Effects of dietary levels of vitamin A on the egg yield and immune responses of laying hens.

This research, which was designed and carried out as two consecutive experiments, investigated the effects of four different levels (0, 4,000, 12,000, and 24,000 IU/kg) of vitamin A supplementation on egg yield, plasma vitamin A levels, and immune responses of laying hens. Transmission of maternal immunity to their descendants was also studied. In the first experiment, egg yield, blood vitamin A levels, and various parameters of the immune system such as T lymphocyte levels in the peripheral blood, plasma cell counts in the spleen, and antibody titers against Newcastle Disease Virus (NDV) in the sera were investigated for a 1-yr period. A total of 864 Hisex-brown laying hens were used in this experiment. The chicks were reared as commercial flocks until the 18th wk of age. No significant differences occurred among the parameters of the different diet groups. In the second experiment, maternal immunity was assessed in the chickens, supplied by hatching the eggs from hens in the first experiment. Maternal immunity was assayed by using the parameters as in Experiment 1. For this purpose, both blood and tissue samples were taken on the 2nd, 7th, and 10th d posthatch. Vitamin A supplementation had no significant effects on maternally, derived antibody titers or histologic structure of the lymphoid organs.