This work studies the factors responsible for amaranth (Amaranthus retroflexus) growth suppression by different cover crops (CC). In field trials with two shading levels amaranth biomass was similar, demonstrating that light interception by CC was not the primary mechanism responsible for amaranth growth suppression. We could show that below a threshold of 3 t/ha of CC biomass, amaranth growth suppression was negatively correlated with CC biomass (R2 = 0.41) and that this correlation was influenced by the CC species. Brassicaceae and black oat (Avena strigosa) did not follow this relation and effectively controlled amaranth even with a low biomass. The effects of root interactions between amaranth and CC on amaranth growth were further tested in the absence of competition for light, water and nutrients under controlled conditions. We could show that phacelia (Phacelia tanacetifolia) had no growth repressive effect, whereas buckwheat (Fagopyrum esculentum), black oat and forage radish (Raphanus sativus var. longipinnatus) significantly suppressed amaranth growth by 46, 37 and 49% through indirect root interactions and by 68, 41 and 62% through direct root interactions. We deduce that this was due to allelopathic root exudates. We conclude that in order to describe and predict the weed suppressive ability of CC it is not sufficient to only study biomass production and shading.
