The effect of infectious bovine rhinotracheitis vaccine on reproductive efficiency in cattle vaccinated during estrus.

Nineteen nulliparous, sexually mature heifers free of antibody to infectious bovine rhinotracheitis (IBR) virus were given two 35 mg intramuscular injections of Lutalyse 10 days apart to synchronize estrus and randomly divided into control and vaccinated groups. On the day of the last Lutalyse injection, modified live IBR virus vaccine was administered intramuscularly to each animal in the vaccinated group and each group was placed with a proven sire for 35 days. After the vaccination, samples were taken for isolation of virus and for determination of serum neutralizing antibody titers. No virus shedding occurred after intramuscular vaccination. However, the conception rate was markedly lower in the vaccinated group than in the control group. These results suggest that the intramuscular inoculation of modified live IBR virus vaccines into cattle during estrus is contraindicated.