Weed populations, weed control methods, and soybean yields were studied over 4 years. Different weed control practices were tested in fields with high and low weed densities. Keeping plots weed-free for 4 years greatly reduced weed numbers. Mechanical control reduced soybean yield when weeds were not controlled by imazethapyr. Imazethapyr increased soybean yields by 22% in low-density fields and 51% in high-density fields. Weed control history affected the densities of specific weed species.