It was assumed that early antibiotic administration can slow down yolk sac resorption and decrease maternal antibody transfer and lysozyme levels in the yolk sac content and serum, meaning that disrupting the development of humoral immunity in turkeys. The experiment was conducted on female turkeys divided into following group: CON (control) – received no coccidiostat or antibiotics, MON – received monensin in the feed for 56 days; ENR and DOX – received enrofloxacin or doxycycline per os for the first 5 days of life. Additionally, half of the birds in each of this group were vaccinated against turkey rhinotracheitis (TRT) the disease caused by avian metapneumoviruses (aMPV) and Newcastle disease caused by Newcastle disease virus (NDV) at the first day of life (IN), and against omitobacteriosis caused by Ornithobacterium rhinotracheale (ORT) at 28 day of life (SC). On days 1, 3 and 5 of the birds' lives, yolk sacs were collected to assess their resorption. Yolk sac resorption was assessed by calculating yolk sac relative weight based on the measurement of the yolk sac mass and body weight of turkeys. On days 1, 3, 5, 7 and 56, blood was collected to assess anti-aMPV, anti-NDV, anti-ORT antibody titers and immunoglobulin and lysozyme levels. Early administration per os of ENR and DOX or feeding diets containing MON did not inhibit yolk sac resorption, but reduced levels of specific maternal anti-aMPV, anti-NDV and anti-ORT antibodies and IgY and IgM in the yolk sac. Enrofloxacin and doxycycline decreased the titers of anti-aMPV and anti-NDV antibodies and the level of maternal IgY and IgM in turkeys, which could be due to the direct effect exerted by antibiotics on maternal antibodies present in the circulatory system of poults and the inhibition of post-vaccination synthesis of specific antibodies. The administration of antibiotics in the early rearing period should only be implemented in situations of clearly confirmed disease states when the expected health benefits outweigh the risk of weakening immunity.
