Rodenticides are never a good stand alone solution for an established resident rat population in most cases. An aggressive trapping program is best initiated first with several devices of presentation to keep things looking different. Then after a solid removal program you can then go in and clean out the remaining rats with versatile baiting of some kind.
I am sure there is some kind of food source local to the implement shed or the rats wouldnt be there. Find that source and the routes to and from and set them all heavy and secure them.
Making larger,secure feed boxes from plywood top and bottom with rat size door openings and secureing an edible bait in the center interior of the bait boxes to feed/reward them for a week. Then go back and rebait with the same edible bait, add the ZP/tracking powder on this trip to the bottom of the bait box in line with the door openings leading back to the edible bait holder.The plywood floor will keep the powder dry and they should feed aggressively the first nite on the new baiting thus giving all visitors a lethal dose of ZP with which to groom that nite.
Move the bait box around as needed to other activity areas once one location appears cleaned out, or make enough boxes to cover the other positions as well.
A good norway rat population needs a few implemented methods to really make an effective impact or it will continue to flourish.