Evaluation of Sowing Date Effect on Grain Yield and Phenology of Corn Hybrids Using Thermal Indices

In order to evaluate the effect of sowing date and cultivar on accumulated heat unit, phenology and yield of maize (Zea mays L.), a two-year field study was carried out at the Agricultural Research Station of College of Agriculture, Shiraz University, Shiraz, Iran. Treatments consisted of five maize hybrids (SC704, Hido, BC677, BC666 and NS540) and five planting dates (May 15, June 1, June15, July 1, July 15) in a randomized complete block design with three replications. The results of the present study showed that the highest and lowest seed yield was obtained from Hido cultivar at June 1st planting date and NS504 cultivar at July 15th, respectively. Additionally, among the measured indices, crop heat unit (CHU), heat use efficiency (HUE) and relative thermal disparity (RTD) indices were able to estimate heat requirement of maize cultivars accurately in different sowing dates and different phenological stages. These indices provided stable trends and reasonable estimates of the thermal requirements of maize hybrids cultivated on early and delayed sowing dates and hence, they had relative superiority over the other indicators such as growing degree days (GDD), helio-thermal unit (HTU) and the pheno-thermal index (PTI). RTD and CHU had the lowest coefficients of variance which it showed that these indices have enough accuracy to estimate heat requirement of maize hybrids in different planting dates.