In the subtropical highlands of Central Mexico,where the main crop is maize(Zea mays),the conventional practice(CP) involves tillage,monoculture and residue removal,leading to soil degradation and unsustainable use of natural resources and agricultural inputs.Conservation agriculture(CA) has been proposed as a viable alternative in the region,based on reduction in tillage,retention of adequate levels of crop residues and soil surface cover and use of crop rotation.This study began in 2009 when the highlands of Central Mexico suffered from a prolonged drought during vegetative maize growth in July-August,providing an opportunity for the on-farm comparison of CA with CP under severe drought conditions which 21 climate change models projected to become more frequent.Under dry conditions,CA resulted in higher yields and net returns per hectare as early as the first and second years after adoption by farmers.As an average of 27 plots under farmers’ management in 2009,the maize yields were 26%higher under CA(6.3t ha~(-1)) than under CP(5.0 t ha~(-1)).2010 was close to a normal year in terms of rainfall so yields were higher than in 2009 for both practices;in addition,the yield difference between the practices was reduced to 19%(6.8 t ha~(-1) for CA vs.5.7 t ha~(-1) for CP).When all the 2009 and 2010 observations were analyzed in a modified stability analysis,CA had an overall positive effect of 3 838 Mexican Pesos ha~(-1)(320 $US ha~(-1)) on net return and 1.3 t ha~(-1) on yield.After only one to two years of adoption by farmers on their fields,CA had higher yields and net returns under dry conditions that were even drier than those predicted by the analyzed 21 climate change models under a climate change scenario,emission scenario A2. In the subtropical highlands of Central Mexico, where the main crop is maize (Zea mays), the conventional practice (CP) involves tillage, monoculture and residue removal, leading to soil degradation and unsustainable use of natural resources and agricultural inputs.Conservation agriculture ( CA) has been proposed as a viable alternative in the region, based on reduction in tillage, retention of adequate levels of crop residues and soil surface cover and use of crop rotation. This study began in 2009 when the highlands of Central Mexico suffered from a prolonged drought during vegetative maize growth in July-August, providing an opportunity for the on-farm comparison of CA with CP under severe drought conditions which 21 climate change models projected to become more frequent.Under dry conditions, CA resulted in higher yields and net returns per hectare as early as the first and second years after adoption by farmers. As an average of 27 plots under farmers’ management in 2009, the maize yields were 2 6% higher under CA (6.3 t ha -1) than under CP (5.0 t ha -1). 2010 was close to a normal year in terms of rainfall so yield were higher than in 2009 for both practices ; in addition, the yield difference between the practices was reduced to 19% (6.8 t ha -1 for CA vs.5.7 t ha -1 for CP). All of 2009 and 2010 observations were analyzed in a modified stability analysis, CA had an overall positive effect of 3 838 Mexican Pesos ha -1 (320 $ US ha -1) on net return and 1.3 t ha -1 on yield. After only one to two years of adoption by farmers on their fields, CA had higher yields and net returns under dry conditions that were even drier than those predicted by 21 climate change models under a climate change scenario, emission scenario A2.