We've got one pasture that has countless thousands on a hillside where I have a thrower mounted. We've never worried about them or ever had a problem. Like Dun said they are biodegrade fairly quickly.
One thing that is important is picking up shotgun hulls. The cows will chew them and occasionally swallow them. I've heard they are attracted to the sulfur in the spent gunpowder. Whatever the reason, they should be picked up – especially by dove hunters who naturally concentrate around stock tanks. It's amazing how a cow can crunch up a shell, even the brass. It will be mauled and have sharp parts projecting. If she swallows it, you've got pretty good odds on having a case of hardware. Make any hunters on your place pick their shells up. They don't rot like the old paper shells and can make a place look trashy, not to mention that they can be expensive when you factor in a cow or two.
Craig-TX