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

Summary

Intercropping, the 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 competitive nor allelopathic interactions, as neither of the crops were growth depressed when intercropped compared to being grown as sole crops. In the pot experiment, an increased shoot dry matter production of intercropped barley plants compared to sole cropped plants was observed from the second week after emergence. Pea shoot dry matter production was depressed from two weeks after emergence, whereas a clear effect on oilseed rape was not found until four weeks after emergence. Thus, under these experimental conditions, interference from barley started as early as two 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. A greater root deployment of intercropped barley plants indicated the importance of early below-ground interference exerted by this species. 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.