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Have you tried rolling one of those long magnets on wheels over the ground? Builders and Roofers use them to clean up nails after they finish a project and we periodically roll one around the area where the hooved animals are, after a good rain has come and gone. Nails seem to float to the surface of the soil after a hard rain and I am amazed we keep finding bits of wire and nails in the same area, after several years of picking them up. We also have a hand held one that we drag along the ground, but it doesn't cover as much area at one time nor is it as strong magnetically, as the one that is 3 foot wide on wheels.
ETA - they are called magnetic sweepers