Impact of Agronomic Practices on Weed Communities: Fallow Within Tillage Systems

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Abstract
Continuous-cropping conservation tillage systems may provide a viable alternative to the practice of summer fallow; however, concerns have been raised regarding potentially negative changes in weed communities in continuous cropping. Field experiments were established in Saskatchewan at three locations to determine the nature of weed community differences between a crop sequence with and without fallow in zero-, minimum-, and conventional-tillage systems from 1986 to 1990. Weed communities in continuous-cropping treatments tended to have greater total densities and were more similar in composition than crop-fallow treatments. 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 growth in all tillage systems. Differences in weed community composition were generally characterized by fluctuational changes in species associations. Volunteers of summer-annual crops, such as canola, flax, and barley, were associated with continuous cropping, but other species including perennial weeds, such as Canada thistle, perennial sowthistle, and quackgrass, were not strongly associated with the presence or absence of fallow. The practice of fallowing land to manage weeds may not be necessary.