The National Wildlife Federation yesterday offered to foot the bill if ranchers near Yellowstone National Park will vaccinate their cattle against brucellosis. Bison are sometimes carriers of the disease, and when bison wander out of the park they are often shot out of fear that cattle will be infected. No case of Yellowstone bison infecting cattle has ever been documented, but in late 1995 the Montana Department of Livestock shot and killed more than 1,000 bison that wandered out of Yellowstone in search of food.