In southern Australia, intercropping, pasture cropping and overcropping have evolved as techniques to address environmental problems such as dryland salinity and wind erosion and to utilise soil water outside the conventional winter-dominant growing season. We paired three winter-dormant pastures, including two subtropical C4 perennial species (Rhodes grass, Chloris gayana; Gatton panic, Megathyrsus maximus) and the summer-active perennial C3 legume siratro (Macroptilium atropurpureum), with a conventional barley (Hordeum vulgare)–lupin (Lupinus angustifolius) rotation to explore the extent to which different summer-active species reduced crop yields. We also examined whether the competition for resources could be altered by supplying increased nitrogen to the crop and changing the row spacing of the pasture.
Under high-input conditions, pasture reduced cereal crop yields by up to 26% and lupin yields by up to 29%. Under low-input conditions, pasture cropping did not significantly reduce crop yield, and frequently increased crop yields. With low inputs, barley yield increases in 2011 ranged from 23% to 31%. In lupins under low-input conditions, yield increases ranged from 91% to 106% in 2010 and from –6% to 39% in 2012. The impact of the crop on the pasture was less pronounced, where the timing of pasture growth was delayed by the crop, but absolute levels of production were not influenced by the crop. Row spacing altered the temporal dynamic of pasture production; initially, the pasture produced less than the narrow spaced equivalent, but after 2 years, production exceeded that in the narrow row. Across all pasture species in 2009 and 2012, winter pasture production reduced crop yield by 0.32 and 0.4 t grain/ha pasture biomass produced, implying that moderate yield losses occurred because pasture production was also moderate. In the other two years, winter pasture production did not affect crop yield, suggesting that the pasture was able to utilise resources surplus to crop requirements. In this environment, with this combination of crops and summer-active pastures, higher levels of inputs did not enhance crop yield in a pasture-cropping system. We suggest that grain yield losses are lower in the low-input system and this implies that, at some level, competition between the species was reduced in a nitrogen-limited environment and the extent of the competition depended on season.
