Linseed (Linum usitatissimum) as an oil crop in organic farming: abiotic impacts on seed ingredients and yield

Linseed contains substantial amounts of desired ingredients, such as omega-3 fatty acids (α-linolenic acid) and antioxidants, as well as undesired ingredients such as cyanogenic glycosides and cadmium. This paper presents and discusses the abiotic factors which influence quality and yield of linseed seeds in organic cropping systems. In temperate climates, conventional farmers can harvest more than 2.0 t seed ha⁻¹, with oil contents ranging from 35 to 44 %. Organic trials usually had lower yields with up to approximately 1.8 t seed ha⁻¹ in temperate, continental, and Mediterranean conditions. Cool climatic conditions in temperate regions lead to high shares (>60 %) of omega-3 fatty acids related to total oil content in organic farming as well as in conventional farming. Linseed achieved maximum yields of seeds and omega-3 fatty acids with a fertilization level of less than approximately 110 kg nitrogen ha⁻¹. High levels of nitrogen fertilization did not benefit the content of omega-3 fatty acids; increasing the nitrogen fertilization from 52 to 260 kg ha⁻¹ reduced the content of α-linolenic acid by 7.25 % in a trial. The content of cyanogenic compounds turned out to be positively correlated with the protein content of the seeds and the nitrogen fertilization; therefore, the usually moderate level of nitrogen supply in organic crop rotations could keep the content of cyanogenic glycosides on a comparatively low level, compared to cropping systems with higher nitrogen levels. Linseed is a hyper-accumulator of cadmium; the total cadmium content in the seeds is determined by the cadmium content of the soil and the linseed variety. Generally, the choice of linseed variety influences the quality of the seed, and respective varieties must be chosen to improve product quality. Harvest, post-harvest, and processing conditions additionally affect seed and oil quality. Heating can reduce the unwanted cyanogenic glycosides, but it also reduces the content of desired anti-oxidants and of omega-3 fatty acids.All in all, low overall nitrogen levels in organic cropping systems may offer the option to produce linseed seeds and oil with high levels of omega-3 fatty acid and low cyanogenic glycosides, with yields similar to conventional farming. In organic farming systems, the risk of elevated cadmium contents in linseed is low as synthetic P fertilizers are prohibited and raw phosphates are rarely used, thus minimizing the input of cadmium via fertilizers. As high product quality can be maintained or even improved by organic farming practices, linseed is suitable as an oil crop in organic farming that allows diversification of the crop rotation.