About yodeling: I don't know what year Jimmie Rodgers started doing it but there was a blues style of yodeling that was popular around Jackson Miss. in the 1920's. One prominent performer who used it was Tommy Johnson (e.g. Cool Drink of Water Blues, Canned Heat Blues--both recorded in 1928) but others imitated him. Stylistically it is as different from cowboy yodeling as blues singing is from cowboy singing--which is to say, a lot different.

There was a thread on yodeling several months ago.