Five field experiments located at four sites (Burraja, Grogan, North Boorhaman and Roseworthy) across south eastern Australia compared cereal grain yields in the presence of lucerne (companion cropping) and absence of lucerne (cereal monoculture). Top-dressed nitrogen (N) was applied to subplots at Burraja, Grogan and North Boorhaman, while in-crop lucerne suppression was applied to plots at Burraja and two separate, but adjacent experiments at Roseworthy, to determine if these management strategies could improve cereal performance in the presence of lucerne. In addition annual lucerne and cereal biomass was measured at North Boorhaman from a companion crop and compared with both a lucerne and cereal monoculture. Over the three years and four sites, cereals growing with lucerne yielded between 19% and 57% less (P<0.05) grain than cereals growing alone. 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 same yield responses. Top-dressing N at North Boorhaman in 2003 and 2005 resulted in a 14% and 40% respectively, increase (P<0.05) in grain yield across all cereal crops. The absence of a response at Burraja and Grogan was probably due to sub-optimal growing season rainfall. In-crop lucerne suppression did not increase grain yields at either Burraja or Roseworthy, but in some seasons reduced (P<0.05) cereal grain contamination by lucerne pods and flowers. Companion cropping increased (P<0.05) total (cereal and lucerne) annual biomass production by 20-41%, compared with the lucerne monoculture, and 16% more (P<0.05) than the cereal monoculture. Demonstrating that while grain yield reductions reduce the attractiveness of companion cropping, this practice does offer other advantages in terms of improved yearly water use and quality out-of-season feed supply that cereal monocultures can not deliver.Rob Harris, Tim Clune, Mark Peoples, Antony Swan, William Bellotti and Wen Che
