Objectives Producing vegetable and oilseed is an important approach for improving production efficiency of rapeseed. This study aimed to investigate the suitable nitrogen topdressing rate that can mostly promote the rapeseed yield and economic benefits in the vegetable-oilseed dual-purpose rapeseed system.

Methods Two consecutive field experiments were conducted during the 2023−2025 growing seasons at the Shayang Experimental Station of Huazhong Agricultural University. The experiment adopted a two-factor split-plot design, with topping amount (0, 3.00, 6.00 t/hm²) and nitrogen topdressing rate (0, 22.5, 45.0, 67.5, 90.0 kg/hm²) as the main factors. The rapeseed yield, yield components, aboveground N uptake, nitrogen fertilizer use efficiency and economic benefits were analyzed.

Results Both topping amount and nitrogen topdressing rate significantly affected seed yield, yield components, aboveground N uptake, nitrogen use efficiency, and economic benefits. Topping significantly reduced seed yield, the seed yields decreased by an average of 3.1% and 21.6% under topping amounts of 3.00 t/hm²and 6.00 t/hm², respectively, relative to that under non-topping treatment. Nitrogen topdressing partially offset this yield loss, and the seed yield initially increased and then decreased with increasing nitrogen topdressing rate. Compared with N0 treatment, the seed yield peaked at the N45.0 under topping amount of 0 and 3.00 t/hm², increasing seed yield by 17.1% and 17.3%, respectively; while at a topping amount of 6.00 t/hm², the seed yield peaked at the N67.5, with a yield increase of 21.5%. Compared with the non-topping treatment, topping decreased harvest density, siliques per plant and 1000-seed weight. Specifically, the treatments with topping yields of 3.00 t/hm² and 6.00 t/hm² reduced the number of siliques per plant by an average of 7.3% and 27.8%, while the seed number per silique increased by 12.2% and 14.8%, respectively. N topdressing did not significantly affect harvest density, but greatly increased siliques per plant by an average of over 20%. Topping reduced aboveground N uptake, whereas nitrogen topdressing promoted it. Compared with the non-topping treatment, topping 3.00 t/hm² and 6.00 t/hm² reduced aboveground N uptake by an average of 5.9% and 23.0%, respectively. Relative to N0, nitrogen topdressing increased total aboveground N uptake by an average of 21.6%, 24.5%, and 29.6% under topping intensities of 0, 3.00, and 6.00 t/hm², respectively. With increasing nitrogen topdressing, the proportion of nitrogen accumulated in stems and pod shells increased, while the proportion in seeds decreased. Topping reduced nitrogen agronomic efficiency but enhanced the yield-increasing effect of nitrogen. The highest nitrogen contribution rate (14.5%) was achieved at N45.0. However, topping at 6.00 t/hm² required topdressing N 67.5 kg/hm² to achieve its maximum contribution rate (17.6%). Topping and nitrogen topdressing after topping increased net income by over 90% and approximately 10.0%, respectively.

Conclusions The optimal agronomic management strategy for the vegetable-oilseed dual-purpose rapeseed system is a topping amount of 3.00 t/hm² combined with a N topdressing rate of 45.0 kg/hm². This rate minimizes seed yield loss and achieves a high income of 12664 yuan/hm² and a relatively high nitrogen agronomic efficiency. Topping 6.00 t/hm² of stalks requires a relatively high N topdressing rate of 67.5 kg/hm² to achieve high yield but still can not offset the resulted rapeseed yield loss, reduced nitrogen use efficiency.