oksewglad

I was waiting :) A full explanation from Wikipedia.
A cow magnet is a preventive veterinary medical device for cattle. Cow magnets are strong alnico magnets about 1cm by 8cm (0.4 by 3.1 inches) in the shape of a smoothed rod.
A rancher or dairy farmer feeds a magnet to the animal; the magnet settles in the rumen or reticulum and remains there for the life of the animal.
When the cow grazes, it often consumes and swallows what is called tramp iron: baling and barbed wire, staples, nails, and other metallic objects. These objects are indigestible and would lodge in the reticulum and cause inflammation resulting in lower milk production (for dairy cattle) or lower weight gain (for feeder stock). This condition is called hardware disease.
The cow magnet attracts such objects and prevents them from becoming lodged in the animal's tissue. While the resultant mass of iron remains in the cow's rumen, it does not cause the severe problems of hardware disease. Cow magnets cannot be passed through a cow's 4th bonivial meta-colon.

And there you have it. (Cows have four "stomachs" which is why it is called a ruminant animal.)

Sorry QuiltE, didn't see your post! Your explanation better than Wiki!!! We give all our cows a magnet after they calf for the very first time (aka first calf heifers). We keep a record of each animal that's been given a magnet. And yes the compass trick ususally works. Works better when the GK put the compass back after they have been playing with it!! :roll: