg a.e. ha−1 in the first year, reduced perennial sowthistle density from 9.2 shoots m−2 to ≤0.1 shoots m−2 Increasing the rate of clopyralid in the first year resulted in decreases in perennial sowthistle density in the second and third years following application but not in the fourth year. In unweeded zero tillage check plots of the two experiments, perennial sowthistle increased to 41.8 and 36.0 shoots m–2, respectively, over the same period. Applying metsulfuron at 0.0045 kg a.i. ha–1 in the second year and dicamba plus the potassium salt of MCPA (1:4) at 0.54 kg a.i. ha−1 in the third year, in place of clopyralid plus MCPA, produced similar reductions in perennial sowthistle density. Under minimum tillage, where tillage was limited to a single preseeding discing before seeding and a single mid-to-late October deep tillage cultivation, reductions in perennial sowthistle density following in-crop applications of clopyralid in the first year and clopyralid plus MCPA in the next 2 yr were similar to those following application of the same in-crop herbicides under zero tillage. However, in minimum tillage plots where metsulfuron and dicamba plus MCPA were applied in the second and third years, perennial sowthistle density was not reduced or declined at a slower rate than in other sprayed plots. Canola and barley yields increased as a result of reductions in perennial sowthistle density. Key words: Perennial sowthistle, Sonchus arvensis L., tillage, clopyralid, metsulfuron, dicamba