Cover crop-based ecological weed management: exploration and optimization

Keywords: organic farming, ecologically-based weed management, cover crops, green manure, allelopathy, Secale cereale, Brassica napus, Medicago sativa Cover crop-based ecological weed management: exploration and optimization. In organic farming systems, weed control is recognized as one of the main production-related bottlenecks. System-oriented approaches for ecological weed management are needed and cover crops may form an important component of such an approach. Inclusion of cover crops in crop rotations introduces two important mechanisms through which the development of weed populations may be hampered. In late summer and autumn the successful introduction of cover crops can prevent growth and seed production of weeds through competition. In springtime, cover crop residues incorporated in the upper layer of the soil may suppress or retard weed development and growth due to, among others, allelopathic effects. The main focus of research was put on the weed suppressive effect of cover crop residue material in spring and particularly on identifying management options to maximize this effect. To better appreciate the potential of cover crop residue material the investigations were focused on three aspects, namely allelochemicals in the cover crop, the residence time of the residue-mediated inhibitory potential in the soil and the variability in inhibitory effects on receptor plants. The study was conducted with representatives of three plant families: Secale cereale L. (winter rye), Brassica napus L. (winter oilseed rape) and Medicago sativa L. (lucerne). Mechanical injuring of field grown cover crops enhanced the allelopathic activity per unit biomass. However, this increase was often just sufficient to compensate for loss of plant material resulting from damaging, implying the limited practical significance of damaging. Different options for pre-treatment and incorporation of cover crop residue material were compared and these were found to influence the size and persistence of an inhibitory effect on seedling emergence. Results were found to be cover crop specific. With regard to species’ sensitivity our results suggest that for inhibition of a receptor plant not just seed size is important. Only if the time course of sensitivity of the receptor plant matches with the time course of residue-mediated inhibitory potential, significant reductions in seedling establishment can be expected. In light of this, it was postulated that variation in synchronicity of receptor species’ sensitivity and potential residue effects may well explain the large degree of variation often noted in field studies of allelopathy.