Technical Abstract: Little is known about the impact of crop rotation and post-harvest residue management on the agricultural sustainability of conventional grass seed cropping systems in the Willamette Valley of western Oregon, U.S.A. The effects of a six-year rotation (continuous grass versus diverse crop species rotation), and post-harvest residue management (straw remaining or removed) on soil quality were investigated at three locations, each contrasting in soil drainage classification. The crop rotations were continuous grass seed production, grass/legume, and grass/legume/cereal. The species of grass grown at each site were different and represented those most commonly produced in each environment. They included perennial ryegrass, tall fescue, and creeping red fescue. Residue treatments included low residue (grass straw removed by raking and bailing) versus high residue (flail chopped onto the field). All grass seed crop rotations maintained high soil quality under conventional tillage and each residue treatment, but under some situations, soil quality was enhanced with continuous grass rotation and high residue management, particularly under moderately drained soil conditions where tall fescue seed was produced. These studies suggest that straw removal for value-added use like bioenergy production or livestock feed can be accomplished without appreciably affecting soil quality. Furthermore, grass seed cropping systems play an important role in soil carbon (C) storage and enhancement, a valuable ecosystem service in this region where grass seed is produced in on land that is not suitable for production of crops that require better-drained soils. Western Oregon’s marine climate and poorly drained soils offer a unique environment to study crop production and soil quality due to the mild temperatures and high amounts of annual precipitation often associated with semi-flooded field conditions. No such climatic and soil conditions like this exist elsewhere in the world.