Energy, not protein.
Personally, I'd try raising heifers on pasture before putting steers on it unless there's freezer or special market where you're at. Since you have the pasture available, I would go with corn and a limiter and not keep them on grass too long. You'll get a big framed Holsteins and have trouble at the packer. Especially these days. Holsteins are not like angus, maintenance is 10-12% higher so to finish at a reasonable weight you'll need lots of energy.
You could ask 10 people on implants and get 11 answers. I would start lighter calves on a synovex and finish with one that has TBA (Rev S, Rev IS, or Synovex Plus.) Try finding someone that has raised Holsteins around you ask what works for them. I'd check around for a consultant that works with Dairy Beef too, a lot of them know more about Holstein Steers than most vets and will come out for free.