Plasticity of seed weight in winter and spring rapeseed is higher in a narrow but different window after flowering

•Source reduction after the beginning of flowering increased seed weight in different windows for winter (15–30 DAF) and spring rapeseeds (0–15 DAF).•Seed weight increased 33 % for winter and 39 % for spring rapeseeds in response to the source-sink reduction.•Seed oil changed very little in response to source reductions after flowering.•Seed protein concentration showed a negative association with oil concentration.•Seed protein concentration was associated positively with seed weight and negatively with seed number.