An organic cereal row cropping system: A combination of relay undersown cover crop and inter-row hoeing for a balanced Nitrogen and weed management

Research output: Book/ReportPh.D. thesisResearch

Thi Thuy Hong Phan

Nitrogen (N) and weeds are the two main yield limiting factors in organic arable crop production. The intensive autumn tillage is the common practice for perennial weed control in the Nordic countries, but does not comply with the nutrient management strategies in organic farming. The autumn tillage restricts farmers to grow cover crops for soil fertility improvement and on the other hand this practice poses high risks of N loss to the environment. In the context of organic agriculture has moved towards the sustainable development, it requires improving the internal N cycling capacity of arable cropping systems and reducing the reliance on the external inputs (i.e. animal manure). This leads to an increasing demand for a more robust cropping system with a self-sufficient nutrient supply based on N-building crops and a balance between weed control and nutrient management for an optimum yield. This PhD project investigated a new row cropping system, having an increased row spacing to allow the implementation of one or two passes of inter-row hoeing for weed control before cover crop establishment, for its ability to produce a sufficient N supply, weed control and crop yield. Different sowing times and cover crop species were tested to identify an optimum sowing time for a high biomass production and the suitable undersown cover crop species for delayed sowing time. In the undersowing condition, the potential failure of cover crops due to traffic damage was concerned, therefore the tolerance capacity of different cover crop species to traffic damage was studied. The possibility to apply rototillage, a reduced tillage method, to terminate specific cover crop species were also tested in the field condition. In the meantime, to better understand the effect of different fragment lengths and soil cover depths, induced by different termination methods, on the regeneration and the emergence of different cover crop species was investigated under greenhouse condition. In order to increase the knowledge of the creeping thistle control, I investigated the mechanisms of how the two control strategies (cover crops vs. autumn tillage) affect the N competition of subsequent crop and thistle with the mean of 15N isotope dilution technique. The results showed that the new row cropping system is feasible for a combination of inter-row hoeing and delayed undersown cover crops with a sufficient N supply for subsequent crop and without negative effects on the companion crop yield. The improvement in the grain yield (0.3–1.7 Mg DM ha-1) and grain N content (10–22 kg N ha-1) of the succeeding crop was obtained in some cover crop treatments with both one and two hoeing passes, as compared with the autumn tillage. The optimal growth and N uptake of legume-nonlegume mixtures in the row cropping system was obtained with a short delay in showing time to allow one hoeing pass before cover crop establishment, but not significantly different from those obtained with synchronized sowing or further delayed sowing. Some cover crops such as black medic/plantain and mixture with winter radish were more effective than clover-grass mixtures when the sowing time was delayed further to allow two hoeing passes. From the mechanical tolerance experiment, Fabaceae and Brassicaceae species were generally low tolerant to traffic damage during their young stages, therefore, to avoid a potential failure of a cover crop, at least a high tolerance species (i.e. grass and other forbs such as plantain and chicory) should be included in the cover crop mixture with legumes. Cover crops were effective to deplete autumn NO3-N, without significant difference between cover crop mixtures. The effect of cover crops on the initial N availability in subsequent spring was different between cover crop mixtures and years, which depended on incorporation time and their regrowth capacity in the spring. The row cropping/row hoeing/cover crop system was less effective in managing thistles than the autumn tillage. There were encouraging results with the red clover/orchard grass treatment, but not much with other cover crops. The result from the three year experiment with two consecutive years of growing cover crops and inter-row howing (unpublished data) showed that the effect of cover crop strategy on thistle control was increased over the years with the continuous present of undersown cover crops, and inclusion of deep rooted species showed a tendency to improve the effect on thistle control.In the greenhouse experiment, the emergence of cover crops was significantly reduced when the soil covering depth increased from 2 to 5 cm while the effect of fragment length on the emergence of cover crops was various depending on species. Leaving full roots was suitable to control red clover, lucerne, ryegrass and dyer’s woad while smaller fragmentation is suitable for the control of other species, except kidney vetch had a low regeneration capacity regardless of fragment length. The field termination showed rototillage strategies were effective to terminate most of cover crop species, except while clover, and the double rototilling strategy increased the mortality of all cover crops after termination compared to the single rototilling strategy. The 15N experiment unfolded the mechanism behind the effect of two post-harvest practices (cover crops vs. autumn tillage) on the N competition between subsequent crop and thistle. Cover crops retained more N in the topsoil, thereby postponed its availability to the deep-rooted C. arvense, while making it available to the shallow-rooted spring barley, thus strengthening the barley’s ability to compete against C. arvense. In contrast, autumn tillage enhanced the leaching of N to deep soil layers (much of it leached below the barley root zone) which gave thistle an advantage to access the leached soil N. This result goes against the application of the intensive tillage for perennial weed control in current organic system and support for a potential effective long-term effect of cover crop strategy on thistle control.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Publication statusPublished - 2019

ID: 217937989