THE INSTRUCTIONAL ROUND-ROBIN IFR FLIGHT HAD CANCELLED IFR ON ITS FINAL LEG. THE PILOT-IN-COMMAND INSTRUCTOR EXCHANGED SEATS WITH THE RATED PASSENGER PILOT OCCUPYING THE RIGHT REAR SEAT. AFTER 3.5 HOURS ELAPSED TIME THE RATED PILOT UNDER INSTRUCTION BECAME THE PIC AND CONTINUED SIMULATED INSTRUMENT FLYING WITH THE RATED PASSENGER ACTING AS SAFETY PILOT. THE FUEL PLANNING HAD BEEN CONDUCTED BY BOTH PILOT'S. THE ENGINE FAILED DUE TO FUEL EXHAUSTION AND DURING THE FORCED LANDING THE AIRPLANE COLLIDED WITH A COW. TOTAL FLIGHT TIME WAS 4.3 HOURS. DURING THE ROUND ROBIN, THE FLIGHT LANDED AT CORSICANA, TEXAS, BUT NO FUEL WAS PURCHASED. IN THE VICINITY OF TEXARKANA, TEXAS, THE FUEL GAUGES INDICATED ABOUT 5 GALLONS APIECE AS THE FLIGHT INITIATED TIS FINAL LEG TO ITS HOME AIRPORT. FUEL EXHAUSTION OCCURRED ABOUT 10 MILES SHORT.