Abstract
A total of twenty local and foreign maize (Zea mays L.) hybrids in five different maturity group were studied in Mashhad for two years (2007-2008). Temperature variations were created by two planting dates (5 May and 20 May). Results indicated that both indices, GDD and CHU, estimated heat units of different maturity hybrids and based on, created similar groups of hybrids. Both indices showed some changing in grouping of some hybrids. In general, the GDD to maturity discriminated different FAO maturity groups. Late hybrids accumulated more GDD during grain filling period. Required CHU was higher for the period to flowering than do from flowering to maturity. By increasing the maturity group, the required CHU for reproduction period increased as well. In general, based on the similar results in using both indices, regarding the simple calculation of GDD, we can recommend the use of GDD in grouping maize hybrids based on heat units in cold-temperate regions.
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