The aim of this study was to evaluate the effects of dietary Piriformospora indica (PI) and clove essence (CE) and a combination of them (synbiotic) on performance and biochemical parameters after induction of stress using dexamethasone. The experiment was performed with 320 male, one-day-old chicks (Ross 308) using eight experimental groups and five replications. In each replication, eight chickens were fed a maize–soy-based diet. Experimental groups included group 1 (T1): control diet (based on maize and soybean without any feed additives); group 2 (T2): control diet with 0.2 cc dexamethasone injection; group 3 (T3): control diet with 11.6 cc CE; group 4 (T4): control diet with 10 mg PI; group 5 (T5): control diet with 11.6 cc of CE with 0.2 cc injection of dexamethasone; group 6 (T6): control diet with 10 mg PI and 0.2 cc of dexamethasone; group 7 (T7): control diet with synbiotic containing 10 mg of PI with 11.6 cc of CE; group 8 (T8): control diet with synbiotic containing 10 mg of PI and 11.6 cc of CE with 0.2 mg dexamethasone injection. PI and CE were poured into the drinking water during the rearing period. After 31, 33, 38, and 40 days, 2 mg/kg BW of dexamethasone was injected in the right breast muscle of experimental groups 2, 5, 6, and 8; after 35 and 42 days, samples were taken. The addition of CE, PI, and the synbiotic, caused an increase in body weight gain, feed intake, and reduced FCR compared to the control group in experimental groups 3, 4, and 7. The addition of 11.6 cc CE and 10 mg of PI caused an increase in body weight gain and feed intake and reduced FCR compared to the control group. Dexamethasone (0.2 mg) led to damage of the lymph organs, body weight loss, a reduction in humoral immunity, and side effects on performance parameters. Clove extract and the prebiotic reduced the adverse effects of dexamethasone.
