We use One Shot. They took the One Shot/Ultra combo off the market here. We give the Ultrabac 8 separately. The vaccine works very well, but it can give a pretty good lump if you accidentally get it in the muscle.
We do calves as young as a month old with the whole Pfizer program, and it works just fine. Very seldom have sick calves at weaning. In fact, now that I think about it, we haven't had one since we went on the program, other than calves that were born in the pasture and didn't get spring vaccinations.
We wouldn't be without it.