Fortunately this is knowable because sticking magnets in cows is something that we&#x27;ve done for many, many years.  So we know what it costs and how effective it is.<p>Whereas a robot to solve the problem doesn&#x27;t yet exist.  And the important bit here is to realize what the word &quot;solve&quot; means here.<p>I&#x27;m not saying a workable solution will never exist, but in order to be as effective it would need to be orders of magnitude better at its job than a roomba is at its job.  And what the roomba does isn&#x27;t very hard to begin with.  You would have to cover a very large area of sometimes pretty rough terrain every N days, and you would need to have a very high success rate at both detection and removal for the solution to be able to compete with the established practices.<p>To understand the problem I can only recommend spending some time on a farm where they keep grazing cattle.