We have but little OSR here. I am aware that swarming at the rape or soon afterwards is common and can understand why that most likely occurs. What puzzles me is why supercedure is also common at this time?
If the hives are located well into the OSR field, the yield will be higher than if the hives are some distance outside the field. I wonder if this is because bees go elsewhere in preference to the OSR, I know that being in a rape field can be a distinctly unpleasant experience; pollen and dust everywhere and an unpleasant smell.