The ability ofMedicago sativato suppress weed growth may provide a viable alternative to chemical weed control and allow crop producers to reduce herbicide inputs. Quantitative information regarding the suppressive effect ofM. sativaon weed populations in current cropping systems is lacking. A survey was conducted in Manitoba, Canada, in 1993 and 1994 to investigate weed populations in commercial cereal fields that had been preceded by eitherM. sativahay or cereal grain crops. A total of 117 fields were surveyed; approximately half from each field type. Principle component analysis indicated that the inclusion ofM. sativain crop rotations resulted in weed communities different from those of continuous cereal fields. Naturally occurring populations ofAvena fatua, Cirsium arvense, Brassica kaber, andGalium aparinewere lower in cereal fields that had previously containedM. sativathan in cereal fields that had been preceded by a cereal crop. Lower field uniformity values forC. arvenseandAvena fatuaindicated that these weeds were also more patchy in theM. sativarotations. Population differences between field types were nonsignificant forAmaranthus retroflexus, Chenopodium album, andPolygonum convolvulus, and although populations ofTaraxacum officinaleandThlaspi arvensewere greater inM. sativa/cereal fields than in continuous cereal crops. No consistent effect of field type onSetaria viridispopulations was observed. These results show thatM. sativaeffectively suppressed some, but not all, of the weeds found in the study area. IncludingM. sativahay crops in crop rotations can be part of an integrated weed management strategy for weeds such asA. fatua, B. kaber, andC. arvense.

) were usually lower (P < 0.05) in the undisturbed (i.e., no-till) system, compared to where tillage was used. It was concluded that the combination of alfalfa in rotation and no-till management can provide significant weed control benefits to cropping systems. ) est habituellement plus faible (P < 0,05) quand le sol n'a pas été perturbé (non-travail). On en conclut que, combinés, l'assolement de luzerne et le non-travail du sol concourent de manière appréciable à la lutte contre les mauvaises herbes dans les systèmes culturaux.