Software systems are continuing changing because of new functionality has to be incorporated or the exiting one has to be changed. Systems have to be well evolved otherwise the will not be easily changed. Aspect orientation provides mechanisms to evolve and maintain systems by encapsulating systems crosscutting concerns in aspects. We have developed an object oriented grazed-based beef cattle simulator which has maintenance and evolution problems. For this reason, in order to take advantages of the aspect orientation we present in this work the migration of the simulator to the aspect orientation. The crosscutting concerns identified in the object-oriented system are presented and how these crosscutting concerns are transformed into aspect in the target aspect-oriented system.