We've got the same problem in TN. I've got ranchers that want every one of them shot on sight, but it is illegal to kill them here. They have been seen attacking calves during birthing.
Have your ranchers reach out to USDA Wildlife Services for assistance with the black vultures.
Even though they are covered under a migratory bird treaty with Mexico, there are ways to address calf attacks and it doesn't cost the rancher a dime if Wildlife Services does the work.
For a long time the wildlife biologists denied that the black headed vultures were predatory, insisting that, like turkey vultures, they were scavengers.
In the Midwest, ranchers and Wildlife Services gathered enough evidence to show calf mortality and damage and there are now options to help with the problem.
There always has been, and still is, a process to follow if the rancher wants to do control himself, but they don't make it easy.
IMHO and experience, contacting USF&W (the folks who administer and enforce the treaty) about the problem is a waste of time.
Ed