Model-based assessment of maize cropping under conventional and conservation agriculture in highland Mexico Rolf Sommer * , Patrick C. Wall, Bram Govaerts International Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico Received 25 April 2005; received in revised form 4 July 2006; accepted 10 July 2006 Abstract The CropSyst crop–soil-simulation model was used to assess the performance of conservation tillage in comparison to conventional tillage during 13 years of continuous maize cropping in highland Mexico. We tested if the calibration and validation requirements for CropSyst could be met using data sets, which were routinely collected by agronomists. Highest maize yield was observed under zero-tillage with retained residues. Simulation results indicated that this was due to more favorable moisture conditions, attenuating water stress in adverse years. Soil mineral N concentration measured in 1998 indicated the likelihood for N- stress under zero-tillage with residues retained. CropSyst additionally predicted N-stress as a yield limiting factor in other years, despite a seemingly optimal N supply by mineral fertilizer. CropSyst could predict yield under conventional tillage with residues retained and under zero-tillage with residues removed reasonably well, indicated by a modified Nash-Sutcliffe coefficient of efficiency (E 1 ) of 0.32 and 0.48. Yield predictions for conventional tillage with residues removed were poor (E 1 = 0.05) and those for zero-tillage with residues retained insufficient (E 1 = 0.20). Nonetheless, simulation results highlighted systematic differences between treatments with regard to water and N-dynamics. CropSyst lacks routines to account for soil crusting, the temporal impact of tillage on soil hydraulic conditions and the effect of surface residues physically restraining surface water runoff. These model shortcomings and the lack of detailed and continuous field measurement constrained detailed analyses and discussion of quantities produced by the model. # 2006 Elsevier B.V. All rights reserved. Keywords: CropSyst; Crop–soil-simulation model; Model efficiency; Zero-tillage; Infiltration; Surface runoff 1. Introduction Conservation agriculture (CA) has been promoted for over 20 years in North and South America and Australia with remarkable success and benefits for the environment. There are now over 70 Mha of zero-tillage (one of the key components of CA) worldwide, with >90% of this land in the aforementioned regions. Three practices underpin CA: (1) minimizing soil disturbance by reduced or zero-tillage, (2) retaining residues on the soil surface and (3) using crop rotations (Ekboir, 2002). By reducing tillage, farmers save labor and money that would otherwise be invested in implements and tractor power (Smart and Bradford, 1999). In addition to the economic benefits it brings, CA can improve soil health by increasing soil organic matter and biological activity as well as macroporosity, water infiltration and the amount of plant-available soil water (Unger, 1986; Saxton et al., 1988; Steiner, 1994; Fabrizzi et al., 2005; Wright et al., 2005). In addition to decreasing soil www.elsevier.com/locate/still Soil & Tillage Research 94 (2007) 83–100 * Corresponding author. Present address: Center for Development Research-ZEF, University of Bonn, Walter-Flex-Str. 3, 53113 Bonn, Germany. Fax: +49 228 731889. E-mail address: Rolf_Sommer@uni-bonn.de (R. Sommer). 0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2006.07.007