Impact of Plant Density and Nitrogen Fertilizer on the Yield and Quality of Rapeseed Flowering Stalks Harvested at Various Plant Heights

To develop innovative, high-value utilization models for green manure rapeseed, this study investigated the effects of planting density (PD) and nitrogen (N) application rate on the yield and quality of rapeseed flowering stalks harvested at different plant heights. A factorial experimental design was employed, incorporating three factors, PD, N application, and harvesting height, each at three levels. Morphological traits, including stem diameter, fresh weight per plant, flowering stalk yield, and fresh biomass yield after stalk harvesting, were evaluated, together with biochemical traits comprising cellulose, soluble protein, soluble sugar, and vitamin C concentrations in the flowering stalks. The results showed that stem diameter, individual plant fresh weight, and flowering stalk yield generally declined with increasing PD and delayed harvesting height, whereas responses to N application varied depending on PD level. Flowering stalk yield ranged from low to high values across treatments, with the highest yields consistently recorded at a PD of 202,000 plants ha−1 (PD2) across all N levels and at 303,000 plants ha−1 (PD3) combined with 17.25 kg N ha−1 (N1). Variations in PD and N application significantly influenced both yield performance and biochemical composition of the flowering stalks. Elevated concentrations of soluble protein, soluble sugar, and vitamin C were observed at a harvesting height of 25 cm under PD3N1 and at 30 cm under PD2N1 and PD2N2, whereas lower cellulose concentrations were detected at 30 cm under PD2N1 and PD2N2. Overall, these findings demonstrate that coordinated optimization of planting density, nitrogen input, and harvesting height can effectively balance yield and quality traits, providing a practical dual-purpose production strategy for rapeseed cultivated as both a vegetable and green manure in the Shanghai region.