Cows, being descended from some primal herbivore that had to be on constant alert from predators on open grassland, eat rather quickly and with little discrimination. They will consume large quantities of grass or hay without chewing it. The food ends up in the first stomach or forestomach and the cow will sit around later and patiently 'chew its cud' by regurgitating the contents of the forestomach back into its mouth.
One problem with this method of eating is that most pastures, especially after windstorms, are littered with stray pieces of metal: nails, bits of baling wire, staples and other ferrous materials also referred to as tramp iron. Cows fed a steady diet of hay are even more subject to this problem as the hay harvester chops up the metal into very small pieces making it easier for the cow to ingest. Some hay harvesters have magnets to collect this debris, but they are sometimes missing or improperly installed. The metal the cow consumes tends to accumulate in the folds of the reticulum and rumen, the two chambers of the forestomach, causing irritation and infections and if passed further, can pierce the stomach walls. Metal can travel beyond and pierce the diaphragm and even lead to death. Cows suffering symptoms of Hardware disease, as it is commonly called, develop a characteristic hunched over stance that is clearly identifiable. The cow loses her appetite and decreases her milk output (dairy cows), or her ability to gain weight (feeder stock).
The solution to this problem is simple and ingenious;
farmers feed the cow a very strong
magnet. The two to three inch
cylindrical magnet is delivered into the cow’s stomach via a
balling gun, the same device that is used to pill the cow. The
magnet stays in the stomach, usually for the life of the cow, and attracts all the stray bits of ferrous metal and stops it from traveling any further. Metal also tends to lie flat against the magnet and hence has less of a chance to perforate anything. The strong
acids in the cow’s stomach will usually attack the metals and dissolve them (somewhat) over time. A popular
patented (no. 5128644) magnet is the Ru-Master 5 which is composed of five strong ceramic doughnut magnets connected by a steel rod and capped by rounded steel ends. Because the magnets are powerful and inexpensive, they are often used in
science museum exhibits.
http://www.magnetsource.com/Solutions_Pages/cowmags.html
http://www.extension.umn.edu/extensionnews/1998/JO1072.html.