Originally, "Mess Call" was a bugle call sounded for breakfast, lunch and supper. Somewhere along the way people started singing lines, such as --
"Come and get your beans now,
Come and get your beans."
or
"Come and get your chow now,
Come and get your chow."
Later, it became a stadium football cheer with the added words --
Eat 'em up,
Eat 'em up,
Rah, rah, rah!
or
Eat 'em up,
Eat 'em up,
Raw, Raw, raw!