Karin wrote at 2012-12-25 19:20:34
Magnets are not just for those cows that are grazing out on pasture where there is a lot of metal pieces that can get in a cow's stomach. A lot of folks (mostly dairy producers) give their cows magnets because the feed may have metal pieces that have been picked up or broken off from the machinery that were used to harvest or store feed.
One magnet weighs around 5 lbs, and they do come in two sizes, a smaller one for younger cattle, and larger ones for mature cows and bulls. The magnet has a VERY powerful magnetic field, strong enough to collect a lot of metal bits and keep them in place throughout the cow's lifetime. The metal does break down with the gastro-intestinal juices from the cow's digestive tract. Some cows need to be given subsequent magnets because one magnet can fill up with so much metal that it cannot catch anymore, and another magnet is needed to catch the metal that the initial one couldn't.
Cows don't eat magnets, they have to be given orally, (some may call it "forcing" a cow to take a pill, sorta like you have to force pills down your dog's or cat's throat), like what Becky mentioned, with a bolus gun.
Note though, that a magnet is not necessary if you are grazing your cattle in a clean, metal-free pasture and feed that is estimated to be metal-free. It's also not necessary if you are adamant about going out to the corral or the pasture and picking up any metal that is found and disposing it in the proper place. That's what I've done plenty of in the drylot where we kept our stocker steers over winter, and the incidences of foot-rot went down to being non-existent.