Objectives The combined application of organic and chemical fertilizers can enhance crop yield and improve crop quality. Exploring the effect and optimal application rate of organic fertilizer on rapeseed yield and quality will provide technical support for the green and high-quality production of rapeseed.

Methods A two-year field experiment was conducted in Jinmen City, Hubei Province from 2018 to 2020. On the base of applying chemical fertilizer N 200 kg/hm², P₂O₅ 75 kg/hm², K₂O 90 kg/hm², five manure dosage were set, including 0 (M0), 1125 (M1), 2250 (M2), 3375 (M3) and 4500 (M4) kg/hm². Rapeseed yield and its component were investigated, shoot nutrient accumulation, seed oil and protein yield were analyzed. The linear plus platform model and the monic quadratic function model were used to explore the optimal amount of organic fertilizer.

Results Compared with M0, manure application significantly increased rapeseed yield, M3 and M2 treatments achieved the highest seed yields, with increment by 109.0% and 48.3% in 2018−2019 and 2019−2020 growing seasons, respectively. The pod number per plant and seed number per pod were positively correlated with seed yield (P<0.05). The highest values of the two yield components were recorded in M2 treatment, which were 8.0%−50.3% and 10.0%−36.9% higher than M0, respectively. Manure dosage had remarkable influence on seed N and P content, few on seed K content. The seed N, P, and K accumulation raised by 2.1%−72.1%, 12.5%−91.7% and 22.9%−71.8%, respectively. Seed oil and protein content changed oppositly with the increase of manure dosages. Increase of manure dosage decrease seed oil content by 2.6%−10.9%, but increased the seed protein content by up to 18.2% and 27.1%, respectively compared with M0. However, both oil and protein yield generally increased with yield, and arrived the platform level at manure dosage of 1758 and 2819 kg/hm², respectively. The highest profit were made by selling rapeseed at manure dosage of 2250 kg/hm² in this experiment.

Conclusions On the base of chemical fertilizer, manure application can further increase seed yield by increasing the pod number per plant and seeds per pod. Rapeseed oil and protein contents are negatively correlated, relatively low manure rate benefits high oil yield and the opposite for protein yield. Consequently, the manure dosage should be proposed according to the main purpose of rapeseed production.