The importance of in-crop lucerne suppression and nitrogen for cereal companion crops in south-eastern Australia § R.H. Harris a,c,f, * , T.S. Clune a,f , M.B. Peoples b,f , A.D. Swan b,f , W.D. Bellotti c,f , W. Chen d,f , S. Norng e a Department of Primary Industries Victoria, Rutherglen Centre, RMB 1145, Rutherglen, Vic. 3685, Australia b CSIRO Plant Industry, P.O. Box 1600, Canberra, ACT 2601 Australia c University of Adelaide, School of Agriculture and Wine, Roseworthy, SA 5371, Australia d Murdoch University, School of Environmental Sciences, South Street, Murdoch, WA 6150, Australia e Department of Primary Industries Victoria, Tatura Centre, Private Bag 1, Tatura, Vic. 3616, Australia f CRC for Plant-based Management of Dryland Salinity, University of Western Australia, M081, 35 Stirling Highway, Crawley, WA 6009, Australia Received 20 October 2006; accepted 25 May 2007 Abstract Five field experiments located at four sites across south-eastern Australia found cereal grain yields were less in the presence of lucerne (companion cropping) than in the absence of lucerne (cereal monoculture). Top-dressed nitrogen (N) and in-crop lucerne suppression, generally did not enhance cereal crop yields in the presence of lucerne compared with cereals growing in monoculture. Grain yield reductions from cereals growing with lucerne were found at four of the five sites, with reductions ranging from 16 to 26% compared with cereals growing in monoculture. In regard to cereal production, there was no main treatment by top-dressed N interaction at all sites, indicating that applying N to cereals irrespective of whether they were growing with or without lucerne, resulted in the same yield responses. With favourable growing seasons (decile > 6) and low available soil N levels, top-dressing N resulted in a 31% and a 0.8 unit increase in grain yield and grain protein, respectively, across all cereal crops and years. However, the absence of a grain yield response to top-dressed N at one site was due to excessive cereal biomass production from N application, causing extensive crop lodging in 2003, and decile 2 growing season rainfall in 2004. At another site, high available soil N levels and low growing season rainfall (decile 3) resulted in a 12% decline in grain yield across all cereal crops and years in response to top-dressing N. We therefore conclude that N application to cereals growing with lucerne can increase cereal grain yields, but only when accompanied by favourable growing season rainfall and low available soil N levels. In-crop lucerne suppression was effective at reducing cereal grain contamination by lucerne pods and flowers in companion crops, but was less effective under dry seasonal conditions, demonstrating that soil moisture will affect herbicide efficacy and the effectiveness of this practice. Economic analyses of companion cropping based on grain yields alone, will not be adequate without an assessment of summer lucerne production, until such data exists across a range of environments, it would be premature to conclude whether and or where this practice has commercial merit. Crown Copyright # 2007 Published by Elsevier B.V. All rights reserved. Keywords: Inter-cropping; Companion cropping; Grain yield reduction; In-crop lucerne suppression; Top-dressed nitrogen; Lucerne; Wheat; Barley 1. Introduction Lucerne (Medicago sativa) companion-cropping (also known as inter-cropping or over-cropping) involves sowing an annual crop directly into an existing lucerne stand (Willey, 1979; Harris et al., 2003; Robertson et al., 2004). In comparison with conventional cropping systems, companion cropping promotes greater use of rainfall by maintaining a perennial plant throughout the year, and therefore reduces the risk of excess rainfall leaking below the root zone and contributing to the harmful effects of dryland salinity (Crawford and Macfarlane, 1995; Angus et al., 2001; Dunin et al., 2001; Latta et al., 2001; Ward et al., 2002). While lucerne’s ability to dry soil profiles to depth is beneficial for reducing dryland salinity (Ridley et al., 2001), the www.elsevier.com/locate/fcr Field Crops Research 104 (2007) 31–43 § The herbicide clopyralid used in our study for in-crop lucerne suppression was used for research purposes only, and is not currently registered for the suppression of lucerne. The authors and the organisations we represent do not endorse the use of this product for lucerne suppression. * Corresponding author at: Department of Primary Industries Victoria, Rutherglen, Vic. 3685, Australia. Tel.: +61 2 6030 4500; fax: +61 2 6030 4600. E-mail address: rob.harris@dpi.vic.gov.au (R.H. Harris). 0378-4290/$ – see front matter. Crown Copyright # 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2007.05.013