The Wisconsin Integrated Cropping Systems Trial (WICST) was established in 1990 to compare the productivity, profitability and environmental impact of six cropping systems of increasing biodiversity.  In this poster we report on yield comparisons between organically and conventionally grown corn and soybeans. Specifically, corn grown in two three-phase rotations (c-s-w/rcl without manure; o/p/a-a-c with manure) was compared to corn produced in two conventional rotations (continuous corn; no-till c-sb).  The soybean comparison was between one organic (c-sb-w/rcl) and one conventional system (no-till c-sb).  The results from large plots (0.3 ha, 4 blocks) with thirteen years at one location and eight years at the other show that organic grain crops produce 85 to 93% as well as conventional systems.  More in depth analysis of the data showed that organic corn and soybean yields are only 60 to 80% of conventional yields when mechanical weed control is poor, but 90 to 98% when mechanical weed control is successful.  Years of poor mechanical weed control at both sites were related to wet spring weather that impeded timely mechanical weed control.  For example, at one site, in four years out of ten with corn and five years out of thirteen with soybeans poor weed control was a problem.  In the case of soybeans, the impact of wet spring weather increased with time, and we hypothesize it was due to the gradually increasing weed seed bank in the plots.