I found an article on JSTOR titled The Antebellum South: What Probably Was and What Should Have Been by Stanley L. Engerman. Much of the piece discusses problems that the author has with previous work on the topic, and discussions of the efficiency of Southern farms and the affects of slavery on agriculture. The author also states that obtaining hard miners is difficult because no records remain from distribution by certain means such flatboat shipments and overland trade.
However, he does state that at least parts of the South were “deficit areas in grain.” Apparently 1860 was a bad year for grain production in at least parts of the South. He also writes “That’s why the possibility of regional self-sufficiency is indicated by the “better estimates,” they remain substantial room for interregional flows in both types of commodities (as well as in livestock on the hoof.”
So I think it’s highly likely that at least some of the food consumed by both the Southern population and its livestock came from areas outside the Southern and border states.
Perhaps more interesting though is this line:
“Regional distributions of federal receipts and expenditures by John Legler indicate that in most antebellum years federal expenditures in the south exceeded revenues collected from that region.”
Hmmm....
Doubtless true, but misleading since it was also true of every other region.
That's because, as DiogenesLamp's graphic endlessly points out, virtually all of Federal revenues in, say, 1860, came from import tariffs collected at major ports, most notably New York.
From these ports imports were shipped around the country, making it nearly impossible to say who actually paid for what.
But that never stops Lost Causers like DiogenesLamp from fantasizing that some great percentage -- take your pick, 60%, 70%, 85% -- were "paid for" by "Southern exports".
In reality, what "Southern exports" paid for was "imports" of manufactured products from the North -- commodities like woolen clothing, shoes & iron manufactures from stoves to farm implements & railroad equipment.
These Southern "imports" contributed to the South's roughly 15% (that's 15%, not 50%, much less 80%) of the U.S. 1860 GDP of roughly $4.4 billion, and so 15% was the Federal loss of revenue in 1861, after secession.
But Southern propagandists in 1860 and Lost Causers today insist that "Southern exports" we're vastly more important before 1861 than they were after, say, 1866.
They weren't.