I have two full blood female healers and a full blood female pit (non hunting dogs) and the heelers catch hard they love the pigs and my pit she looks and walks away and lays down

Yea I know them not acting a fool on the lead is good but I figured they would do something with a pig literally few feet away but they just sit there an do absolutely nothing lol.  An on the fence as soon as they started ripping on the fence I put them on leashes and walked them away. 

First off I'm new to having my own dogs.  I have two 5 month old pit pups both seen there first pig a few days ago and they tore that sucker up.  Raised hell an caught the ears both hold on like champs and the pig is prob twice there size.  After some research on here and reading how others trained puppies I decided not to let them catch it anymore till there adult teeth set in with that being said I tried to put them by the hog pen to get them excited they tear the fence up trying to get in so I got in the pen with them on leashes and they won't do anything on the leash I mean nothing but as soon as I let them off the leash they want that pig bad!  How do I continue training without them hurting therselves by biting the fence cause the leash is a no go