MLV for BVD, PI3, BRSV are fine. MLV for rotavirus, coronavirus, etc are fine.
IBR as a virus has the ability to destroy the ovarian CL, and cross the placenta to cause fetal death. In animals that are naive or do not have sufficient protection, vaccination with a MLV may cause disease. Doesn't appear to happen before 5-6 months of gestation.
IBR as a virus goes latent- it is a herpes virus after all- and the virus itself can reappear years later if the cow is stressed. So she's in late-gestation and a big snow storm comes through - the virus could reappear and cause disease. It's not known if the MLV in the vaccine can go latent. Differentiation between the virus strain and the vaccine strain is very difficult in the laboratory. Maybe you work your cows and 30 days later some of them abort. Was that due to the vaccine you gave or did the stress from working the cows cause reappearance of a virus she acquired naturally as a calf?
Don't get the impression that MLV as a category are bad - they're not. They tend to provide better protection than killed vaccines and sometimes are necessary to create an appropriate immune response. Don't throw out the baby with the bathwater. Brucellosis (Bangs vaccine) is a MLV and we sure wouldn't have erradicated it this far without vaccination. MLV BVD vaccines are still probably your best bet for preventing BVD-PI calves based on the vaccine trial research I've read. MLV vaccines do stimulate strong responses to common respiratory viral pathogens- which is extremely important.
I'd still advocate using MLV vaccines. I wouldn't advocate vaccinating pregnant cows with a BVD/PI3/BRSV/IBR MLV vaccine in the last half of pregnancy - but why would you vaccinate then anyway??? It's too late to protect the calf from BVD. Vaccination should ideally be done *before* breeding, in which case yearly vaccination is perfectly appropriate.
