A simulation model to predict body weight gain in growing steers grazing tropical pastures Beatriz Tobias C. a , Germa ´n D. Mendoza M. a,c, * , Emilio Aranda I. a , Sergio Gonza ´lez M. a , Enrique Arjona S. b , Fernando Plata P. c , Luis Vargas V. a a Programa en Ganaderı ´a, Colegio de Postgraduados, Km 36.5 Carretera Me ´xico – Texcoco, C.P. 56230, Montecillo, Mexico b Programa en Estadı ´stica, Colegio de Postgraduados, Km 36.5 Carretera Me ´xico – Texcoco, C.P. 56230, Montecillo, Mexico c Departamento de Produccio ´ n Agrı ´cola y Animal, Universidad Auto ´ noma Metropolitana, Xochimilco, Calzada del Hueso 1100, C.P. 04970, DF, Mexico Received 19 July 2004; received in revised form 23 September 2005; accepted 28 November 2005 Abstract A simulation model was developed to predict performance of growing steers grazing trop- ical pastures. The model is deterministic and integrates the effects of protein and energy intake from forages and supplements. Protein deposition was predicted by the PDI system. Energy balance was based on the California Net Energy System with modifications for ani- mal activity and heat stress. Three output results of average daily gain (ADG) were obtained with the model: from energy (ADGe), from PDIN (ADGpdin) and from PDIE (ADGpdie). The model was validated using data from five experiments (29 treatments) conducted in the humid tropics of Mexico. The model did not accurately predict weight loss in steers based on energy or protein intake. Estimation of ADG from energy intake was successful in only six treatments, under-estimating gain by 30% with ADGe. Gain was predicted by ADGpdin in only one treatment, showing a general under-estimation of 40%. Values of ADGpdie were similar to the observed values in 10 treatments, under-estimating gain in 70% of the 0308-521X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.agsy.2005.11.006 * Corresponding author. Tel.: +52 595 95 2 02 00; fax: + 52 595 95 2 02 79. E-mail address: gmendoza@colpos.mex (G.D. Mendoza M.). www.elsevier.com/locate/agsy Agricultural Systems 90 (2006) 99–111 AGRICULTURAL SYSTEMS