This experiment was conducted to evaluate the effects of dietary protein reduction levels and supplementation with thyme extract and probiotics on growth performance, blood parameters, intestinal morphology, and litter quality in broiler chickens. A total of 300 commercial Arian broilers were used in a completely randomized design with a 3 × 2 factorial arrangement, including two protein levels and three additive levels, resulting in six treatments with five replicates per treatment and ten birds per experimental unit. The experimental treatments were as follows: (1) diet formulated according to the strain protein recommendation without additives (control); (2) diet containing 5% less protein than the strain recommendation without additives; (3) diet formulated according to the strain protein recommendation with 1 mL of thyme extract per liter of drinking water; (4) diet containing 5% less protein than the strain recommendation with 1 mL of thyme extract per liter of drinking water; (5) diet formulated according to the strain protein recommendation with 0.1 g/kg probiotic; and (6) diet containing 5% less protein than the strain recommendation with 0.1 g/kg probiotic.
Based on the results, reducing dietary protein by up to 5% did not adversely affect performance parameters, including body weight, feed intake, weight gain, and feed conversion ratio, and no significant differences were observed. Moreover, protein reduction had no significant effect on protein efficiency ratio, energy efficiency ratio, or the European Production Index throughout the entire experimental period. The effects of a 5% reduction in dietary protein on blood parameters, carcass and organ traits, litter characteristics, and intestinal morphology were also evaluated. This reduction led to decreased serum albumin, increased serum uric acid, increased body weight, reduced breast weight, increased thigh weight, and increased ready-to-cook carcass weight, with these differences being significant comp