Objectives This study investigates the effects of nitrogen (N) application rates on the yield of winter oilseed rap (Brassica napus L.) in normal and freeze years, to clarify the relationship between key yield-reducing traits and N application rates, and provide nutrient management to mitigate the freeze damage on oilseed rape production.

Methods Field experiments were conducted in Hubei Province, located in the middle reaches of the Yangtze River, during the 2022/2023 and 2023/2024 growing seasons. Five N fertilizer rates (0, 90, 180, 270, and 360 kg/hm², denoted as N₀, N₉₀, N₁₈₀, N₂₇₀, and N₃₆₀) were set. The duration of growth stages in both years were recorded, and the corresponding heat indices were calculated. At maturity, indices including aboveground dry matter weight and distribution ratio, yield and its components, and rapeseed particle size were measured. The freeze damage degree of winter rapeseed under different N treatments was evaluated by principal component analysis and membership function method.

Results In February 2024, severe low-temperature freeze damage occurred during the bud and bolting stage of winter rapeseed in Hubei Province, designating this year as the freeze damage year, while 2023 was the normal climate year. Compared with normal years, each nitrogen fertilizer treatment in the 2023/2024 season extended the budding and stem elongation stage by approximately 7 days, resulting in a shortening of the flowering stage by 3−9 days and the silique−maturity stage by 1−4 days, respectively. Compared with N₀, other N treatments increased rapeseed yield by 947.0−1588.2 kg/hm² in 2022/2023 and 615.3−1334.8 kg/hm² in 2023/2024, with yield increase rates of 101.66%−170.50% and 179.91%−390.29%, respectively. The freeze damage caused yield reduction in N₀ exceeded 60%, whereas that in N₉₀, N₁₈₀, N₂₇₀, and N₃₆₀ treatments were declined to 49.04%, 33.46%, 36.59%, and 40.70%, respectively. The minimum decrease of N use efficiency caused by freeze damage was also observed at N₁₈₀, with the partial factor productivity of N and agronomic efficiency of N₁₈₀ decreased by 33.46% and 15.96%, while the decrease at N₃₆₀ were by 40.70% and 39.34%, respectively. The impacts of freeze damage on key traits at maturity varied with N rates. Compared to normal year, N₀ treatment in freeze year was observed significant decrease in plant height, number of branches, seeds per pod, and 1000-seed weight, N₁₈₀ were observed smaller reductions in these traits, while N₃₆₀ exhibited more serious damages in traits, with an average reduction of 143 pods per plant and a 41.53% decrease in branch pods, and freeze stress also reduced the overall seed size by 0.2 mm in N₃₆₀. Comprehensive analysis revealed that the key factors for evaluating the degree of freeze damage at maturity are the aboveground dry matter weight, stem dry matter distribution ratio, 1000-seed weight, and number of pods per plant.

Conclusions In the middle and lower reaches of the Yangtze River, freeze injury damages the aboveground dry matter accumulation and its allocation to seeds, thereby reducing the pod number and 1000-seeds-weight, and consequently the yield. Both low or excessive nitrogen application will exaggerate the damage. Appropriately increasing the nitrogen application rate will effectively alleviate yield loss and improve fertilizer use efficiency. The proper rate is N 180.5 kg/hm² in normal climate years, and should be increased to 227.7 kg/hm² in freeze years, while exceeding 227.7 kg/hm² will exacerbate the damage.