What happened as more land in the U.S. was placed under cultivation after 1865?

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.

For
example:

• 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.