Which number should replace the question mark?

A farmer has three sons. One day, his oldest boy comes to him and pleads with him that he is graduating from school and would really like to get a car. His father says, "Son, come with me." He takes him to the barn and points to the farm tractor and says, "That tractor is needed here on the farm and I promise that as soon as it's paid for, we'll get you a car." The boy was not too happy but he did understand that situation and said, "Okay, Dad."A week later, his second son (10 years old) approaches him wanting a new two-wheel bicycle. Well, he gets the same excuse ... "as soon as that tractor is paid for..."
Shortly, a few days later, son no. 3, his youngest, comes bugging him for a tricycle. Again, 'ol Dad gives him the lecture about the tractor being paid for first.
While leaving the barn, the young boy, more than a little disgusted with the whole thing, sees a rooster mating with one of the hens and promptly goes over and kicks the rooster off the hens back,mumbling to himself the whole time.
His dad says, "Son, why on earth would you do something like that? He didn't do anything to you to deserve that!"
The little boy looks Dad right square in the eye and says,"Hey, nobody rides anything around here until that damn tractor is paid for.