Effects of repeated clover undersowing in spring cereals and stubble treatments in autumn on Elymus repens, Sonchus arvensis and Cirsium arvense

Cover crops are included in cropping systems in order to achieve various ecological benefits. In stockless organic cereal systems in Scandinavia, nitrogen is commonly supplied by undersowing a legume shortly after sowing. Retarding the growth of annual weeds is considered an additional benefit of using cover crops. However, studies on the influence of undersown cover crops on the growth of perennial weeds are lacking. In this paper we present data from a four- year field experiment on the growth of Cirsium arvense (L.) Scop., Sonchus arvensis L. and Elymus repens (L.) Gould., in cereals with and without undersown red clover (Trifolium pratense L.), combined with contrasting stubble treatments after harvest. Clover did not significantly reduce the growth of any of the weed species. Regression analyses showed, however, that the growth of C. arvense and S. arvensis increased with increasing clover biomass at low densities of clover, whereas at higher densities it decreased with increasing biomass of clover. Amongst the stubble treatments, shallow ploughing followed by harrowing gave the best control of all weed species. The effect was most pronounced on E. repens, and least on S. arvensis, for which neither the number nor the weed biomass were significantly different from the untreated control. Rotary tilling gave a similar effect, but again less pronounced on S. arvensis. Mowing suppressed E. repens to some degree, tended to suppress S. arvensis (n.s.) but had no effect on C. arvense. The growth and survival of seed-propagated plants of the same species were studied in a one-year field experiment and in a greenhouse experiment, the latter only including the species S. arvensis and C. arvense. The use of a cover crop reduced the number of leaves per plant in S. arvensis both in the field and in the greenhouse. The same effect was found for C arvense, but only in the greenhouse experiment. In the latter, above-ground biomass of the cover crop explained 67% of the variation in total biomass of S. arvensis and 47% of that for C. arvense. From a practical point of view, the study has shown that undersown red clover has only marginal effects on perennial weeds, in contrast to various stubble treatments which influence strongly on weed growth. In general, soil tillage suppresses perennial weeds best, but mowing may be a more environmentally friendly alternative for some weed species, such as. E. repens. (C) 2011 Elsevier Ltd. All rights reserved.