Casting one's withers

Do you know the meaning and origin of "casting one's withers"?

I'm curious about the context you've heard this used in. "Casting withers" is an expression that means a prolapsed uterus. Sometimes, when a cow (or a sheep) give birth the womb is turned inside out and forced out through the birth canal. She is said to have "cast her withers". Farmers have their own unique language. If it's used metaphorically, I can't imagine what is being meant....Yikes!