Here's my transcription from the sound sample at http://www.brewgrass.com/sounds.html

Well, I wake up in the morning when the sun comes out,
Grab a cup of coffee and some food for my mouth,
Get the milk bucket, and away I go,
Down to the barn to see sally-o.

Down to the barn to see sally-o.
She's my favorite girl. Oh, don't you know?
Her hair is brown and her eyes are too.
She's so pretty and she gives milk too.
She's so pretty and she gives milk too.

As sung by Gary Brewer and the Kentucky Ramblers on "Money to Ride the Train," Copper Creek CD 129, 1996.

I've found references to 2 other recordings called "Sally O" but they are both Irish, and I don't know if they're about cows.