Modeling tillage effects on soybean growth and yield

The effects of tillage and cooler soil temperatures under surface residue were incorporated into CROPGRO-Soybean. An energy balance-based soil temperature model was integrated into CROPGRO-Soybean and compared to the original soil temperature model, which is driven primarily by air temperature. The new model gave an 8.6% higher R2 (0.88 and 0.81 for new and old models, respectively) and 20.6% lower RMSE at 10 cm depth (2.7°C and 3.4°C for new and old models, respectively) under bare soil conditions for 5 years of validation. The new model generally gave lower RMSE and higher R2 values compared to the old model at all soil depths. It gave excellent predictions of emergence under Central Iowa conditions, with an average error of 0.6 day for 8 planting dates in spring of 1997. The old model had an average error of 1 day. Under cool conditions, the new model gave better predictions of emergence dates. However, under warm periods, both models typically gave the same accuracy, and were within 1 day of the measured emergence date. A tillage component was integrated into CROPGRO-Soybean and tested for conditions at Ames, IA. Predictions of changes in soil physical properties were consistent with the literature. The model gave good predictions of soil temperature at 6 cm depth under moldboard (R2 = 0.81), chisel plow (R2 = 0.72), and no-till (R2 = 0.81) for 1997 and was able to simulate cooler soil temperatures under no-till in early spring and consequent delays in emergence. Excellent predictions of soybean phenology and biomass accumulation (e.g. R2 = 0.98, 0.97, and 0.95 for pod weight predictions under moldboard, chisel plow, and no-till, respectively) were obtained for 1997. Limited testing of the model for central and northeast Iowa conditions showed that slightly lower yields from no-till were primarily due to delayed emergence. Predicted yields from no-till were slightly higher when no-till had better water conservation (i.e. in drought years) and negligible delays in emergence. Generally, the tillage model confirmed field experiments showing that tillage did not significantly affect soybean yields in years with little or no water stress