Estimation of Heat Units Requirements for Different Maturity Groups of Grain Maize Hybrids in Kermanshah

Twenty Iranian and exotic maize hybrids in five different maturity groups were studied in Kermanshah in 2007-2008 growing seasons. Temperature variations were created by four planting dates (20 April, 30 April, 14 May and 01 June). Estimated heat units from planting to flowering, using both thermal indices, GDD and CHU, were reliable in different maturing maize hybrids and nearly created similar grouping for maize hybrids. Both indices, changed hybrids NS 540 and OSSK 552 from former FAO 500 maturity group to later maturing group, FAO 600. BC404 and OSSK499 hybrids were also changed from ealier maturity group FAO 400 to later maturity group FAO 500. However, BC 504 hybrid changed from FAO 500 to earlier maturity group, FAO 400. Although GDD and CHU were reliable for estimating planting to silking (flowering), CHU did not provide a reliable thermal units for estimating planting to maturity. Based on GDD for planting to maturity, all hybrids grouped in five maturity groups, which was similar to groups based on estimated GDD for planting to flowering.