Pretty much every Beckton bull will work on heifers, very good blood line, Buffalo Creek had a few big players as well, goes back to Cherokee.  Used both in natural service with no major problems.  Can't remember who the main ones are that ABS is pushing currently, mainly stay with a high accuracy 80%+ and a good 10+ CED  BW below 0 and you should still be able to get the good WW and YW's out of them by going AI.  Think most the AI guys are also rating heifer bulls with stars or some other measure, can usually be fine staying with the best rated ones but may need to watch the accuracy.