What farming practices have depleted grasslands in North America?

Cultivation of crops and tillage practices.

Explanation:

Farming of crops and tillage practices on the grasslands depleted grassland that are present across North America because the lands are converted and used for the production of food crops in order to fulfill the feed the existing population. More and more land is converted from grassland to the cultivation of crops that leads to lower area for the grazing of animals which results in decrease population of animals such as sheep, goat and cow etc which depends n these grazing regions for food.