Response of Grain Yield of New Bread Wheat (Triticum aestivum L.) Cultivars to Sowing Date Based on Agro-Climatic Indices Under Gorgan Environmental Conditions

To study the response of grain yield of commercial bread wheat cultivars to different sowing dates based on agro-climatic indices, a field experiment was carried-out in Gorgan agricultural research station, Gorgan, Iran, in 2017-18 and 2018-19 cropping cycles. . The experimental design was split plot arrangements in randomized complete block design with four replications. Five sowing dates; 01 November, 16 November, 01 December, 16 December and 31 December were assigned to main plots, and four spring wheat cultivars; Ehsan, Tirgan, Meraj and Kalateh were randomized in subplots. The results showed that grain yield and number of spike per square meter were not significantly different in two cropping cycles. The highest grain yield (6157.9 kg ha-1) was obtained in the third sowing date, however, the grain filling period was not affected (42.37 and 42.21 days, respectively) despite one-month difference, with the first sowing date. It implies that the third sowing date (16 December) provided favorable conditions for crop growth and no lodging was observed in different cultivars. In the third sowing date, optimal conditions; temperature, day length, relative humidity and energy use efficiency during grain filling period were established for different wheat cultivars, therefore, the highest values were obtained for agro-climatic indices; growing degree days (GDD), day length (DL), photo-thermal units (PTU), hydro-thermal units (HYTU) and energy use efficiency. Kalateh and Tirgan cultivars had the highest grain yield, respectively, and their grain yields were significantly greater than Ehsan and Meraj cultivars. Also, the number of spike per square meter was significantly greater in these two cultivars. The growth period in cv. Kalateh was shorter than the other cultivars, no difference in duration of grain filling period, which caused the plant to escape terminal heat and drought stresses, increasing energy use efficiency and consequently increasing grain yield. According to the results of this study, suitable sowing date and adapted cultivar played key roles in managing grain filling period and increasing grain yield.