"Cow Viagra" joke

Farmer: I've got a bull that's right off it duties. It's got to service
300 cows and all it wants to do is eat.
Vet: Give it one of these little pills in its feed and stand back.

So 2 weeks later the farmer comes back to the vet:

Farmer: WOW, what a pill! I gave the pill to the bull like you said and
POW! It jumped over the gate, ran down the lane and fucked 70 cows in
30 minutes.
Vet: So, what's the problem - why have you come back?
Farmer: Well, I was wondering, I am meeting this 18-year-old tonight -
could you give me one of those tablets? I'm not as young as I was.
Vet: Oh, no! Sorry, it's too strong but I will give you a quarter of a
pill.

So the farmer takes the pill and goes off to prepare for his date.
Several days later, the farmer goes back to the Vet.

Farmer: Hello, Vet. Wonderful. 40 Times.
Vet: So, why have you come back?
Farmer: Ah! I need something for my wrist - she never showed up!