Modelling management strategies for wheat-soybean double crops in the south-eastern Pampas

Favourable prices and new technologies have recently stimulated the adoption of wheat-soybean double crops in the south-eastern Pampas. To assess the largely unknown long-term performance of this strategy, we used CERES-Wheat and CROPGRO-Soybean with slight adaptations to account for double cropping. Models and climate records were combined to analyse management practices allowing for earlier soybean sowing, which was assumed to be the key for high soybean yield and whole-system performance. Practices investigated were: (i) short-season wheat cultivars; (ii) non-selective herbicides for faster drying of wheat before harvest; (iii) relay double cropping, where soybean is sown on standing wheat before crop maturity in contrast to sequential cropping where soybean is sown after wheat harvest. Models reproduced phenology and yield of crops in double cropping systems to the level required for our investigation. Advancing wheat harvest with herbicides and using current early wheat varieties had little impact on soybean yield. A hypothetical frost-resistant wheat allowing for earlier wheat sowing and harvest highlighted the potential to increase the yield of sequential soybean by 23%. Average soybean yield increased from 1738 kg ha(-1) in sequential to 2099 kg ha(-1) in relay cropping, whereas wheat yield was similar in both cropping variants. Seasonal variability in rainfall and simulated yield was partially accounted by the Southern Oscillation Index (SOI), indicating the potential for this signal to be used in risk management. The amount of rainfall during the period of yield generation in wheat and the remaining soil water at the end of the wheat growing season were higher during consistently negative SOI phase, and lower during consistently positive phase in September. In the period between 1971 and 2003 average October-December temperature increased at 0.038 degrees C year(-1). Higher temperature accelerated simulated wheat development and advanced harvest (0.15 day year(-1)) with no impact on yield; this allowed for earlier sowing of double cropped soybean, which together with more favourable temperature increased simulated soybean yield at a rate of 26.2 kg ha(-1) year(-1). Hence, our modelling study indicated that: (i) wheat-soybean relay double cropping outperformed its sequential counterpart, (ii) seasonal variability in rainfall and yield could be partially accounted by Southern Oscillation Index (SOI) and (iii) long-term trends in crop phenology and yield could be associated with increased mean temperature. (c) 2006 Elsevier B.V. All rights reserved.