A multivalent in ovo vaccine (MIV) was tested for safety and efficacy in a commercial broiler complex. The MIV comprised five replicating live viruses including serotypes 1, 2, and 3 of Marek's disease virus (MDV), an intermediate infectious bursal disease virus (IBDV) and a recombinant fowl poxvirus (FPV) vector vaccine containing HN and F genes of Newcastle disease virus (NDV). The performance of MIV-vaccinated broilers was compared with that of hatchmates that received turkey herpesvirus (HVT) alone (routinely used in ovo vaccine in the broiler complex). The chickens that hatched from the MIV-injected and HVT-injected eggs were raised under commercial conditions in six barns. Barn 1 housed 17,853 MIV-vaccinated chickens and each of the barns 2–6 housed 18,472–22,798 HVT-vaccinated chickens. The HVT-vaccinated chickens were given infectious bronchitis virus (IBV) and NDV vaccines at hatch and at 2 wk of age. The MIV-vaccinated chickens received IBV vaccine at hatch and IBV + NDV at 2 wk of age. The relative values of hatchability of eggs, livability and weight gain of chickens, and condemnation rates at processing were comparable between the MIV and the HVT groups (P > 0.05). Chickens from the MIV- and the HVT-vaccinated groups were challenged with virulent viruses under laboratory conditions. The resistance of vaccinated chickens against Marek's disease could not be assessed because of high natural resistance of unvaccinated commercial broilers to virulent MDV. The relative resistances of the MIV- and the HVT-vaccinated groups, respectively, against other virulent viruses were as follows: IBDV, 100% for both groups; NDV, 81% vs. 19%; FPV, 86% vs. 0%. The successful use of MIV under field conditions expands the usefulness of the in ovo technology for poultry.

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