Legend has it that sometime toward the end of the Civil War(1861—1865)a government train carrying oxen traveling through the northern plains of eastern Wyoming was caught in a snowstorm and had to be abandoned. The driver returned the next spring to see what had become of his cargo. Instead of the skeletons he had expected to find, he saw his oxen, living, fat, and healthy. How had they survived？
The answer lay in a resource that unknowing Americans had trampled underfoot in their haste to cross the "Great American Desert" to reach lands that sometimes proved barren. In the eastern parts of the United States, the preferred grass for forage was a cultivated plant. It grew well with enough rain, then when cut and stored it would cure and become nourishing hay for winter feed. But in the dry grazing lands of the West, that familiar blue joint grass was often killed by drought. To raise cattle out there seemed risky or even hopeless.
Who could imagine a fairy-tale grass that required no rain and somehow made it possible for cattle to feed themselves all winter？ But the surprising western wild grasses did just that. They had wonderfully convenient features that made them superior to the cultivated eastern grasses. Variously known as buffalo grass, grama grass, or mesquite, not only were they immune to drought; but they were actually preserved by the lack of summer and autumn rains. They were not juicy like the cultivated eastern grasses, but had short, hard stems. And they did not be cured in a barn, but dried right where they grew on the ground. When they dried in this way, they remained naturally sweet and nourishing through the winter. Cattle left outdoors to fend for them selves thrived on this hay. And the cattle themselves helped plant the fresh grass year after year, for they trampled the natural seeds firmly into the soil to be watered by the melting snows of winter and the occasional rains of spring. The dry summer air cured them, much as storing in a ham cured the cultivated grasses.
What does the passage mainly discuss？

A. Western migration after the Civil War
B. The climate of the western United States
C. The raising of cattle
D. A type of wild vegetation