Knowledge of the long‐term effects of tillage on weed flora will provide useful information to improve weed management in agroecosystems. Field studies were conducted from 1991 to 1997 to evaluate the effects of tillage systems on weed density and species composition before control methods in rotations including wheat (Triticum aestivum L.), soyabean [Glycine max (L.) Merr.] and maize (Zea mays L.). In wheat, annual broad‐leaved species showed higher populations in conventional tillage in 4 out of 6 years, and grassy annuals and perennial species showed an erratic response with tillage systems. In summer crops, broad‐leaved populations were higher under conventional tillage than non‐tillage for the last 5 years in the wheat/soyabean rotation and for the last 4 years in the maize/soyabean rotation. The weed spectrum changed rapidly in non‐tillage plots. With time, in the absence of tillage, grassy annual populations increased in the maize/soyabean rotation, and wind‐dispersed weed populations increased in the wheat/soyabean rotation. Perennial weeds showed an inconsistent behaviour in relation to tillage systems in the maize/soyabean rotation.

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