It's a joke about ED and cows.

So there's farmer, Who is having trouble getting his cows to reproduce. No matter how he tries, they just won't get busy.

So the man needs a cow expert, and consults the foremost cow scientist in the country. After an explanation of the problem, the expert tells him he needs to stick his hands into the vaginal opening of the female cow, and smear it directly under the bulls nose.
The farmer, is hesitant, but decides to go ahead and give it a shot. And they go at it immediately. It's crazy! The farmer is stunned at just how effective the method was, and he's thinking about it later that night in bed next to his wife.

As he tries to sleep, in perfect darkness, he has an idea. You see, this particular farmer has severe ED, and hasn't gotten an erection in years. So he, remembering the cows, slowly starts touching his sleeping wife in her slot, but she remains asleep. He rubs it under his nose, and within seconds, his member stands straight up! He can't believe it! So he wakes his wife, says honey, honey look!

She turns on the light and says "You woke me up for a bloody nose?"