1. Introduction
The best strategy to optimize production and reproduction in poultry species while also mitigating the harmful results of environmental conditions is proper nutrition [
1,
2,
3]. One of the pillars of nutrition is the use of amino acids in poultry diets. Methionine represents the first limiting amino acid in broilers. Bunchasak [
4] summarized the several roles of methionine as follows: (1) an essential amino acid for the synthesis of protein; (2) a sulfur donor; (3) an amino acid involved in the synthesis of polyamine; (4) a precursor of main intermediates in metabolic pathways, for instance, carnitine or cystine; and (5) a methyl donor group for the normal formation of co-enzyme S-adenosylmethionine and normal cellular metabolism. Elnesr et al. [
5] stated that methionine’s main function is as an antioxidant, and reported that the improvement in the antioxidant system activity is one of the solutions available to increase productivity in the poultry industry. Synthetic sources of methionine such as DL-methionine (DL-Met) are included in poultry feed to optimize the dietary level of methionine. Methionine plays an essential role in energy production and boosts the livability, performance and feed efficiency utilization in poultry [
2,
6]. Kidd et al. [
7] pointed out that healthy poultry responded positively to the inclusion of amino acids as feed additives and had an affirmative impact on performance. Methionine supplementation can alter the immune response and is beneficial in reducing immunologic stress [
8]. The addition of methionine boosted the reproduction performance, egg quality and egg production of broiler breeders [
9,
10].
An adequate amount of methionine must be introduced to meet the nutritional and physiological requirements to maintain the health and performance of birds [
11,
12]. Thus, updating the nutritional supplies of modern Japanese quails may be an imperative approach in commercial rearing systems. Abou-Kassem [
13] found that the performance was significantly improved for quails fed a diet containing a methionine level higher than the recommended level. It is hypothesized that the use of high methionine levels in diets is expected to have beneficial impacts on quail breeders. Therefore, this study aimed to study the effects of different dietary DL-methionine levels on the productive and reproductive performance, egg quality, hematology, liver and kidney function, lipid profile, and antioxidant and immune parameters of quail breeders (8–16 weeks of age).
4. Discussion
Methionine supplementation of the basal diet improved the egg production of quail breeders [
17]. Similarly, Liu et al. [
18] and Bunchasak and Silapasorn [
19] stated that the addition of DL-Met in the basal diet improved broiler breeders’ egg weight and hen-day egg production. Kalvandi et al. [
20] illustrated that supplementation of methionine increased egg weight and egg production compared to the basal group. Meng et al. [
21] clarified that breeders’ egg weight and laying rate were significantly augmented after the addition of methionine in the diet. The explanation could be that methionine supplementation augments protein deposition, thus encouraging egg production [
22]. Gonzalez-Esquerra and Leeson [
23] exhibited that supplementation of methionine in the diet boosts the performance through polyamine metabolism pathways.
Kalvandi et al. [
20] illustrated that supplementation of methionine increased the daily feed intake and improved the feed conversion ratio compared to the basal group. Meng et al. [
21] elucidated that breeders’ feed conversion ratios were significantly improved after the addition of methionine in diet. The improvement of feed efficiency with methionine levels may be elucidated by the fact that the nutrient requirement of quail based on National Research Council (NRC) [
22] are different from the present commercial birds. These birds have a better performance than quail because of the management practice and genetic selection [
23]. Thus, birds require more amino acids to meet their requirements.
The supplementation of DL-methionine significantly enhanced the hatching rate and fertility rate compared to the control group [
10]. Quails fed with methionine diets showed higher hatchability and fertility than those fed the basal diet [
20]. The current results showed that Met has an affirmative role in encouraging reproduction performance. This may be due to the role of Met in the synthesis of a glutathione precursor that can clear reactive oxygen species (ROS) and then alleviate the harmful impacts of ROS on the protein, lipid, and DNA structures. The excessive ROS created in the embryo tissues cause deleterious influences, leading to high embryo mortality. Thus, the supplementation of Met can boost the antioxidant indices of the chick embryo and increase the hatchability [
10,
20].
Moreover, the methionine requirement determined by NRC [
22] is for quail breeders and this may be insufficient for supporting their productive and reproductive performance (especially under hyper-thermo neutral conditions). It has been shown that the dietary addition of methionine above NRC [
22] recommendations improved cellular immunity in birds [
10,
12].
It is well known that methionine has favorable effects on the physiology, metabolism of egg production, egg quality and general health status of birds. Eggshell quality is a major concern for egg producers and consumers alike [
9,
11,
12]. The current study stated that Met improved the egg quality of laying quail, in agreement with Xiao et al. [
10], who indicated that the supplementation of methionine enhanced eggshell strength, relative eggshell weight and thickness compared with the control group, but did not affect albumen height, egg shape index and Haugh unit. Kalvandi et al. [
20] clarified that the supplementation of methionine in quail diets increased eggshell thickness and Haugh unit (HU) score, but did not affect the yolk and albumen percentages. The improvement of egg quality in groups fed different levels of Met may be due to the fact that Met enhance the antioxidant performance within the body [
24,
25].
Dietary DL-methionine levels did not affect most hematological indices. These results are in agreement with studies by Zhang [
26], who stated that Met supplementation did not influence packed cell volume, red blood cell or white blood cell differentiation counts. However, Kalvandi et al. [
20] clarified that breeder quails that received Met treatments had higher lymphocyte and lower heterophil and heterophil/lymphocyte (H/L) ratios than quails fed a basal diet, but eosinophil and monocyte did not differ among treatments. The levels of methionine treatments led to a significant decrease in heterophils and an increase in blood lymphocytes and heterophil/lymphocyte ratio as a stress index [
27].
Biochemical indicators in the blood can be used to display the health status of poultry. The concentrations of total protein, globulin, albumin, ALT and AST can be indicators of hepatic function, while uric acid can provide signs about renal function. Blood protein in the current study was similar to the results of Hadinia et al. [
28], who revealed that dietary Met levels increased levels of globulin, producing no alterations in total protein concentrations. Xiao et al. [
10] stated that Met declined uric acid concentration in serum. This may be attributed to the antioxidant and anti-hepatotoxic activities of DL-methionine.
Most investigations of avian species indicated that Met supplementation affected apolipoproteins and lipids in the blood [
29,
30]. Methionine is associated with lipid metabolism, including fatty acid oxidation, steatosis and de novo lipogenesis [
31,
32]. Hydrophobic lipids (free fatty acids, cholesteryl esters and triacylglycerols) during the laying cycle are produced and collected to form egg yolk precursors, including vitellogenin particles and very low-density lipoprotein [
30]. Thus, it would be better to supply Met in laying hens’ diets. The addition of Met in quail diets increased plasma levels of HDL–cholesterol and declined triglycerides total cholesterol and LDL–cholesterol [
20]. Met supplements facilitate efficient lipid metabolism in the liver and its transportation to the tissues of birds [
33]. The potential mechanisms of the lipid-depressing impacts of Met might be correlated to its antioxidant properties.
Previous studies have displayed that birds’ immune systems are stimulated by dietary Met levels that far exceed the levels recommended to meet birds’ performance needs [
24,
34]. Supplemental methionine can stimulate immune responses [
35]. The immune response was improved with the supplementation of Met by enhancing the proliferation of immune cells and antibodies [
36]. Bouyeh et al. [
27] indicated that methionine plays four main roles related directly or indirectly to immune system responses, as follows: (1) methionine participates in the synthesis of protein; (2) methionine is a glutathione precursor, a tripeptide, which decreases ROS and thus protects cells from oxidative stress; (3) methionine is required for the polyamine (spermine and spermidine) synthesis that take part in the nucleus and cell division events; and (4) methionine is the most important methyl group donor for the methylation reactions of DNA and other molecules.
Methionine plays a part in the main functions in the body, including ROS elimination and GSH precursors [
37], and thus methionine is sensitive to oxidative modification [
38,
39,
40]. The results of previous investigations pointed out that augmented dietary Met levels exerted antioxidant impacts in poultry. The current results confirmed these studies. Swennen et al. [
41] denoted that Met enhanced the activities of essential antioxidant enzymes (glutathione peroxidase (GSH-Px) and SOD) in blood. Zhang et al. [
42] indicated that increasing methionine levels above NRC recommendations led to an increase in total glutathione in the blood. Moreover, Park et al. [
43] stated that the supplementation of different methionine levels increased the total glutathione level and reduced MDA in plasma. Lai et al. [
44] showed that the addition of dietary methionine levels augmented GSH-Px activity in chicken serum. Furthermore, supplementation of DL-2-hydroxy-4-methylthiobutanoic acid (HMTBA) as a source of methionine improved the activities of glutathione peroxidase, the total antioxidant capacity, and the concentration of reduced glutathione in duck muscles compared with DLM [
45]. On the other hand, both DL-HMTBA and DLM increased the concentrations of MDA in the muscles compared with a basal diet [
45].