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Rascal Flatts' J.D. Rooney My DUI Bust ... Oh Deer, Lemme Explain!!!

Joe Don Rooney -- lead guitarist for the country music trio Rascal Flatts -- allegedly told cops a wild story about what led to his DUI bust. Literally, wild.

According to a police affidavit, obtained by TMZ, cops say Rooney told them he was driving his BMW in Williamson County, TN back in September at around 4 AM when a deer ran out in front of him.

Cops say Rooney blamed the deer for causing him to veer off-road and crash into some trees, which deployed his airbags. Cops say Rooney smelled like booze ... and that he told them he played golf earlier before going to a friend's house for drinks. The officer says he admitted to "one more than I should have."

What's more ... cops say Rooney bombed several field sobriety tests. As we reported ... he was ultimately arrested and taken to county jail where he was booked on a misdemeanor DUI charge. He was later released after posting $2,500 bond.

Rooney -- a founding member of the group -- is due back in court next month.

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