7th World Congress on Genetics Applied to Livestock Production, August 19-23, 2002, Montpellier, France APPLICATION OF A BIO-ECONOMICAL MODEL TO ESTIMATE ECONOMICAL WEIGHTS FOR TRAITS USED IN SELECTION INDEX IN BEEF CATTLE R.C. Brumatti, J.B.S. Ferraz, I.B. Formigoni and J.P. Eler Grupo de Melhoramento Animal, FZEA/ Universidade de São Paulo, Cx. Postal 23 CEP 13.635-900 Pirassununga, SP, Brazil INTRODUCTION All productive activities should have, as basic premises, the search of efficient results in production, technically in economical terms. One cannot imagine a productive and business activity that does not have as objective the search of results economically solid and convincing. Therefore it is comprehensible that the administration of the rural property comes to get relevant importance and, according to Lôbo et al. (1999), to increase the beef cattle production efficiency in Brazil, one necessarily has to go through improvement of the genetic quality of the herd. According to Euclides Filho (1999), it is important to emphasize that for selecting individuals, the first task is to obtain good estimates of the genetic values of the animals that are available for selection. That is the starting point to develop a selection index, which, according to Eler (1999), is a method that looks for the establishment of relationships among the more important productive traits, in such a way that in the use of the index, a number that will be the estimate of the individual's genetic value shows up, and based on that number, the animals will be ranked. However the choice of the characteristics that will compose the index, takes into account the economical importance, the variability and the heritability of each trait, besides the genetic and phenotypic correlations among them. Although several methodologies have being used among the animal geneticists, the characterization of the selection objectives is fundamental for the development of the referred selection programs (Ponzoni and Newman, 1989). According to Harris (1994) positioning the selection objectives in the mathematical form, in a complete economical base, is the key to integrate the development of the animal selection in industrial programs and goals. According to Ponzoni and Newman (1989), the selection objective is defined as the combination of traits economically important for beef cattle in a specific production system and the selection criteria are the characteristics used in the estimate of the values of production of the animals. So, selection criteria will be influenced naturally by the traits included in the production objective. Doing the opposite could eliminate characteristics economically important (Ponzoni and Newman, 1989). For application of the selection indexes it is necessary that the genetic parameters of the characteristics and their respective economical values are known (Weller, 1994). The first step to estimate economical weights is the definition of each genetic parameter and the incomes and costs of the production system in a fixed period of time (Bittencourt et al., 1998). Weller (1994) pointed that the profit estimate can be quite complex under certain circumstances and that it will depend on the characteristics under selection and on the market forces, mainly in relation to future prices. However, that author presented two main methodologies to obtain the economical weights : a) the use of profit equations and b) the use of simulations with bio- Session 23. Design of breeding programmes Communication N° 23-19