Cover crops may provide numerous ecosystem services: they retain residual soil nitrate (NO3--N), reduce soil erosion, increase soil organic matter, sequester carbon (C), and improve general soil health. Cereal rye (CR) is by far the most dominant cover crop grown in field crop production systems in Iowa because it easily establishes, overwinters well, and thus can produce large amounts of spring biomass. However, there are negative tradeoffs associated with CR cover crops, including the potential to reduce corn yields (‘yield drag’). We are working with colleagues in the Dept. of Agronomy and USDA-ARS, to understand the factors play a role in yield drag. We have shown seedling disease and allelopathy likely play a role.