Livestock is one of the most important sectors of the Uruguayan economy. In 2008 Uruguay ranked 7th in beef export and 4th in sheep meat export in the world. Agricultural research has generated a large amount of scientific information over the past years. However, the scientific information developed in different areas, often lacks an integrated approach in order to evaluate the productive and economic impact of different technological options, on the production system as the basic unit. In addition, the national genetic evaluation program for cattle is developing a selection index for the beef breeds used in Uruguay. To develop a selection index, the application of a systemic approach for assessing the economic weights of the variables affecting the structure of revenues and costs of livestock production systems, is essential.
This thesis develops a dynamic bio-economical model. The objective was to create a flexible tool, to evaluate the productivity and economic return of livestock systems - that apply different productive, reproductive and management alternatives- and of the Uruguayan meat chain as a whole.
The model is centrally deterministic, incorporating stochastic components in the simulation of certain events. It allows the simulation of the stages of breeding, stocking and fattening either independently or as an integrated process within a production system.
Considering technical coefficients and pasture characteristics, the model simulates processes at the individual level (feed intake, body weight change), generating a monthly growth rate, thus the evolution into the next category, which are integrated at a herd level. As a consequence, a sale volume is produced, consisting of surplus females from the breeding herd and fattened animals for slaughter. This sale volume is valued at market prices and determines the income of the system. 
The results of applying the model highlight the importance of key variables with a great impact over the whole system, such as age at first mating. The high sensitivity of all the systems to sales prices is also relevant.
Complex interactions have been detected between variables like pregnancy rate whose effect and magnitude are highly dependent on the category, culling strategies and sales prices.
Fattening is the alternative of greater economic return since it’s a biologically more efficient process. However, its potential is highly sensitive to the availability of forage resources.
Regarding the relative importance of characters capable of genetic improvement, the diversity in production systems make difficult to arrive to general conclusions. Pregnancy rate, growth rate in males and dressing percentage, appears to be the most important variables, although further research about the potential variation of these characteristics, is needed. The low cost of feeding in extensive range systems and the relevance of fattening culled cows in cow-calf systems, determine a positive economic value of characters such as feed intake and adult weight, which are reported to have a low or even negative impact according to the international literature. 
Pregnancy diagnosis and culling of barren cows still seem to be alternatives of low cost and high impact, mostly in extensive production systems.
The model allows simultaneous consideration of many variables affecting beef production systems. It can be used as a research and extension support in livestock production systems and for the evaluation of policies at a regional or national level.