Christopher Columbus brought some cattle with him when he discovered the New World, and some of them escaped and ran wild as early as 1493. They ran wild on the Texas plains and multiplied for about four centuries. They were massive herds by the time that Jim and his team of cowboys discovered them and started trail breaking them to herd them north to Abilene.
A team of cowboys who were veterans of the Civil War forms up and decides to go to West Texas to gather some wild horses. They do so but notice the massive herds of wild longhorn cattle roaming the range. After saddle breaking the horses, they decide to go round up a herd of wild cattle, trail break them, and drive them to market up at Abilene.