Abstract Background: Under commercial conditions, chickens are fasted for the first 36 to 72 h post-hatch. Delayed access to feed and water causes delayed development of the intestinal tract and retarded performance. Objectives: This study focused on the chick’s life as affected by in ovo feeding (IOF) and the animal’s interaction with the feeding procedure. The birds in a factorial arrangement (2 × 3) were placed into six treatment groups in a completely randomised design. The treatment groups differed in feed procedure, 6 h [early feeding (EF)] or 36 h [common feeding (CF)] post-hatch, with or without IOF with beta-hydroxy-beta-methyl butyrate (HMB) or calcium gluconate (CG) in a saline solution, and were examined for hatchability and performance parameters until 24 h post-hatch. In addition, physical and histological characteristics of breast, jejunum and serum indices in 14 days post-hatch and performance criteria until 35 days of age were recorded. Methods: On day 17 of the incubation period, eggs were subjected to the IOF procedure. One mL of sterile IOF solution including 0.1% HMB or 0.4% CG dissolved in 5% saline solution was injected into the eggs. Results: Results showed that IOF groups had lower (p < 0.05) FCR than the control group. The highest mortality rate was noted in the control-CF group. The lowest myofibril density was related to the HMB-CF group. Myofibril periphery, area and diameter for the HMB-CF group were larger (p < 0.05) than those of the other groups. Conclusions: Results indicated that injection of HMB increased hatchability by almost 15%. The IOF of HMB improved the digestive tract and breast muscle development and improved FCR
