Bull nurses on cow...

So a little back story. Last year I had posted a question about a bull throwing crappy calves being Grounds For culling. He wasn't really the friendliest bull, he had horns, and he nursed on an old Hereford cow that we have. So this last fall he got a one way ticket to vacation. And then this December I bought a nice shorthorn bull little less than 2 years old. This morning leaving for work I look in the pen and I noticed now he's nursing on that Hereford... I don't want to get rid of him he's a nice bull both in temperament and looks but how do I break these guys of nursing on this one particular cow. She's the last cow from my mom's herd she always gives a nice calf. So she's kind of sentimental and she pays for herself. I've heard putting a ring in the Bull's nose might help. Any suggestions? Having this happen with two bulls on the same cow it's kind of bizarre to me.

Bull nurses on cow...