I am not trying to say that Sabu is not vital, when he is. I am trying to say Johnson is more effective when it comes to mating. He has more experience being in a breeding situation for many more years and cows than Sabu has. Sabu is a proven producer because he has produced two known calves. Johnson has the experience to get around the cows, for example, I know of a certain bull that is quite proven in Europe with many calves, he would use his one tusk to dominant over the cow and mount her when he is ensured he has control, and with experience he was ablle to predict her cycle where she is ripe for fertilization (remember a cow is only fertile for a few days per cycle). It may be one reason Rudy is not pregnant, Sabu is not able to dominant over her due to different personalities. Granted they may have mated, but it may not be at the right time of the cycle. The list goes on.
Another thing: Sabu has not been in a breeding situation for +/- 20 years.