First clean up feed or any feed source they are using.   Second,  BAIT,  Like Mark says get the good bait chunks.  And buy or make quality bait stations.   I prefer the T-shaped bait stations.    We/I am on a schedule rotation of 3 baits.   I use:  Hawk,  March -  August. .   Jaguar  September - December and Rampage  January - February.   Since you have a problem  start with the Rampage.   It is the most expensive but it is one bite and they die.    Be prepared to pick up dead rats.    Check the bait stations daily until you can back off to weekly and eventually to monthly.   

Now start cleaning.  No stacks of old feed or seed sacks,   clean up the junk where they are living and growing. Old junk machinery needs to hauled to the recycler.    Pick it up and then start mowing.   Start by shredding it but work to mowing it with a lawnmower.  I do the mowing of the yard and farmstead.   I've got a big Grasshopper and try to make it so there is no place for the rodents to hide.   Put gravel around machine sheds,  grain bins and any other livestock buildings.  And Spray Roundup on that gravel multiple times during the summer.     If bad enough you can give them a shot of anhydrous in their holes.  But, they might just scurry to other spots. 

Maintain  a clean farmstead.   Rotate your bait.   If the women in your operation abhor rodents as much as I do they might be willing to take up the mowing.   I do the baiting,  mowing and spraying of roundup.   Haven't seen a live rat in years!   Mice yes,  but no rats.