Here&#x27;s a BBC article from a Tiverton farmer saying the magnets sometimes come back up.<p><a href="http:&#x2F;&#x2F;www.bbc.co.uk&#x2F;news&#x2F;uk-england-devon-12549492" rel="nofollow">http:&#x2F;&#x2F;www.bbc.co.uk&#x2F;news&#x2F;uk-england-devon-12549492</a><p>I guess serial numbering would help get the magnets back to the right cow.  But it doesn&#x27;t matter if one or two come back up - you&#x27;re aiming at &lt;st&gt;herd immunity&lt;&#x2F;st&gt; reducing the number of cows affected. Stopping most cases is good enough.<p>(Edit because omginternets is right)