Effect of sowing date and agro-climatic indices on grain yield of new bread wheat (Triticum aestivum L.) cultivars in Gorgan region, Iran

To determine the optimal sowing date for new bread wheat cultivars as well as to investigate the relationship between grain yield and agro-climatic indices, a field experiment was carried out as split plots layout in randomized complete block design with four replications in two cropping seasons (2020-2021 and 2021-2022) in Gorgan agricultural research station, Gorgan, Iran. Seven sowing dates (from 01 November to 31 December, with 10-day intervals) were assigned to main plots and four spring bread wheat genotypes (cv. Araz, cv. Arman, cv. Taktaz and N-93-9 promising line) were randomized in subplots. Combined analysis of variance showed that the effect of sowing date was significant on grain yield, biological yield and spike no. m-2. While there was no significant difference between cultivars for the aforementioned traits. The highest grain yield obtained from 11 November (6235 kg.ha-1) and 21 November (6105 kg.ha-1) sowing dates, respectively. In delayed sowing dates (11 December, 21 December and 31 December), grain yield of cv. Taktaz was greater than other genotypes. The three sowing dates, 01 November, 11 November and 21 November, were not significantly different for grian filling duration (42.5, 42 and 40.3 days, respectively). However, grian-filling duration significantly decreased in delayed sowing dates (33, 34.1 and 35.6 days, respectively). The highest cumulative growth degree-days (GDD) for grain filling period was calculated for sowing dates 11 November  and 21 November  (782 and 773 GDD, respectively). In conclusion, the results of this experimebt showed that the highest grian yield and maximum energy use efficiency may obtain from the sowing date window of 11 November to 21 November for Gorgan region in Iran.