Effect of sowing date on yield and yield components of three oilseed rape varieties

In order to elucidate the effect of sowing date on oilseed rape (Brassica napus L.) yield and yield components, three varieties including Tower, Rafal and Global were sown from November 7 to December 22, 1998-99 in four dates with 15-day intervals in Dezful region. A split-plot design based on randomized complete blocks with four replications was used in which levels of sowing dates and cultivars formed the main and sub-plots, respectively. Variables including plant height, axillary branches/plant, pods/plant, seeds/pod, single seed weight, biomass, seed oil content and seed yield were measured. Results showed that the sowing date had a highly significant effect on morphological characteristics, yield components, oilseed rape yield and seed oil content. Delay in sowing date caused a reduction in all yield components, especially pods/plant and oilseed yield by 285.23 g/m2 to 135.57 g/m2 on November 7 and December 22, respectively. Variations in sowing dates had different effects on yield components and reduction rates of pods/plant, seeds/pod and single seed weight. The significant effect of variety on all characters, except single seed weight indicated that there were genetic differences among the studied cultivars. Oilseed yield had the significant positive correlations with pods/plant (r = 0.93), single seed weight (r = 0.83) and seeds/pod (r = 0.66). The results of path analysis showed that pods/plant and seeds/pod had the highest positive and negative effects on oilseed yield, respectively. Finally, considering the susceptibility of pods/plant to variations of sowing date and the important role of this character in oilseed yield production, the delay in sowing date in Dezful region reduced the oilseed yield by decreasing pods/plant.