Dog urine killing grass. Help!

We have two female dogs that are killing our new sod. I know qe could take them out on a leash and take them to a good place to go to the bathroom but that is not going to happen. We want them to be able to go outside any time that they want. That is why have a dog door so that they can go in or out as they please. We have a big back yard and they are causing brown spots (dead grass) everywhere. Please help. Thanks!