Early interference dynamics in intercrops of pea, barley and oilseed rape

Intercropping, a practical application of ecological principles such as diversity, interference, competition and coexistence in agriculture, is gaining increasing interest in organic farming. To be able to improve the management of intercrops, a more detailed knowledge is required on aspects of interference between crops. A bioassay and a pot experiment were conducted to assess both the mechanisms and the time of expression of processes involved in the early interference between pea (Pisum sativum L.), barley (Hordeum vulgare L.) and oilseed rape (Brassica napus L.) when grown as intercrops. Conditions of non-limiting nutrient and water supply were chosen to eliminate competition for these growth factors. The bioassay gave no indication of either competitive or allelopathic interactions, as none of the crops were growth depressed when intercropped compared with when grown as sole crops. The effect of barley interference, as a depression in pea dry matter production in the IC, was observed in the pot experiment as early as 2 weeks after emergence. Although competition was supposedly eliminated, there were indications of strong above-ground competition among sole cropped barley plants as well as among pea and oilseed rape plants intercropped with barley. Growth characteristics that allow for rapid establishment of barley plants are likely to be the determining factors leading to the dominance exerted by this species when intercropped.