Continuous-cropping conservation tillage systems may provide a viable
alternative to the practice of summer fallow; however, concerns have b
een raised regarding potentially negative changes in weed communities
tn continuous cropping. Field experiments were established in Saskatch
ewan at three locations to determine the nature of weed community diff
erences between a crop sequence with and without fallow in zero-, mini
mum-, and conventional-tillage systems from 1986 to 1990. Weed communi
ties in continuous-cropping treatments tended to have greater total de
nsities and were more similar in composition than crop-fallow treatmen
ts. Inclusion or exclusion of fallow within the rotation had a greater
impact on weed community composition than did tillage system at Ituna
and Waldron, but the reverse was true at Tadmore due to poor crop gro
wth in all tillage systems. Differences in weed community composition
were generally characterized by fluctuational changes in species assoc
iations. Volunteers of summer-annual crops, such as canola, flax, and
barley, were associated with continuous cropping, but other species in
cluding perennial weeds, such as Canada thistle, perennial sowthistle,
and quackgrass, were not strongly associated with the presence or abs
ence of fallow The practice of fallowing land to manage weeds may not
be necessary.