Table 1. Summary of studies on the impact of feed additives in aged hens

Type of feed additives	Age (weeks)	Feed additives	Major observations	References
Probiotics	69—74	Bacillus subtilis	↑ Laying performance, egg quality, gut health	Kim et al. (2023)
Probiotics	60—68	Clostridium butyricum	↑ Feed efficiency and yolk color	Wang et al. (2020)
Probiotics	64—68	Clostridium butyricum and Bacillus subtilis	↑ Intestinal structure, gut microbiota composition↓ Sandpaper—shelled eggs	Khogali et al.(2022)
Probiotics	50—57	Lactobacillus acidophilus and Bacillus subtilis	↑ Intestinal barrier function and immune homeostasis	Chen et al. (2023)
Prebiotics	74—83	Xylo-oligosaccharides	↑ Laying performance, egg quality	Xiao et al. (2020)
Prebiotics	52—62	Chitooligosaccharide	↑ Laying performance, egg quality, antioxidant activity, immune ability↓ Serum lipids	Xu et al. (2020)
Prebiotics	69—74/74—79	Mannan-oligosaccharides	↑ Laying performance, feed efficiency, nutrient digestibility↓ Pathogenic intestinal bacteria	Ghasemian and Jahanian (2016)
Prebiotics	43—51	β-glucan	↑ Eggshell color, fertile egg hatchability, immune function	Zhen et al. (2020)
Prebiotics	64—75	Inulin	↑ Laying performance, egg quality, feed efficiency, tibia traits, small intestine morphology, microflora composition	Abdelqader et al.(2013)
Organic acids	53—70	Blends of formic acid, butyric acid, and lactic acid	↑ Live body weight	Soltan (2008)
Organic acids	67—74	Blends of fumaric acid, butyric acid, propionic acid, and lactic acid	↑ Feed efficiency	Rahman et al. (2008)
Organic acids	67—74	Blends of fumaric acid, butyrate, propionate, and lactic acid	↑ Feed efficiency, eggshell quality	Rahman et al. (2011)
Exogenous enzymes	63—73	Phytase	↑ Laying performance, peak laying period, eggshell quality	Sun and Kim (2021)
Exogenous enzymes	42—47	Phytase	↑ Laying performance	Kim et al. (2017)
Exogenous enzymes	60—69	Protease	↑ Laying performance, feed efficiency, cecal microbiota	Chen et al. (2021)
Vitamin	56—61	Vitamin A	↑ Laying performance, egg weight, feed intake↓ Oxidative stress	Lin et al. (2002)
Vitamin	90—97	Vitamin A and vitamin K	↑ Yolk color, eggshell quality, antioxidative status	Guo et al. (2021)
Vitamin	42—47/44—49/52—57/72—77	Vitamin D3	↑ Eggshell quality, vitamin D3 contents in eggs	Cho et al. (2023)
Vitamin	60—72	Vitamin D3 and 25-OHD3	↑ Laying performance, eggshell quality, plasma calcium levels	Jing et al. (2022)
Vitamin	74—81	25-OHD3	↑ Egg weight	Kakhki et al. (2019)
Vitamin	57—63	Vitamin D3, vitamin D2, and 25-OHD3	↑ Utilization of calcium and phosphorus	Adhikari et al.(2020)
Vitamin	66—72	Vitamin E	↑ Laying performance, egg quality, antioxidant capacity	Linghu et al. (2021)
Vitamin	70—80	Vitamin E	↑ Immune function, antioxidant capacity, ovarian functions, estrogen secretion, follicle development	Wang et al. (2024)
Vitamin	46—55	Vitamin E	↑ Laying performance, tocopherol deposition, antioxidant capacity↓ Serum cholesterol concentration	Zhao et al. (2021)
Vitamin	46—52	Vitamin C	↑ Laying performance	Reyes et al. (2021)
Herbal extracts	70—80	Oregano extract	↑ Laying performance, egg quality, intestinal morphometry	Ramirez et al. (2021)
Herbal extracts	64—72	Moringa oleifera	↑ Laying performance, egg quality↓ Serum cholesterol and triglyceride, excreta ammonia concentrations, uric acid, creatinine	Abdel-Wareth and Lohakare (2021)
Herbal extracts	72—78	Ligustrum lucidum	↑ Laying performance, caecal microbial composition	Li et al. (2017)
Herbal extracts	52—64	Lonicera confusa and Astragali Radix	↑ Egg quality, antioxidant capacity↓ Systemic inflammation	Xie et al. (2019)
Herbal extracts	60—64	Mentha piperita L.	↑ Laying performance, egg quality, immune response, antioxidant capacity, intestinal microbiota	Bai et al. (2023)