MORGAN HILL, Calif. — In the half-light of a winter evening here, a tawny calf skitters across the pasture after its mother, a Lazy T brand visible on its right hip. The brand, used by the Tilton Ranch since Janet Burback’s parents settled on this land in 1917, appears on the ranchers’ shirts, their trucks and their business cards.

To Ms. Burback, the brand is a matter of pride and tradition. “Anybody who’s still branding their cattle, that’s the last hold on something their grandparents and great-grandparents started,” she said.

But it is also a matter of necessity. When a cow strays or falls into the hands of rustlers — still a significant threat — it is the brand she counts on to bring the animal home.

So, like many other ranchers in California and other Western states, Ms. Burback looks with suspicion on a federal plan to institute an identification system for cattle, one that emphasizes numbered ear tags rather than brands as the official markers of a cow’s identity. Ranchers worry that the new regulation, in the final phase of revision, represents a first step toward ending branding, a method they regard as the most visible, permanent and reliable way of identifying who owns which cow.