Integrated nitrogen management strategies to improve seed yield, oil content and nitrogen efficiency of winter oilseed rape (Brassica napus L.): A review

Winter oilseed rape (Brassica napus L.) is the dominant oilseed crop in northern Europe. Generally, it requires high amounts of nitrogen (N) but is characterized by low N-efficiency, which is defined as produced seed dry weight per unit of accumulated N-fertilizer. Consequently, there is a strong need to resume all the relevant information on N in relation to cropping of winter oilseed rape and environmental impact to improve the efficiency of rape production. To enhance the productivity of winter oilseed rape cropping, integrated N-management strategies are needed including optimized N-supply due to varied crop rotations or fertilization, best soil and crop management practices. This paper reviews various N-management strategies in relation to seed yield and N-efficiency of winter oilseed rape. Comparing different previous crops, winter oilseed rape yielded considerably lower after cereal crops than following legumes. The seed yield is not only affected by the position within the crop rotation but also by the length of the break between two winter oilseed rape crops and type of cultivar. The use of N-efficient cultivars with reduced N-demand led to lower N-balance surpluses. Since the optimal N-treatment varies with cultivar, year and site condition, the maximum quantity of N-fertilizer for highest seed yield are discussed. A comparison of N-fertilizers reveals that N-fertilizers work different due to their chemical composition. Moreover, rate of N-recovery varies among methods and timing of N-application. Adjusted to the timing of optimum N-demand of the crop, timing of N-doses effectively increases production efficiency of winter oilseed rape. Apart from these integrated N-management strategies soil cultivation, seeding, application of plant protection agents and plant growth regulators as well as soil fertilization and harvesting are closely interacting with N-efficiency of winter oilseed rape. Altogether, amount and timing of N-fertilizer as well as cultivar selection had the strongest influence on productivity followed by smaller effects due to previous crop and type of fertilizer. Using N-efficient management strategies like choice of variety, form and timing of N-application adapted to site conditions, a remarkable reduction in fertilizer N-demand (up to 50% of fertilizer input) is possible leading to lower N-balance surpluses in winter oilseed rape production, thus minimizing environmental pollution.