<p><pre><code>    ...it really is much simpler to just stick a magnet in every cow.
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Is it though? If you have twenty cows, sure, I can believe that twenty magnets are simple and cheap - but what about 200? 2,000? And what happens when one of your cows turns out to have a larger than usual pyloric sphincter, and the magnet gets into its intestines? Or a wider than standard trachea, leading to it regurgitating the magnet when it tries to chew the cud?<p>Essentially what I&#x27;m envisaging is an outdoor Roomba that just gets left to do its own thing until it needs human assistance. Arable farming - at least at the larger end of the scale - is getting pretty comfortable with drone equipment, with driverless combine harvesters guided by GPS and similar things; livestock farmers are not going to be averse to it either (not least because farmers, in my experience, really dig flashy toys! :) ).