Productivity and water use in intensified forage soybean-wheat cropping systems of the US southern Great Plains

Agro-ecological intensification of continuous dryland wheat (Triticum aestivum L.) systems with forage soybean [Glycine max (L.) Merr] in lieu of summer fallow could provide nutritious summer forage for livestock, reduce requirements for inorganic N fertilizer for winter wheat, and ameliorate sustainability issues associated with summer fallow, such as soil erosion and low precipitation use efficiency (PUE). This modeling study utilized DSSAT-CSM to assess: the yield and water use of three cultivars of forage soybean of diverse maturity groups (MGs) harvested at three different dates [60, 90, and 120 days after planting (DAP)]; and their influence on performance and water productivity of subsequent winter wheat, and overall double-cropping systems in comparison to the fallow-wheat system. Crop yield and evapotranspiration (ET) data, collected from continuous field experiments at El Reno, Oklahoma (35.57 degrees N, 98.03 degrees W) during 2002-2006, were used for calibration and validation of soybean and wheat models. Long-term simulations conducted using historical weather data (1994-2019) suggested that mid-maturity group (MGV) soybean could provide greater forage yields and higher water use efficiency (WUEB) than later-maturing soybeans (MGVI & MGVII) when harvested at 90-120 days after planting (DAP). Double-cropping forage soybean and winter wheat caused a reduction of 1.4-2.1 Mg ha(-1) grain and 4.7-5.6 Mg ha(-1) and biomass by wheat, and higher seasonal ET losses of 77-132 mm than the fallow-wheat system. However, trade-offs in yields between forage soybean and winter wheat were effective when precipitation was >180 mm during the first 60 d of growth by soybean. Despite yield losses in winter wheat, double-cropped forage soybean-wheat systems could be a viable option based on its economic competitiveness and other ecological benefits.