Most cattle owners brand their livestock for security reasons. But in addition to just having the ownership known producers need ways to identify a specific cow. Beef packagers have to keep documentation about the origins of their cows and they require them from “downstream” cattle producers so the result is everyone in the industry needs to keep records about their cows. Most significant producers were already doing this on an individual cow basis.
The usual practice is to put a numbered ear tag on a calf and the stockman records information about that cow throughout its life with breeding information, special feeds and supplements, weight gains, shots, etc. This is in the producers self interest because it can yield valuable information about what is working and not working with the herd as different experiments are done with breeding and feed. And if there were ever a disease outbreak it would be easier to trace back.
The newest version ear tags have a radio frequency tag inside them (at a cost of $5/cow). This allows the tag to be read by a specialized reader as the cow goes by. You often see these in cattle chutes so they record the number of the cow, then a system electronically weighs it and the cowboy makes keyboard notes about shots, problems, etc. You can also place them at a gate opening to automatically count the number of cows passing – and identify which ones are not there.