Serological Response and Reproductive Performance in Nellore Heifers Vaccinated Against Infectious Bovine Rhinotracheitis, Bovine Viral Diarrhea, and Leptospirosis

This study evaluated the serological profile, vaccine-induced antibody response, reproductive performance, and associated reproductive factors in Nellore heifers vaccinated against bovine alphaherpesvirus 1 (BoAHV-1), bovine viral diarrhea virus (BVDV), and Leptospira spp. in a commercial herd in Central Brazil. Heifers (n = 1925) were allocated to four groups: non-vaccinated control (n = 485), Leptospira-vaccinated (n = 478), viral-vaccinated (n = 484), and combined viral–leptospiral-vaccinated (n = 478); a 400-heifer subset was bled before vaccination and 50 and 120 days later. Antibodies were measured by virus neutralization (BoAHV-1, BVDV) and by microscopic agglutination test (Leptospira spp.). Data were analyzed by generalized linear mixed models. Baseline seroprevalence was 49.8% (199/400) for BVDV, 11.2% (45/400) for BoAHV-1, and 1.2% (5/400) for the Leptospira serovars included in the vaccine formulation. Viral-containing vaccines induced marked seroconversion against BoAHV-1 and BVDV in seronegative heifers, sustained to 120 days. The Leptospira spp. response was transient, peaking at 50 days. Pregnancy per artificial insemination after the first fixed-time artificial insemination (FTAI) was 51.4% (990/1925) at 30 and 48.4% (931/1925) at 60 days, with 6.0% pregnancy loss. Cumulative pregnancy over two FTAI did not differ among groups (70.0%; 1317/1881; p = 0.18). Corpus luteum presence (p = 0.0002) and inseminator (p = 0.01) were associated with pregnancy outcome. Direct pathogen detection was not performed, so active infection or viral reactivation could not be ruled out. Pre-breeding vaccination induced pathogen-specific humoral responses, particularly against the viral agents, but was not associated with improved pregnancy outcome under the conditions evaluated.