CONTROL OF CANADA THISTLE (CIRSIUM-ARVENSE) WITH GLYPHOSATE APPLIED AT THE BUD VS ROSETTE STAGE

The interaction of cultivation and photoperiod on the initiation of Ca nada thistle rosettes, and the effect of growth stage on control of Ca nada thistle with glyphosate were determined in a series of field expe riments, Under the natural photoperiod occurring in the southern Canad ian Prairies, rosettes of Canada thistle were initiated by cultivation to remove the shoot growth during the last week of July, Regrowth rem ained as rosettes without any stem elongation and formed dense cluster s of leaves, Application at the ''August rosette stage'' improved the effectiveness of the glyphosate treatment and resulted in consistent c ontrol of Canada thistle with less than half as much herbicide as reco mmended for control when applied at the bud-stage, The number of shoot s of Canada thistle on plots treated with glyphosate was less than on the intensively summer-fallowed check plot, Check plots received 5 cul tivations during the summer-fallow season, One year after application of glyphosate at the rosette stage, the reduction in shoot density was 99% compared to the summer-fallowed check, By year three, without gly phosate applications in years two and three, the benefits of applying glyphosate at the rosette stage rather than the bud-stage were very ev ident, When applied at the bud-stage the shoot density on plots treate d with glyphosate at 2.25 kg ha(-1) was 24 m(-2) compared to only 10 s hoots m(-2) when applied at the rosette stage, (35% vs 72% control), G lyphosate at 0.9 kg ha(-1) or less than half of the rate recommended f or application at the bud-stage, applied to Canada thistle in the rose tte stage, resulted in consistently fewer shoots 2 and 3 yr after trea tment, Physical removal of shoot top growth, simulating an in-crop her bicide treatment, improved the consistency of control. Two years after application of 0.9 kg ha(-1) of glyphosate at the rosette stage, the control of Canada thistle was 98% compared to the summer-fallowed chec ks.