ABSTRACT.- The aim of this study was to determine if different mixtures and monocultures of cover crops (CC) reduce aerial dry matter, number and diversity of autumn-winter weeds. Two experiments with different CC species were implanted in the INTA Pergamino Agricultural Experimental Station (Pergamino, Buenos Aires, Argentina, 33° 51'S, 60° 34'W) in 2018 and 2019. The species used were:Avena sativa, Triticosecale and Vicia villosa. Mixtures and monocultures of CC were used, in different proportions and densities, and were compared with a chemical fallow. Two cuts of CC were made aboveground at tillering and milk grain stages (Z 2.5 and 7.3, respectively) to obtain aerial dry matter of CC (ADMcc, g.m-2), ADM of weeds (ADMw, g.m-2) and number of weeds (weeds.m-2). Moreover, weed species were identified and Shannon diversity and evenness indices and effective specific richness were calculated. In 2018, the highest ADMcc was recorded in Avena sativa at Z 2.5 as well as the highest weed density. At Z 7.3, the highest ADMcc was recorded in Avena sativa, Triticosecale and their mixture. At that stage, the highest ADMw was recorded in the chemical fallow. In 2019, the 3:1 Avena sativa/Vicia villosa mixture showed the highest ADMcc and diversity of weed community showed non-significant differences from the chemical fallow at Z 2.5. There were non-significant differences in ADM among CC and diversity of weed community in the CC was practically null at Z 7.3. At both stages, the highest number of weeds and ADMw was observed at chemical fallow, whereas in CC, non-significant differences were detected. The results obtained indicate that CC reduce the number, ADMw and diversity of the weed community, regardless of the mixtures used. © 2022 Informa UK Limited, trading as Taylor & Francis Group