A Decision Support System for smallholder campesino maize–cattle production systems of the Toluca Valley in Central Mexico. Part II— Emulating the farming system Octavio A. Castela´n-Ortega a, *, Roy H. Fawcett b , Carlos Arriaga-Jorda´n a , Mario Herrero b,c a Centro de Investigacio ´n en Ciencias Agropecuarias of the Universidad Auto ´noma del Estado de Me ´xico, Instituto Literario No 100, C.P. 50000, Toluca, Mexico b Institute of Ecology and Resource Management, University of Edinburgh, West Mains road, Edinburgh EH9 3JG, Scotland, UK c International Livestock Research Institute (ILRI), PO Box 30709, Nairobi, Kenya Received 23 October 2000; received in revised form 12 April 2001; accepted 13 September 2001 Abstract This paper describes the functioning and validation of the Decision Support Systems describedinthefirstpartofthepaper.TheDSSranthreecasestudieswithdifferentfarmsizes that represent the range of farmers found in the Valley. The DSS results were validated against survey data for the same cases. Traditional technologies for maize and milk produc- tion were reproduced by the DSS including land use and cattle feeding systems. The generic nature of the DSS was demonstrated as well as its capacity to deal with the system’s socio- economic and biological aspects. The results suggest that the DSS was successful in reprodu- cing the functioning of the farming system’s main components. More importantly it simulated the complex interactions observed between the farmers and their crops and cattle. Finally, it is acknowledged that despite the size and complexity of the DSS, it only was able to emulate the functioning of the main components of the farming system. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Model validation; Campesino farmers; Traditional technologies; Cattle feeding systems 0308-521X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0308-521X(01)00110-X Agricultural Systems 75 (2003) 23–46 www.elsevier.com/locate/agsy * Corresponding author. Tel.: +52-722-29-65552; fax: +52-722-2-156-489. E-mail address: oaco@coatepec.uaemex.mx (O.A. Castela´n-Ortega).