Prebreeding vaccination should provide fetal and abortive protection against bovine viral
diarrhea virus (BVDV) and bovine herpesvirus 1 (BoHV-1) but not impede reproduction
when administered to cattle before estrus synchronization and breeding. The objective was
to assess reproductive performance when naive beef heifers were vaccinated with modified-
live viral (MLV) vaccine 2 days after unsynchronized estrus, and then revaccinated with MLV
vaccine at 10 or 31 days before synchronized natural breeding. Sixty beef heifers naive to
BVDV and BoHV-1 were randomly assigned to one of four treatment groups. Groups A and B
(n ¼ 20 per group) were vaccinated with MLV vaccine containing BVDV and BoHV-1 at 2 days
after initial detected estrus, and then revaccinated 30 days later, which corresponded to
10 days (group A) or 31 days (group B) before synchronized natural breeding. Groups C and D
(n ¼ 10 per group) served as controls and were vaccinated with an inactivated vaccine that
did not contain BVDV or BoHV-1 at the same time points as groups A and B, respectively.
Estrous behavior was assessed using radio frequency technology. Estrus synchronizationwas
performed, with initiation occurring at revaccination (groups A and C) or 21 days after
revaccination (groups B and D). After synchronization, heifers were submitted to a bull
breeding pasture for 45 days. At the end of the breeding period, heifers were assessed for
pregnancy using ultrasonography. Progesterone concentrations were evaluated at estrus and
10 days after unsynchronized and synchronized estrus, at initial pregnancy check, and at the
end of the study. All pregnant heifers in groups A and B and five pregnant heifers in group C
were euthanized between 44 and 62 days of gestation and ovarian and conceptus tissues
were assayed for BVDV and BoHV-1. Vaccination with MLV vaccine did not result in signifi-
cant negative reproductive impact based on the duration of interestrus intervals, proportion
of heifers exhibiting estrus within 5 days after synchronization, serum progesterone con-
centrations, pregnancy rates, and pregnancies in the first 5 days of the breeding season.
Bovine viral diarrhea virus and BoHV-1 were not detected in luteal tissue, ovarian tissue, or
fetal tissues. Use of MLV vaccine did not impede reproduction, when revaccination was
performed at 10 or 31 days before synchronized natural breeding.