Those are nice finds! My cows wear earrings, plastic but not cheap. That's a day's work piercing ears. Hopefully you'll find the bull's nose ring on your next outing.
I grew up on a dairy (milking 300 cows) so hopefully I know a few things about cows.
What you found is a cow tag. I have never heard them called 'cow necklaces'. (Trying not to laugh here!) If you only have 40 or 50 cows you can likely identify them by sight but once you get 100 or more cows you need tags.
Why is it important to know one cow from another? There are many reasons.
If a cow has been treated with antibiiotics (or other medications) her milk cannot be sold. If her milk gets mixed in with the milk of the rest of the herd it all goes down the drain. (Been there, done that.)
Cows are tested regularly on how much milk they produce and the quality of the milk (butter fat content). Cows that are not producing get culled.
Cows are bred when they are in heat. You can cross-check to see if it is time for a cow to be bred when she is in heat. Generally a cow is bred once a year. Assuming she is bred 3 months after calving she will have calves at yearly intervals. She startes producing milk when her calf is born. After a few days her milk can be sold. She will continue to produce milk as long as her milk is taken regularly (say twice a day) so milking a cow fools its body into thinking it is producing milk for the calf. After 9 or 10 months the cow is no longer milked and is allowed to go "dry". So she gets a couple of months of rest before getting back to work.
I don't recall brass tags like that; we used plastic tags. Later on (early to mid '70s) we started using ear tags. Ear tags are better for a number of reasons:
Won't fall off
Won't choke the cow
Easier to read (get the cow to look at you and you can see her number)
Great tag, WTG!!!
That would be a good tag to clip on your detecting belt for luck, No.1...
I like finding things like that almost as much as finding silver...
Great info also, Klaatu
Good luck, & Happy hunting~