Actually, they do work. Growing up on a farm, I helped my Dad administer many a magnet to cows with what he termed "hardware," meaning the dumb #@%&^ swallowed something she wasn't supposed to. For the most part, they perked right back up and went on their merry way with nothing more than a magnet.
BTW, whoever said the magnets were a couple of centimeters long... They're more like 3" long, about the same size as a large animal pill, if you've ever seen one. Strong too. We've found many a lost metal part in the cow feed, in the grass, or in the gravel driveway by sweeping the area with one of these little marvels.