Influence of conservation tillage and zero tillage on arable weeds in organic faba bean production

The field experiments were conducted in 2008, 2009 and 2010 on a Gleyic Cambisol near Goettingen, Lower Saxony, Germany. A crop sequence of summer barley, winter cover crops (intercropped oat and sunflower) and summer faba bean was examined under organic farming conditions. Emphasis was given to the studying of arable weeds in faba beans. However, enhancing symbiotic nitrogen fixation of summer faba beans by accumulation of soil-nitrogen by winter cover crops was a second objective in these experiments. The faba bean field plots had been cultivated with three different tillage systems: 1. zero tillage, sowing with cross-slottechnique, 2. conservation tillage (wing share cultivator, rotary harrow) sowing with cross-slot-technique and 3. conventional tillage with mouldboard plough followed by rotary harrow, sowing with precision monoseeder. In plots with zero tillage preceding cover crops were left as mulch on the soil surface. Cover crops accumulated adequate nitrogen amounts and following faba beans reacted with significant increase (up to 10%) in symbiotic nitrogen fixation. Maximum of arable weed biomass was observed in zero tillage-plots at the end of May or early in June. The abundance of the predominant weed wild mustard (Sinapis arvensis) increased with tillage intensity, whereas the abundance of creeping thistle (Cirsium arvense) increased in 2010 with decreasing tillage intensity. Average grain yield of faba beans was low with only 3.0 and 2.4 t ha-1 in 2009 and 2010, respectively.