The importance of in-crop lucerne suppression and nitrogen for cereal companion crops in south-eastern Australia

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.