Effects of Reduced Tillage on Weed Pressure, Nitrogen Availability and Winter Wheat Yields under Organic Management

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Abstract
Reduced tillage reduces soil erosion and increases topsoil organic matter compared with conventional tillage. However, yields are often reported to be lower, presumably, due to increased weed pressure and a slower N mineralization under organic farming conditions. The effects of reduced tillage compared with ploughing on weed infestation and winter wheat performance at four different crop stages, i.e., tillering, stem elongation, flowering, and harvest, was monitored for a single season in an eleven-year-old organic long-term tillage trial. To disentangle the effects of weed presence on crop yield and potential crop performance, subplots were cleaned from weeds during the whole cropping season. Weed biomass was consistently higher under reduced tillage. Soil mineral nitrogen contents under reduced tillage management were higher, which could be explained by the earlier ley termination in autumn compared with the conventional tillage system. Nitrogen status of wheat assessed with SPAD measurements was consequently higher under reduced tillage throughout the season. At harvest, wheat biomass and grain yield were similar in both tillage systems in the presence of weeds, but 15–18% higher in the reduced tillage system when weeds were removed. The negative impact of weeds on yields were not found with conventional tillage with a low weed infestation. Results suggest that reduced tillage can provide equivalent and even higher yields to conventional tillage in organically managed winter wheat if weed management is improved and good nutrient supply is assured.

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