I vaccinate all our heifers with Electroid 7 (Blackleg) and Virashield 5 + VL5 (IBR, BVD, PI3, vibrio, lepto, etc) at weaning time, or 6-7 mos old. They get 5 cc of each vaccine, one on left side of neck, and the other on the right side, given SQ. They are then taken to the vet for their OCV (bangs). Before I turn them out of the weaning pen (after 4 weeks), they get a booster of the blackelg and the Virashield (same dosage as the first round).
From that time on, they get an annual vaccination of the blackleg and Virashield, along with the rest of the herd.