Results of preliminary experimentation with a computer-based simulation model of beef production in the Eastern Savannas of Colombia are reported. The energetic consequences of various uses of sown pastures are examined with reference to their long-term biological feasibility. The quality of grass-legume mixtures currently under evaluation at the International Centre for Tropical Agriculture is such that the availability of energy in the extensive animal production systems is limiting during the dry season, as it is for traditional savanna-based production systems. A number of options are identified which appear to ameliorate this constraint. Dual-purpose beef and milk systems based on the use of sown pastures are viable and productive owing to the increased availability of energy in the wet season.