Information Below is from Professional Hop Grower Lynn Kemme:

The first largest bines to emerge on 2nd year or older hop crowns are not necessarily the best to train. These "bull" shoots have a large hollow core, like a straw; and easily kink or are damaged by late spring storms. Most crowns put out two to three of these, at the most; and crews can be trained to identify them and prune them out as they twirl climbing bines. "Bull" shoots are often a light olive green color with stretched internodes - some field practice clipping a few will reveal which have the hollow stems.  

More Info. By Steve Schmidt

Using the "Bull" Shoots will lower your harvest yield due to the stretched internodes.  With stretched internodes you will get less side arms, by using the 2nd shoots that come out you will have less internode spacing and that in turn increases the number of side arms and increases your harvest yield.