"Yellow-hammer cows" are not specifically animals of the bovine species, but rather a reference to the community of the world (which fits the acronym C.O.W) and how they are trying to keep up with changing technology, comparing this webpage with that webpage or this software with that software, etc.
However, the term "yellow hammer cows" that you are thinking of have come from Patrick Smith's book called "A Land Remembered" where it mentions about wild "yellowhammer" cattle, which may be in reference local name (or the author's coined name) for the Florida Cracker/Pineywoods/Scrub cattle that live[d] there in Florida.
Check out the related links below for more.
The yellowhammer is the official state bird in Alabama.
Alabama.
Yes. It is the Yellowhammer.
The Yellowhammer State and the Heart of Dixie.
Heart of dixie, "Yellowhammer State"
A yowley is a dialect term for a European yellowhammer.
Alabama
Wild "yellowhammer" cattle is a fictional reference in Patrick Smith's book which is called A Land to Remember. There is no breed commonly recognized as yellowhammer.
A Yellowhammer
Florida
The yellowhammer is, of course, yellow.
The Yellowhammer is the state bird of Alabama.