Diversification of crop rotations is important for disrupting the life cycles of detrimental weeds. Perennial and N-fixating crops suitable for mowing can deplete the weed seed bank and hamper the proliferation of some perennial weeds. Cover crops suppress post-harvest weed growth and retain nutrients in the root zone.
Organic crop production acknowledges the importance of crop diversification for the management of diseases, insect-pests and weeds and in particular the supply of nitrogen. Nevertheless, attractive commodity prices and thus reluctance to diversify crop choice at the expense of particular profitable crops mean that crop compositions of crop rotations may not always be optimal. For example, grass-clover and lucerne are often recommended for the supply of nitrogen on organic farms with limited access to animal manure.
Nutrient management is not the only reason for this recommendation but also benefits in terms of weed management have been emphasized. While the benefits for nutrient management are well documented, weed effects are less verified and mainly based on experiences. Still, many growers without livestock manage their nitrogen demand through annual legumes providing short-term revenues but often with adverse effects on weed abundance.
The Core Organic plus project PRODIVA has produced more evidences to the beneficial effects of grass-clover and lucerne. Soil samples from a long-termed crop rotation composing more than 25% of grass-clover / lucerne contained less than half the amount of annual weed seeds as compared to a similar crop rotation where the perennial crops had been replaced by annual legumes. The effect on the seed bank was still significant 3-4 years after abandoning the perennial crop. In the same study, regular mowing of the perennial crops effectively suppressed the growth of Sonchus arvensis; a response similar to that seen for Cirsium arvense.
Perennial weeds, such as Elytrigia repens and Cirsium arvense, can increase their belowground systems of rhizomes and roots substantially unless being suppressed by cultivation or competition from a cover crop. Post-harvest tillage may lead to soil erosion and leaching of soluble nutrients and is not always the best choice for the management of perennials. Cover crops can effectively suppress post-harvest weed growth, if properly established, and provided that a dense and fast growing canopy can be achieved. PRODIVA has tested an assembly of cover crop species for their weed suppressing abilities. Especially, cover crop mixtures with clover species undersown in cereals in spring can produce dense and suppressive canopies.
Clover undersown in winter wheat in spring produces a dense and competitive plant
cover for post-harvest weed suppression (Photo: Jukka Salonen)
Bo Melander, Aarhus University,
Dep. of Agroecology, Research Centre Flakkebjerg,