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Influence of a BRDC vaccine with a MLV or KV IBR component on estrous cycle parameters and anti-müllerian hormone concentration in nulliparous heifers
The objective of this study was to examine the impact of a bovine respiratory disease complex (BRDC) vaccine with a modified live virus (MLV) infectious bovine rhinotracheitis (IBR) component on estrous cycle parameters and the follicular pool. 24 Holstein heifers (mean±SD; 12.4±0.5 months) in two replicates (Spring n=10 and Fall n=14) were synchronized for estrus using a 7-day CIDR protocol with 2 injections of PGF2a, one at CIDR removal and a follow-up injection 16 hours later. Heifers were calf-hood vaccinated with an IBR MLV. Heifers were observed for one complete estrous cycle to establish normal cyclicity. At Heat 2, heifers were vaccinated with either the calf-hood MLV (MLV; n=12) or a BRDC vaccine with a killed (K; n=12) IBR component. Heifers were blocked into treatment groups according to pre-vaccination bovine viral diarrhea virus (BVDV) serum neutralizing (SN) titers. Heifers were then tracked for two complete estrous cycles. Serum samples for estradiol (E2) and progesterone (P4) and ultrasound of ovarian structures were collected to track cyclicity every-other-day. Serum samples for Anti-Müllerian Hormone (AMH) were collected at estrus and mid-cycle to evaluate the follicular pool. Data was normalized with ovulation as day 0. Data were analyzed with the PROC MIXED procedure of SAS with cycle number, season, and vaccine as fixed effects. The model for P4 analysis added day of cycle as a fixed effect. There was no difference (P>0.05) in post-vaccination titers. Vaccination had no impact on P4 concentrations, luteal tissue area, peak E2 production, or estrous cycle lengths (P>0.05). Overall variables that affected AMH concentrations were: season (Spring= 138.92±43.1 pg/mL, P=0.0043), vaccine type (MLV= -92.4±42.9 pg/mL, P=0.0435), and cycle number (P<0.0001). AMH concentration decreased between cycles 1 and 2 and cycles 1 and 3 for MLV vaccinated heifers (P<0.0003). AMH concentrations of cycle 2 were numerically lower between vaccine types (K=308.22±33.3 pg/mL, MLV=181.13±32.9 pg/mL; P=0.0953) though not statistically different. This may be due to low animal numbers, the variability between animals, or the differences observed in the Fall killed vaccine ( -70.93±21.1 pg/mL, P=0.0145) from cycle 1 to 2, but not in the Spring killed vaccine (3.40±45.1 pg/mL, P=0.9969). AMH was weakly correlated with small follicle count (r2=0.15, P<0.0001). Although no differences were seen in overall cycle parameters, these differences in AMH concentrations may indicate a reduction of the follicular pool as a result of vaccination with an IBR MLV.
Keywords:
IBR MLV, Cyclicity, AMH