Retread7 hit the nail on the head. The magnetic sensor is immune to critters walking by but will detect a car or truck driving by, even going slowly.
We have the earlier version of the one he linked to and it has worked well for a number of years. You can get them for less than the price on the manufacturer's web page if you look around on the web.
Also - they claim the cable is "direct bury" but the insulation is not armor. You will thank yourself later if you put the wire in plastic conduit when you bury it. If the wire gets compromised you can indeed get false alarms when a cow (anything heavy) walks on it when the ground is wet.