You'd have a better chance at getting the answer you want if you could be more specific -- what happened to who or to what? As it is, there are so many different ways you could answer this.
- As more land in the West came under cultivation, farming in the East became less profitable and tended to decline.
- More people moved to the West in search of cheap land and a new life.
- The Native Americans were pushed off their ancestral lands and on to reservations.
- Big businesses came to own more of the farm land and small farms started to decline.
- The railroads became more important because farmers needed them to get their crops back to Eastern markets.
All of these are impacts of the opening of more land to cultivation.