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

In order to elucidate the effect of sowing date on the yield and yield components of oilseed rape (Brassica napus L.), three varieties - Tower, Rafal and Global - were sown from 7 November to 22 December 2000-01 on four dates at an interval of 15 days in the Dezful region of Iran. A split-plot design based on randomized complete blocks with four replications was used in the experiment, where the sowing dates and cultivars formed the main and subplots, respectively. Variables including plant height, axillary branches/plant, pods/plant, seeds/pod, single seed weight, biomass, seed oil content and seed yield were measured. The soil of the experimental site had a loamy clay texture. The site had 250 mm annual precipitation and was located in the semi-arid zone. The results showed that the sowing date had a highly significant effect on morphological characteristics, yield components, oilseed rape yield and seed oil content. A delay in the sowing date caused a reduction in all the yield components especially in pods/plant, and in oilseed yield, which dropped from 285 g m-2 when sown on 7 November to 135 g m-2 when sowing was delayed to 22 December. Variations in sowing dates had different effects on the individual yield components, with pods/plant, seeds/pod and single seed weight decreasing to the greatest extent. The significant effect of variety on all characters with the exception of single seed weight indicated that there were genetic differences between the studied cultivars. Oilseed yield showed significantly 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 in sowing date and the importance of this character in the size of the oilseed yield, a delay in the planting date in the Dezful region was found to reduce the oilseed yield through a reduction in pods/plant.