Abstract
Background:
The rising global concern over antibiotic resistance has heightened scrutiny of antibiotic growth promoters (AGPs) in poultry farming, prompting a shift towards alternative feed additives to ensure sustainable and safe poultry production. This trend aligns with the increasing demand for free-range and naturally-raised chicken meat in various regions, including Indonesia. In response, Indonesian breeders have turned to medium-sized male layer chickens (MLC) as substitutes for traditional free-range chickens. This practice, coupled with the need to replace AGPs, highlights the critical importance of exploring innovative and natural solutions to enhance poultry growth and health.

Aim:
This study investigated the effects of probiotics as an alternative to AGPs on the growth performance, carcass traits, and immune organs of male Isa Brown layer chickens.

Methods:
180-day-old male Isa Brown layer chickens were used for the study. The intervention included 6 treatments. T1 basal feed, T2 2.5 grams AGP/kg feed, T3 1 ml probiotic/kg feed, T4 3 ml probiotic/kg feed, T5 4 ml probiotic/kg feed, and T6 5 ml probiotic/kg feed. Probiotics used were Lactobacillus acidophilus, Bifidobacterium sp., and Lactobacillus plantarum at a concentration of 1.2x109 CFU/ml. The feeding trial lasted for 21 days for chickens aged 21 to 42 days, assessing growth performance (body weight, feed consumption, digestibility and feed conversion ratio [FCR]), carcass traits, non-edible organs, and immune organs.

Results:
The findings demonstrate that probiotic supplementation significantly outperformed the AGP-treated group (T2) in enhancing growth performance, carcass weight, pectoral weight development, feed conversion ratio, internal and immune organ weights, nutrient intake, and digestibility. While AGPs showed improvements over the control (T1), probiotic-supplemented groups, particularly T6, achieved superior results across all parameters, indicating that probiotics are not only a viable alternative to AGPs but also a more effective and sustainable approach for poultry production.

Conclusion:
The probiotics used in the study at 4 and 5 ml/kg of feed significantly enhanced the performance, immune organ development, and carcass attributes of male layer chickens (MLC), demonstrating their effectiveness as a viable alternative to AGPs. These findings highlight the potential of probiotics to improve poultry production sustainability by reducing reliance on antibiotics, enhancing growth and health outcomes, and promoting animal welfare through natural and efficient dietary interventions.

Key words: Probiotics, Good Health, Growth Performance, Antibiotic Alternatives, Male Layer Chicken