Montana, Big Sky Country
One of the most naturally beautiful states I've visited. It's contribution to agriculture is mostly cattle ranching. As the land is very arid, it was never very suitable for crops and even when the government was offering free land via the Homestead Act of 1862, they had very few takers. The 4th largest state, but 3rd for density, it averages 7 people per square mile. Basically it's real easy to go for miles without seeing another person, or even buildings at times.
With a population that sparse it's almost hard to include Montana as a place seeing decline, since it can be argued that they never really had a boom in farming. However, even out here since the days of the gold rush, ranching & wheat are still a main stay of the state economy.