We conducted a 9-ha field experiment in central Iowa to determine whether low-external-input (LEI) cropping systems can produce yields and profits that match or exceed those obtained from conventional systems. We compared a conventionally managed 2-yr rotation (corn-soybean) with two more diverse LEI systems: a 3-yr corn-soybean-triticale + red clover rotation, and a 4-yr corn-soybean-triticale + alfalfa-alfalfa rotation, both of which received manure before corn production. Over the period 2003-2005, synthetic N fertilizer use was 50% and 73% lower in the 3-yr and 4-yr rotations, respectively, compared with the 2-yr rotation; similarly, herbicide use was 71% and 78% lower in the 3-yr and 4-yr rotations, respectively, compared with the 2-yr rotation. Despite reductions in chemical inputs, corn and soybean yields were as high (2003 and 2004) or higher (2005) in the LEI systems as in the conventional system. Weed biomass in corn and soybean was low in all years (< 42 kg ha^-1) and did not differ among management systems. Weed seed density in the surface 20 cm of soil was higher in the LEI systems than the conventional system, but declined in all rotation systems between 2003 and 2005. Net returns to land and management over 2003-2005 were highest for the 4-yr rotation ($435 ha^-1 yr^-1), lowest for the 3-yr rotation ($356 ha^-1 yr^-1), and intermediate for the 2-yr rotation ($390 ha^-1 yr^-1). Higher profitability of the 4-yr rotation as compared with the 2-yr rotation derived from a 28% reduction in production costs. Biological nitrogen fixation, nutrient cycling, and weed seed predation by rodents and insects were three ecological processes that promoted cost reductions and maintenance of yields of the LEI systems. Such processes are likely to become increasingly important as petrochemical energy costs rise.