On May 20, 2014 — Mike Hessman of the Toledo Mud Hens connects for his 400th minor league homer, against Scott Diamond of the Rochester Red Wings in an International League contest. Hessman is only the 8th player to reach the mark, and the 4th to do it entirely in U.S.-based leagues, as the top four minor league home run hitters of all time all spent the bulk of their career in Mexico.

Source:
Baseball Reference May 20