Ascorbic acid, widely known as vtamin C, is an essential nutrient for animals such as poultry. Ascorbic acid in poultry feed improves animal health and thus increases the growth performance of birds. Ascorbic acid can be used in the form of synthetic products or can be naturally obtained from fruits and plants. It is soluble in water and can be easily administered in drinking water and the diet. Poultry can synthesize ascorbic acid in the body. However, the performance of the animals can be improved by adding ascorbic acid to their diet. In addition, ascorbic acid is called an antioxidant and an anti-inflammatory. This increases their resistance to disease during the transition season. Ascorbic acid supplementation positively affects the stress response, especially during the dry season in tropical countries. Furthermore, supplementing ascorbic acid in the poultry's diet improves resistance to diseases, regulates stress, and helps in the body's oxidation process. Ultimately, this enhances the laying rate, egg hatch performance, and higher poultry productivity. For layers at the end of the laying period, it helps increase the quality of the eggshell and reduces the proportion of broken eggs. Ascorbic acid has a strong relationship with other vitamins such as vitamin E and other substances such as zinc, safflower oil, folic acid, and a fibrous diet. This review aims to synthesize all the information of ascorbic acid in the poultry's diet, thereby providing the general role of ascorbic acid for the poultry industry.

Tryptophan is an essential amino acid for all animals that was discovered through casein hydrolysis. The use of tryptophan as feed additives has been attracting the attention of many nutritionists because it cannot be synthesized enough in an animal's body. Tryptophan or precursor to the vitamin niacin in the diet is important, and its supplementation for poultry is determined to improve the amino acid balance and promote the poultry's growth performance through enhancing appetite, feed efficiency, and protein synthesis. Moreover, poultry in different growth phases, breeding, and conditions require various amounts of tryptophan. In addition, supplemented tryptophan also improves the immune response or the immunomodulatory activity of poultry to various diseases through the kynurenine pathway, especially diseases in the bursa. Furthermore, tryptophan also has a strong relationship with lysine (the ideal tryptophan/lysine ratio) in improving growth performance. However, tryptophan deficiency could affect the behavioral responses (e.g. pecking behavior and poultry stress) because tryptophan serves as a precursor for the neurotransmitter serotonin and the pineal hormone melatonin in the diet. This paper tried to summarize all information about applying tryptophan in the diets and illustrate the roles of tryptophan in the poultry industry.

The poultry population has substantially grown in recent years, and measures to ensure meat quality have also improved significantly. The exponential growth of the human population eventually resulted in increasing meat demand. In particular, poultry meat has been the more favorable and nutritive option. Research on feed additives has sparked an interest in many poultry specialists looking for ways to increase poultry performance. The use of baker's yeast, Saccharomyces cerevisiae, as an antibiotic alternative by acting as prebiotics and probiotics has received significant attention. Baker’s yeast contains β-glucans and mannan-oligosaccharides as its main components. Recent studies have shown that baker’s yeast, as an alternative protein source, positively affects poultry growth performance, blood parameters, and immune response. Furthermore, the application of baker's yeast as a fermented feed additive showed promising results for poultry production. The use of baker’s yeast in the diet improves the morphological structure of the poultry gut, thus increasing growth performance. More informatively, it stimulates feed intake, increases body weight gain, and improves the feed conversion ratio. Baker’s yeast also improves the immune system of poultry animals by reducing various numbers of harmful microorganisms by combating poultry diseases more effectively and eventually increasing poultry health. This paper aims to synthesize all aspects of the effect of baker’s yeast on the poultry industry and the role and application of baker's yeast in poultry productivity.

Theanine is a naturally occurring amino acid, including two forms: D-theanine and L-theanine. L-theanine is the most common nonprotein amino acid and is soluble in water. Theanine can be found in some plants, such as green tea and some mushrooms. Under stress conditions, theanine is proposed to be a naturally effective additive in preventing reactive oxygen species, thus reducing oxidative stress. In addition, as an immune booster in animal bodies, L-theanine can be applied in the diet to help animals improve their performance, especially their immunity during stress conditions. The digestion of L-theanine improves γδ T cell growth and development. Thus, it is considered an essential compound in boosting the immune function. Moreover, the immune function of L-theanine is also shown in immune-related organs through increasing their weights. Theanine seems to be widely used in pigs, mice, and humans. However, the study of theanine in poultry species is scarce. Therefore, to fill the knowledge gap regarding the use of theanine in enhancing poultry immunity, this study aims to synthesize all information on the application of theanine in poultry, focusing on its immune-boosting role.