Evaluation of the "fall rosette" procedure for perennial sowthistle control in fallow

Field experiments conducted near Grande Prairie, Alberta, compared the effe ctiveness of the fall rosette procedure to other procedures used in a year of fallow for the control of perennial sowthistle (Sonchus arvensis L.). Th e fall rosette procedure, which consisted of discontinuing normal cultivati on in late July, allowing the perennial sowthistle plants to grow for 6 wk and then applying glyphosate plus dicamba at 0.6 + 0.6 kg ha(-1) plus Agral 90 at 0.5% vol/vol of spray volume, reduced shoot density by 84, 90 and 96 % approximately 1 yr after imposition in 1992, 1993 and 1994, respectively. By comparison, the bud stage procedure, which consisted of allowing plants to grow to the bud stage in the spring/early summer, applying the same her bicide treatment and conducting tillage for the rest of the year, reduced s hoot densities by 58, 86 and 85% when imposed in the same years, respective ly. The fall rosette procedure was also more effective than frequent tillag e throughout the growing season without herbicide, or than chemfallow (glyp hosate plus dicamba applied in early July and again in early September, wit h or without late fall tillage) in reducing perennial sowthistle shoot dens ity. Mowing, whenever the perennial sowthistle reached 8-10 cm in height, d id not reduce shoot density.