A substantial loss in yield usually occurs when oilseed rape is sown later than mid-September. Field experiments in 1972–3 tested whether this is because all plants, irrespective of size, initiate inflorescences during the winter, thus restricting the eventual size of the leaf canopy and the yield potential of late-sown plants, which are still very small when the change to the reproductive phase occurs.
Sowing a standard winter variety, Victor, from mid-August to mid-October resulted in initiation occurring from late October to February. When sown after mid-September yields were proportional to plant size at initiation. The winter was mild, and in colder conditions earlier sowing may be required to enable plants to reach a critical size before initiation takes place. When sown in August plants were very large but yield was not improved. A study of the development of the pod canopy showed that the extra yield potential was not realized because heavy losses of pods and seeds occurred, particularly in the more shaded positions.
When a range of winter varieties, chosen to contrast the timing of phasic development, was sown on two dates yield was proportional to the interval between sowing and initiation, particularly when late sown, presumably because late initiators were able to lay the foundation of a more substantial root and shoot system. This has implications for plant breeding programmes because the low erucic acid varieties currently being grown initiate inflorescences early. When spring types were sown in mid-October they formed floral initials very early, and gave particularly small and weak plants, with a low yield. Winter types sown in March did initiate flowers and produce large plants, but yields were no better than spring types as flowering and ripening were irregular and late, the cold requirement for initiation being only marginally fulfilled.