Estimated weed-free corn yield increased linearly as a function of estimated nitrogen availability in years with above-average rainfall (2001, 2004, and 2005) but either did not increase (1998 and 2002) or increased at a lower rate in 1997 when rainfall was below-average. The equation for pooled data across 2001, 2004, and 2005 is Y = 4411 + 36.1X where Y is estimated weed-free yield and X is estimated nitrogen availability (R² = 0.586). The equation for 1997 data is Y = 1638 + 17.7X (R² = 0.368).