I would like to switch over to MLV instead of killed. Looking at the Bovishield 5 VL5. Directions say for primary dose administer one dose to breeding cows and heifers approximately 1 month prior to breeding, followed by another dose 3-4 weeks later.I always thought one benefit of MLV over killed was only one dose was needed? I am guessing if MLV was used on heifer calves before or at weaning, then only one dose is needed a month before breeding? Just a little confused, thanks.