english mastiff?

My wifes house dog is an english mastif and i thought it would be fun to get her to catch the hard part was keeping her from killing the rest of my pigs be care full once she started it was all over from there. I have spent the last five years trying to get her to quit catching. To this day if she get down to the bay pen she still trys to catch. Mind you this is my wifes cuddle dog and my kids ride her and pull her ears and she stilll catches. But she wont even defend here self with the kids picking on her all i can say is she is a great dog we love her and she does not go to the woods but that does not mean that she would not go.