Mixed cropping of barley and durum wheat has been the practice of smallholder farmers in some drylands of Ethiopia even though the reasons for this successful cropping system were not well understood. Therefore, four planting densities, five intercrop proportions and three irrigation water levels were studied in a split-split plot arrangement in RCBD with three replications to determine the competition levels and resource use of barley and wheat mixed cropping under different growth stages. Barley was more competitive at early stages, while wheat dominated towards the reproductive stage. Intra- and inter-specific competition was decreased with increasing irrigation water levels but it was increased with increasing planting densities. Intraspecific competition was increased with increasing barley ratio in the cropping systems at all irrigation water levels, planting densities and harvesting stages. Fast growing nature of barley at early growth stages helps the intercropping system in that barley can capture belowground and areal resources faster, while wheat grows slowly and demands less resource at earlier growth stages. At later stages when wheat becomes dominant and resource demanding, early maturity of barley leaves more space for wheat to satisfy its resource demand. Thus niche differentiation index was consistently more than one in all growth stages and irrigation water levels. Therefore, mixed cropping of these two crop species helps combine important characters in a cropping system so as to enhance productivity through complementary resource uses in drylands.
