Please cite this article in press as: Amossé, C., et al., Relay-intercropped forage legumes help to control weeds in organic grain production. Eur. J. Agron. (2013), http://dx.doi.org/10.1016/j.eja.2013.04.002 ARTICLE IN PRESS G Model EURAGR-25167; No. of Pages 10 Europ. J. Agronomy xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect European Journal of Agronomy jo ur nal homepage: www.elsevier.com/locate/eja Relay-intercropped forage legumes help to control weeds in organic grain production Camille Amossé a,∗ , Marie-Hélène Jeuffroy b,c , Florian Celette a , Christophe David a a ISARA-Lyon, UP SCAB, 23 rue Jean Baldassini, 69364 Lyon 07, France b INRA, UMR211 Agronomie, BP01, 78850 Thiverval-Grignon, France c AgroParisTech, UMR211 Agronomie, BP01, 78850 Thiverval-Grignon, France a r t i c l e i n f o Article history: Received 8 November 2012 Received in revised form 26 March 2013 Accepted 4 April 2013 Keywords: Weed Winter wheat Intercropping Cover crop Medicago Trifolium a b s t r a c t In organic grain production, weeds are one of the major limiting factors along with crop nitrogen defi- ciency. Relay intercropping of forage legume cover crops in an established winter cereal crop might be a viable option but is still not well documented, especially under organic conditions. Four species of forage legumes (Medicago lupulina, Medicago sativa, Trifolium pratense and Trifolium repens) were undersown in six organic wheat fields. The density and aerial dry matter of wheat, relay- intercropped legumes and weeds were monitored during wheat-legume relay intercropping and after wheat harvest until late autumn, before the ploughing of cover crops. Our results showed a large diversity of aerial growth of weeds depending on soil, climate and wheat development. The dynamics of the legume cover crops were highly different between species and crop- ping periods (during relay intercropping and after wheat harvest). For instance, T. repens was two times less developed than the other species during relay intercropping while obtaining the highest aerial dry matter in late autumn. During the relay intercropping period, forage legume cover crops were only effi- cient in controlling weed density in comparison with wheat sole crop. The control of the aerial dry matter of weeds at the end of the relay intercropping period was better explained considering both legumes and wheat biomasses instead of legumes alone. In late autumn, 24 weeks after wheat harvest, weed biomass was largely reduced by the cover crops. Weed density and biomass reductions were correlated with cover crop biomass at wheat harvest and in late autumn. The presence of a cover crop also exhibited another positive effect by decreasing the density of spring-germinating annual weeds during the relay intercropping period. © 2013 Elsevier B.V. All rights reserved. 1. Introduction In organic grain production, weeds are one of the main limiting factors (Bàrberi, 2002; Kruidhof et al., 2008) along with crop nitro- gen deficiency (Casagrande et al., 2009). Weed density, biomass or ground cover are generally higher than in conventional systems (Cavigelli et al., 2008; Ryan et al., 2009; Teasdale and Cavigelli, 2010). Consequently, yields can suffer from competition for trophic resources, i.e. light, nutrients, and water (Cavigelli et al., 2009; Gherekhloo et al., 2010; Ryan et al., 2009). Weeds may compete with the cash crop for light interception (Corre-Hellou et al., 2011). Competition for soil nitrogen may also appear with some high nitrogen-competitive weed species (e.g. Amaranthus retroflexus L.) ∗ Corresponding author. Tel.: +33 427 858 526; fax: +33 427 858 636. E-mail addresses: camille.amosse@gmail.com (C. Amossé), Marie-Helene.Jeuffroy@grignon.inra.fr (M.-H. Jeuffroy), fcelette@isara.fr (F. Celette), davidc@isara.fr (C. David). (Blackshaw and Brandt, 2008). Moreover, weeds are often more competitive than cereals for nitrogen nutrition coming from the soil (Bàrberi, 2002). In some cases, the allelopathic effect of weeds can be harmful to cereal crops as shown by Hamidi and Ghadiri (2011) with wild barley (Hordeum spontaneum Koch) to the detriment of wheat (Triticum aestivum L.). In organic conditions, direct weed control, as listed by Wei et al. (2010), has to be combined with indirect and preventive methods like the diversification of the crop sequence or the use of cover crop (CC) among others (Bàrberi, 2002; Bond and Grundy, 2001; Wei et al., 2010). These techniques aim at tackling weed issues on a long-term strategy basis (Melander et al., 2005) by limiting weed infestation during the crop cycle and thereby reducing the weed seedbank over the following years. The use of forage legume CCs could be an interesting option for limiting weed infestations. Previous research studied different options with CCs being grown between two cash crops (Kruidhof et al., 2008; Liebman and Davis, 2000; Teasdale et al., 2007) or CCs as living (Hiltbrunner et al., 2007) or dead mulches (Kumar et al., 1161-0301/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.eja.2013.04.002