Weed suppressive ability in sole and intercrops of pea and oat and its interaction with ploughing depth and crop interference in organic farming

The cultivation of weak weed competitive pea sole crops after reduced ploughing depth may result in weed problems in organic farming. Intercropping peas and cereals is one option to manage weed problems. However, little evidence exists on the weed suppressive ability of pea-cereal intercrops after differing ploughing depths. The effect of crop stand (pea sole crop, pea-oat intercrop and oat sole crop) and ploughing depth (10–12 vs. 25–27 cm) on the annual weed infestation, PAR transmission and weed nitrogen as well as water supply was investigated in field experiments in Northern Germany. In order to determine causes for the differing weed suppressive ability in pea and oat sole or intercrops, a pot experiment and a bioassay were conducted complementary to the field experiments. Crop stand and ploughing depth did not interact with regard to weed infestation. The weed suppressive ability increased from pea sole crops to oat sole crops, whereas shallow ploughing resulted in a significantly higher weed infestation than deep ploughing. While crop-weed competition for light was not essential for the differing weed suppressive ability, competition for water and nitrogen were detected to be key factors. As root exudates of the examined oat cultivar showed a growth inhibiting potential, allelopathy may also contribute to the weed suppression in oat sole and pea-oat intercrops. Results from this study indicate that pea-oat intercropping is not able to compensate for the higher annual weed infestation after shallow ploughing. Nevertheless, owing to their good weed suppressive ability, intercrops with cereals are of particular suitability for the cultivation of weak weed suppressive semi-leafless peas in reduced tilled soils in organic farming.