Introduction: Climatic parameters, especially temperature, have a significant effect on increasing the yield of plants. Rapeseed has suitable agricultural characteristics such as tolerance to cold, drought and salinity stresses and it is recommended to grow it in most parts of the country due to its higher yield of oil per unit area than other oil seeds. Rapeseed is one of the plants whose performance depends on the right planting date so one of the important issues is determining the appropriate planting date for the important cultivars cultivated in each region. This experiment was conducted in order to investigation and comparison of the yield of rapeseed cultivars in different planting dates. This area has frost stress during winter and heat stress at the end of the season. These stresses affect the length of the phenological stages of development and yield components. The present study also examines the effect of planting date on these variables and by providing regression models, provides a suitable estimate of their effect on seed yield.
Materials and Methods: The experiment was carried out in split plot design in the form of randomized complete block in three replications during two crop years 2019-2020 and 2020-2021 at the central station affiliated to the Agriculture and Natural Resources Research Center of Yazd province. The main factor of planting date was placed in the main plots in three levels: the 7^th of October, the 22^th of October and the 6^th of November and rapeseed cultivars were planted in sub-plots in eight levels including Hayola4815, Zarfam, Nafis, Nepton, Nima, SLM046, Nilofar and RGS003. To determine the yield components including the number of lateral branches, the number of pods per plant, the number of seeds per pod and 1000 seed weight, 10 plants were randomly selected from each experimental unit and the desired traits were counted and measured. The phenological stages of development were performed based on the scale of Zavareh and Imam (2000). SAS software was used to analyze the variance and determine the regression coefficients of the equations. Mean comparisons were also done based on LSD test and 5% probability level.
Results and Discussion: Based on the obtained results, the effect of planting date had a significant effect on the phenological stages and caused a decrease in the number of days until flowering, podding and ripening. The earliest maturity cultivars in this experiment were Hyola4815 and RGS-003. The effect of planting date on grain yield was significant and the highest yield was obtained with 5066.46 kg / ha on 7^th October and the sowing dates of the 22^th of October and the 6^th of November were 11.34 and 26.66% lower than the sowing date of 7^th October, respectively. Among the investigated cultivars, Heyola on the planting date of 7^th Oct., Neptune on the planting date on 22th Oct., and Neptune & Nafis on the planting date of 6^th Nov., had higher yields than the others and recommended for cultivation in the region.
Conclusion: The dependence of seed yield in rapeseed on the appropriate planting date is confirmed by many researchers and it varies on different weather conditions. In hot and dry areas such as Yazd, late planting causes stress at the end of the season, and early planting also causes frost during the winter season, so it is very important to choose the appropriate planting date and early maturity varieties in such areas. According to the regression models, the change in the phenological growth stages coordinated by change in the yield components cause increase the seed yield by 500 to 750 kg/hectare, which can be done by changing the planting date and agricultural operations.