Evaluation of Different Phonological Stages of Corn Affected by Different Densities and Interference Periods of Sorghum Using Heat Indices

In order to compare heat indices at various phonological stages of corn affected by different levels of densities and interference periods of sorghum, a factorial experiment was conducted on the basis of randomized complete block design with three replications in the growing season of 2002 at the Research Station of the Faculty of Agriculture, University of Tabriz, Iran. Studied factors consisted of different interference periods of sorghum (0, 14, 28, 42 days after corn emergence), and sorghum densities of 4, 8, and 12 plant/m2. Corn density was 7.8 plant/m2. Results showed that the best thermal indicator was the classical index to evaluate the effects of density levels and various interference periods of sorghum on corn phonological stages in Tabriz area. This index had the highest correlation with the grain yield at all corn phonological stages such as tasseling, silking and physiological maturity. Also, when winter crops were planted after corn harvesting or ripening or seed filling duration, it can be recommend to use general thermal heat index. If the aim of artifical pollination is to produce hybrid genotypes. Three indices of growth degree day, crop heat unit and heat unit can be used due to their similar results. Finally, if the aim is to determine the silking, it would be better to use crop heat unit or heat unit.