F-1 is the first cross form two different pure breeds. Hereford X Angus = F-1 like Brahman X Angus =F-1. I am not near as good at this as some others on the board. Now if you Cross the F-1 Tigers with F-1 Tigers you get F-2's and so on. After F-7 you have a new breed.
That's because the F1 Brahman x Hereford set the standard... They are the F1's which exhibit the most heterosis, and seem to work best in a commercial environment in the south. Must be the Hereford enfluence, because a study done by Texas A&M with other F1 cattle showed the F1 BH to be superior...specifically compared against the F1 Angus x Brahman (BA), and other crosses.