The first two articles of this dissertation describe field studies carried out to compare agronomic productivity and weed seed decay in a conventional 2-year rotation and two low-external-input crop rotations. The third article comprises a matrix model study of giant foxtail population dynamics in a 2-year corn-soybean and in a 4-year corn-soybean-small grain+alfalfa-alfalfa rotation. These studies found similar or higher corn and soybean yield in low-external-input (LEI) cropping systems that included small grain and legumes than in a simpler 2-year corn-soybean rotation. Economic return was highest in a three year corn-soybean-oat+red clover LEI rotation. In the LEI systems, giant foxtail seed decay was higher than in the 2-year rotation during one year but not in the other two years, which was attributed to differences in the seed lots evaluated, environmental conditions, and seed pathogen dynamics in the soil. Finally, giant foxtail seed bank density was projected to decrease in the 2-year rotation and increase in the 4-year rotation over time, and it was suggested that summer seed decay values above 20% would affect negatively the population growth rate.