Fig. 4. Relationship between plant density and simulated yield by original (broken line) and revised (solid line) CERES Maize model. The top two curves are for simulated yield for the Iowa site in 1999, and the bottom two curves are for the hybrid Pride 5 from Ontario site in 1989. The symbols represent observed yield. The closed symbols are for the Iowa maize yield contest (the year of contest in parenthesis), and open symbols are for Pride 5 in Ontario, Canada (Tollenaar, 1992).