The model developed predict the influences of many parameters on pig growth for the growing and fattening periods. The impact of the feed composition can be simulated on N and P excretion. The simulation results showed that for the growing-finishing period, the multi-phase feeding system ( a mix of 16% and 13% crude protein diets adjusted every week to satisfy the pigs’ requirements according to their growth stage) could reduce the N excretion by 15% compared to the two-phase feeding system (one feed for the growing period and another for the fattening period).