Over-cropping lucerne with wheat: effect of lucerne winter activity on total plant production and water use of the mixture, and wheat yield and quality

Two field experiments in southern Australia investigated a farming system of over-cropping wheat (Triticum aestivum L.) into established lucerne (Medicago sativa subsp. L.) varieties of different winter activity ratings. The study was completed at Roseworthy, South Australia, and Katanning, Western Australia, between August 2000 and May 2003 in seasons receiving below average and average rainfall. Comparative lucerne persistence and biomass, wheat biomass, grain yield and protein contents, and soil water contents were measured. Wheat grain yield was reduced by 13–63% by over-cropping lucerne compared with wheat monoculture. Winter-dormant lucerne (winter activity Classes 0.5 and 2) reduced the yield penalty compared with winter-active varieties (Classes 6 and 10) in 2 of the 4 evaluations. The positive response to applying N at sowing in the second year of over-cropping wheat at Katanning was greatest in the most winter-dormant lucerne treatment (winter activity 0.5). Soil water contents were similar under the lucerne/wheat over-cropping and lucerne monoculture treatments irrespective of lucerne winter activity. Deficits of up to 43 mm at Roseworthy and 88 mm at Katanning were measured in the 0–200-cm soil profile at the start of the third summer of the study. The study shows that it can be more efficient in terms of land area to over-crop wheat into lucerne than to grow monocultures on separate parcels of land akin to phase farming. The improved productivity of over-cropping is associated with the separation of growth patterns of winter wheat and summer-active lucerne. This farming system offers great potential for improving sustainability and productivity in southern Australian cropping rotations.