Table 4.

Impacts of different amino acids on poultry performance.

Antibiotic alternatives	Type	Application	Treated birds	Mode of action	Effects	References
Amino acids	Betaine with different concentrations of 0, 0.75, 1.5, and 2.25 g/kg	Feed	Japanese quail	Decreased levels of liver enzymes such as alanine aminotransferase, alkaline phosphatase, and aspartate aminotransferase and lowered low-density lipoprotein level	Improved birds’ performance, carcass traits, and blood chemistry	Arif et al., 2021
	Total sulfur amino acids	Applied as 1.1 and 0.9 % to starter and finisher diets under a long photoperiod regime	Broiler chickens	Increased the feed consumption and improved feed conversion ratio	Improved broiler performance, carcass traits, and blood parameters	Abou-Kassem et al., 2022
	Betaine	Applied as a dietary supplementation	Poultry	Betaine is an osmolyte and a methyl donor	Improved bird performance	Alagawany et al., 2021
	Threonine	Applied as a threonine dietary supplementation (0, 300, 600, and 900 mg/kg)	Ross broiler chickens	Improved the body weight, weight gain, feed intake, feed conversion ratio, carcass weight, dressing percentage, relative breast weight	Increased birds’ productivity when applied as 900 mg/kg threonine	Khalid and Al-Hayani, 2017
	Glycine	Applied as a 0.62–1.22% from 7 to 20 d of age	Broiler chickens	Increased plasma-free glycine levels	Increased weight gain and feed conversion	Corzo et al., 2004
	Valine	Applied as a 0.52 kg valine/metric ton feed	Broiler chickens	Supported growth and meat yield	Showed the ability to support good production and could potentially offer a useful diet cost-reduction alternative	Corzo et al., 2011
	Valine and isoleucine	Dietary inclusion	Broiler chickens from d 1 to 21 after hatch.	Optimized the effect of leucine supplementation on mRNA expression of mTOR pathway genes in the pectoralis major muscle	Higher valine and isoleucine levels are recommended	Ospina-Rojas et al., 2020
	Phenylalanine, tyrosine, histidine, and leucine	Dietary inclusion	Cobb-broiler chickens	Digestible phenylalanine, tyrosine, histidine, and digestible leucine meets the requirements for the main production parameters of birds during the starter phase	Improved feed intake, weight gain, and breast muscle yield	Franco et al., 2016