Corn GDD Tool to Identify "Safe" Hybrid Maturities for Late Planting. Dr. Bob Nielsen at Purdue University has written an article describing a powerful decision aid, U2U Corn GDD Tool, which can be used to identify "safe" hybrid maturities for late planting. The GDD Tool is currently available for Ohio and it can estimate county-level GDD accumulations and corn development dates based on current and historical GDD data plus user-selected start dates, relative hybrid maturity ratings, GDDs to black layer, and freeze temperature threshold values.