I agree with what you said, and can add a little more context to the subject.<p>I have a little experience with this subject. Some of my family once a month drive through grass fields to harvest hay which is chopped and delivered to dairy farms in the area. They range from several hundred heads to ~10k heads.<p>The fields aren&#x27;t owned by the dairy farms, they&#x27;re owned by field farmers and the hay is sold to the dairy farms. So the field farmers who already use old machines have no money, and no incentive to purchase and outfit a drone to comb their fields for metal.<p>So once a month the fields would have to be &#x27;combed for metal&#x27;, which is hundreds of acres spread out over different locations. However this still wouldn&#x27;t stop everything because you have tractors, equipment, semis and trailers going through the field every month chopping and loading hay. So I suppose you would need to have the magnets on the end of the dump and push the hay through the magnets to make sure you&#x27;re catching everything. This might sound complicated and cumbersome, because it is. On top of that, these trailers also have to be used for hauling corn, potatoes, manure, and everything in between. 
Even if you managed to do all that, there&#x27;s still no guarantee that there won&#x27;t be metal contamination between the time it&#x27;s offloaded and fed to the cows.<p>That was a really long way of saying, it&#x27;d be nice, but it isn&#x27;t practical.