PERENNIAL SOWTHISTLE CONTROL WITH SEQUENTIAL HERBICIDE TREATMENTS APPLIED UNDER MINIMUM AND ZERO-TILLAGE SYSTEMS

Perennial sowthistle (Sonchus arvensis L.) control under minimum and z ero tillage systems, and following several sequential in-crop herbicid e treatments applied in a crop rotation of canola (Brassica rapa L.) f ollowed by 3 yr of barley (Hordeum vulgare L.), was evaluated in two f ield 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 clopy ralid 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 de nsity of 3.9 shoots m(-2) to less than or equal 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 seedin g barley in these plots. In the second experiment, the same herbicide treatments, except for an additional preseeding application of glyphos ate at 0.9 kg a.e. ha(-1) in the first year, reduced perennial sowthis tle density from 9.2 shoots m(-2) to less than or equal 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 yea rs following application but not in the fourth year. In unweeded zero tillage check plots of the two experiments, perennial sowthistle incre ased 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, whe re tillage was limited to a single preseeding discing before seeding a nd a single mid-to-late October deep tillage cultivation, reductions i n perennial sowthistle density following in-crop applications of clopy ralid 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 metsulfur on and dicamba plus MCPA were applied in the second and third years, p erennial sowthistle density was not reduced or declined at a slower ra te than in other sprayed plots. Canola and barley yields increased as a result of reductions in perennial sowthistle density.