So what's a cow magnet, for those that don't know?
A rancher or dairy farmer feeds a magnet to each calf at branding time; 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 as a pseudobezoar (an intentionally introduced bezoar), it does not cause the severe problems of hardware disease. Cow magnets cannot be passed through a cow's 4th bonivial meta-colon. (Wikipedia)
Since cows have magnets inside of them, this was offered as a theory as to why the cows, according to one group of scienitists, seem to align themselves so poetically, as noted in the above mentioned link:
And, as happens in these studies, another group came along and disputed the first study, which was disputed right back by the first group....a group of guys with Biology and wildlife backgrounds were apparently sitting around looking at pictures taken by Google Earth, when they noticed that there seemed to be a pattern in the way some cows in a pasture aligned themselves. After looking at more pictures, a larger pattern began to emerge. Oddly enough, the cows seemed to be aligning themselves with the Earth’s magnetic field lines. The group, led by Hynek Burda wrote up a paper describing what they’d found and had it published in the Proceedings of the National Academy of Sciences.