Crop and livestock production are rarely integrated together in modern farming systems. Reintegrating crops with livestock production has been shown to produce many agronomic and environmental benefits. The objective of this study was to evaluate how an integrated crop–livestock system would influence weed biomass and weed species composition compared with a conventional, continuous corn (Zea mays L.) cropping system. The experimental farming system used in this study was established on a 90-ha site near Pana, IL, in 2002. The integrated system included two phases: (i) a corn and oat (Avena sativa L.) cash crop rotation, grown in summer, and (ii) post-harvest grazing of corn stover with annual cover crops. Over a 4-yr period (2004–2007), weed biomass was approximately 4.5 times higher in the conventional system (8.4 g m–2) compared with the integrated system (1.8 g m–2). Weed species composition was affected by the integrated system and showed a temporal disjunction between the time of year and weed life history. Surprisingly, cattle grazing on cropland had little effect on weed biomass or species composition. The primary drivers that suppress weed biomass and change species composition appear to be use of crop rotation and annual cover crops within the integrated system. Wider adoption of integrated crop–livestock systems, such as the one used in this study, should reduce reliance on herbicides compared with more conventional cropping systems.