The reduction of soil tillage depth is of special interest in organic farming. However, shallow ploughing often results in higher weed abundances in comparison to deep ploughing. Thus, the objective of this study was to evaluate whether the high weed suppressive ability of spring and winter pea-cereal intercrops can compensate for the higher weed growth in shallow ploughing compared to deep ploughing in organic farming. Peas and cereals were grown as sole crops and intercrops under shallow and deep ploughing in experiments on a sandy loam soil in Northern Germany in 2009, 2010 and 2011. Semi-leafless spring peas-oats, semi-leafless winter peas-triticale and regular-leaf type winter peas-triticale intercrops were examined. At harvest, weed shoot biomass in semi-leafless spring and winter pea sole crops under deep ploughing were slightly higher than those of the corresponding intercrops under shallow ploughing. Whereas weed shoot biomass in semi-leafless winter peas-triticale intercrops under deep ploughing was comparable to shallow ploughing, the cultivation of spring peas-oats intercrops under shallow ploughing led to a significantly higher weed growth than did deep ploughing. The regular-leaf type winter pea grown as a sole crop, or an intercrop with triticale, showed a higher weed suppressive ability than the semi-leafless peas. The tillage system had no significant influence on the weed infestation of sole and intercropped regular-leaf type winter peas. Semi-leafless spring peas-oats and winter peas-triticale intercrops could compensate for weed infestation differences under shallow ploughing compared to the corresponding pea sole crops under deep ploughing. Due to the high weed suppressive ability of regular-leaf type winter peas, shallow ploughing and deep ploughing, as well as sole cropping and intercropping, led to comparable weed infestation.
