Summer-annual crop mixtures can be used to increase system productivity, reduce chemical inputs, and manage weeds. The objective of this study was to determine optimal tef and sorghum-sudangrass mixtures for weed suppression and biomass production. Tef and sorghum-sudangrass were planted in monoculture and in mixture with soybean and sunflower in 2008 and 2009. The percentage cover of crops and weeds were measured weekly. Final biomass was measured for component crops and weeds. Percentage cover was used to calculate area cover of crops and weeds. Biomass was used to calculate the land equivalent ratio (LER) and aggressivity indices of crops in mixture. Sorghum-sudangrass mixtures had greater biomass output than tef mixes. Total LER was greater than 1.0 for two- and three-species mixtures, indicating a biomass advantage for polyculture. Aggressivity of grass crops was linearly related to total LER in two- and three-species mixtures. Crop biomass and reduction in weed biomass were exponentially related. All crop mixtures decreased the biomass of weed populations by 12% to 99% in 2008 and 2009. Two- and three-species mixtures decreased weed cover by 24% to 41%.  Using summer annual crops that occupy different ecological niches in polyculture increases biomass production and reduces weed pressure. However, interspecies competition between component crops can reduce the benefits of polycultures.