Al,
We have had a sever infestation of voles and mice the last 1.5 years. There are hundreds of vole holes scattered around the property plus deer mice and other rodent species are out of control. In an attempt to prevent further problems inside the house as the wife does not like furry critters scampering around her house, all food sources have been removed near the house, thus no feeders with seed or grains.. a sad state of affairs..
This last fall I used 35 lbs of zinc phosphide based poison around the property, in vole holes, under out buildings, and in gopher tunnels...I do not think it made much of a difference as when the snow has melted, tunnels and rodent sign is everywhere. Then again if I had not used it I might be up to my ears in mouse poop... The local farmers are getting weary of the nuisance and the damage.
In the placement of poison I made an attempt to limit damage to no target animals by placing the baits in tunnels or under buildings and such.
Matt