Experiments comparing conventional and organic sys-tems often report similar yields despite substantiallyhigher weed abundance in the organic systems. Apotential explanation for this observation is that weed–crop competition relationships differ between the twotypes of systems. We analysed weed and crop yield datafrom the Rodale Institute Farming Systems Trial (FST),which provides a unique 27-year dataset of a conven-tional (CNV) and two organic [manure (MNR) andlegume (LEG)] soyabean (Glycine max (L.) Merr.) andmaize (Zea mays L.) cropping systems. Average soya-bean yields were similar between the MNR and CNVsystems and only slightly reduced in the LEG system,whereas average maize yields did not differ among