Dietary arginine supplementation higher than the recommended levels was shown to improve broiler performance (Murakami et al. 2012; Xu et al., 2018; Zampiga et al., 2018). It suggests that arginine levels needed for maximum performance could be higher in modern broilers, which is probably related to the several functions of arginine on animal metabolism. Besides, arginine enhances the protein synthesis via the activation of the mTOR signaling pathway (Yao et al., 2008; Yuan at., 2015); this amino acid is a precursor for critical molecules such as nitric oxide, creatine (Chamruspollert et al., 2002), polyamines, proline, glutamate, citrulline, and agmatine (Wu and Morris, 1998). Arginine regulates key aspects of animal growth by enhancing insulin, growth hormone, and insulin-like growth release (Cochard et al., 1998; Yu et al., 2018; Xu et al., 2018). However, there is limited research on arginine requirements in broilers, and most research has focused on arginine supplementation in birds receiving some physiological stress (immune challenge, cold, or heat stress), wherein dietary arginine levels could be insufficient to support broiler growth or meet their metabolic requirements during those stress periods. Due to arginine’s role in healing wounds and reducing the carcass incidence of skin scratch infections, arginine supplementation could be a nutritional strategy to reduce the relevant poultry economic losses with carcass condemnation issues.
Optimal dietary arginine levels for broiler performance
A meta-analysis based on 94 observations from eight peer-reviewed papers was conducted to determine the optimal arginine levels in broiler diets. The arginine base data included peer-reviewed papers between 2001 and 2019. It has used data of birds raised in normal conditions and excluded the dose-response data of broilers challenged or raised in suboptimal temperatures. When not given the standardized ileal digestible (SID) amino acid contents of the experimental, they were calculated using digestibility coefficients of ingredients as proposed by Rostagno et al. (2017). Summary references and some experimental conditions used for the meta-analysis are shown in Table 1.
The BW gain distribution of base data for each growth period was described in Figure 1. Arginine requirement during the finisher phase has been seldom studied, data obtained from this period showed a larger variation than the observations from the starter and grower phases where arginine supplementation has shown to increase breast meat yield (Corzo et al., 2003; Jiao et al., 2010) while reducing wooden breast myopathy incidence in modern fast-growing broilers (Bodle et al., 2018; Zampiga et al., 2019).
BW gain data (g/d) plotted against arginine intake (g/d) showed a positive response in BW gain as increasing arginine intake (Figure 2). A significant quadratic regression (P=0.0138) was fitted from the 94 observations to predict the daily SID arginine intake to achieve a desirable BW gain during the different growth phases. For instance, an optimal level of SID arginine of 1.45% would be needed to achieve a daily BW gain of 19.8 g in broilers from 1 to 7 days of age (Table 2).
Considering a SID lysine recommendation of 1.28% and 1.07% for birds from 1 to 21 days and 22 to 42 days according to Rostagno et al. (2017), the ideal SID arginine:lysine ratio could be estimated at 114 and 115 for broilers during the starter and grower periods, respectively.
To standardize and compare performance data from broilers at different ages, dose-response data was expressed as a percentage. The best performance was considered as the reference (100%) and their respective dietary SID arginine level and arginine:lysine ratio were set at zero. The other treatments were expressed relative to the reference treatment. Quadratic regression (QR) and linear response plateau (LRP) models were used to determine the optimum arginine:lysine ratio for the relative BW gain. Also, the first point where the quadratic response curve interested the plateau value was determined (LRP + QR). The SID arginine:lysine ratio was estimated to be at 104, 110, and 127 based on LRP, LRP + QR, and QR models, respectively (Figure 3). The ideal arginine:lysine ratio may be at 114 considering the three approaches. This value is close to the arginine:lysine ratio of 114 and 115 estimated for broilers during the starter and grower periods using a fitted equation in Figure 2.
Recommendations of arginine:lysine ratio range in the literature from 105 to 125, depending on the variable studied, with a mean of 115% (Table 3) (NRC, 1994, Lippens et al., 1997; Baker, 1997; Kidd et al., 2001; Corzo and Kidd, 2003; Jahanian, 2009; Corzo et al., 2012). The SID arginine:lysine ratio to optimize the immune system response is higher than that for optimal feed conversion which is higher than for maximum BW gain (Corzo et., 2003; Jahanian, 2009). Corzo et al. (2012) indicated that broiler chicks optimize their BW gain and feed conversion values at arginine:lysine ratios of 108 and 114, respectively. The SID arginine:lysine ratio estimated from the nutritional recommendations of Jahanian (2009) was 115, 117, and 122 to optimize BW gain, feed conversion, and immune system function, respectively.
Effect of arginine levels on immune response
The higher arginine level needed for maximum immune system function is expected considering the important role of arginine in immunity as it is a substrate for the immune cells. The immune cells use the system y+ cationic amino acid and B0,+ transporters to acquire arginine (Nicholson et al., 2001; Humphrey et al., 2004) where this amino acid promotes lymphocytic proliferation through polyamine synthesis (Ochoa et al., 2001) and improve the phagocytic activity of macrophages by increasing their internal nitric oxide levels (Sung et al., 1991). Consequently, functional aspects of T and B cells are reliant on arginine utilization (D’Amato and Humphrey, 2010).
Dietary arginine supplementation in broilers challenged with Eimeria spp. has shown to reduce mortality (Kidd et al., 2002) and alleviate intestinal villus damage (Tan et al., 2014) as well as attenuate gut injury of broiler chickens infected with Clostridium perfringens (Zhang et al., 2018). Also, Corzo et al. (2012) reported a linear reduction of mortality in broilers supplemented with graded levels of arginine. The positive effects of arginine in challenged broilers (having an increased concentration of CD4+ and CD8+ lymphocytes and B cells, increased antibody titers after vaccination, increased relative weight of the thymus and bursa, and improved T cells response) may be due to improve the general health status (Tayade et al., 2006; Abdukalykova et al., 2008; Jahanian, 2009).
Arginine supplementation as a strategy to reduce broiler carcass condemnation
Interestingly, Corzo et al. (2003) found a linear reduction in infected carcass scratches of broilers fed arginine-supplemented diets (Table 4). Infected skin scratches as cellulitis are considered the first major reason for condemnation at slaughter followed by fractures/contusion (Santana et al., 2008), which is associated with significant economic losses in poultry industry. The reduced infected scratches in arginine-supplemented diets are not only related to the good immunity status as well as to a dermis with greater resistance. The arginine role in enhanced wound healing has been well-documented in mammals by increasing breaking strength and reparative collagen accumulation (Shi et al, 2003; Tong et al., 2004; Stechmiller et al., 2005). For instance, the production of nitric oxide synthase peaks within 24–72 h after wounding (Stechmiller et al., 2005), producing nitric oxide, and citrulline, where nitric oxide plays a major role in antimicrobial activity and improving blood flow to the healing wound (Alexander and Supp, 2014). Thereafter, arginase activity increases with a marked rise in wound ornithine levels which is metabolized to polyamines by ornithine decarboxylase (Shi et al., 2002). The polyamine concentrations increase within 12 h after skin incisional wounding in rats and they are factors necessary for cellular growth involved in tissue repair and wound healing (Maeno et al., 1990). The inhibition of ornithine decarboxylase, an enzyme related to polyamine synthesis, impairs skin wound healing (Lesiewicz and Goldsmith, 1980).
In addition, arginine is a metabolic precursor of proline and hydroxyproline, the major amino acids in the collagen proteins, which are major extracellular components in connective tissues of skin and tendons (Wu et al., 2011). Fernandes (2007) reported a linear increase in broiler dermis thickness as increased dietary arginine levels. It suggests the importance of arginine in skin breaking strength. Scratches with no dermis penetration resulted in a more resistant dermis having less chance to be infected. The effect of arginine supplementation should be further studied considering the huge economic benefits that it may bring to poultry production to reduce carcass condemnation.
Arginine and lysine antagonism effects on broiler performance
When the best performance treatment in each study was considered as the reference (100%) and its respective dietary arginine:lysine ratio was set at zero from dose-response data, birds fed SID arginine:lysine ratios higher than the optimal level did not show pronounced negative effects on broiler performance (Figure 4). The optimal SID arginine:lysine ratios used as reference were above nutritional recommendations in 75% of the peer-reviewed papers used for the meta-analysis. On the contrary, marginal arginine:lysine ratios reduced drastically BW gain and feed conversion. The antagonism between dietary arginine and lysine has long been defined. This effect is clearer with an excess of lysine and marginal arginine level (low arginine:lysine ratio) rather than an excess of arginine (high arginine:lysine ratio) (Balnave and Brake, 2002). Indeed, higher arginine:lysine ratio can improve broiler performance (Murakami et al. 2012; Zampiga et al., 2018).
High lysine levels increase the renal arginase activity and consequently increase arginine degradation (Austic and Scott, 1975). As a result, less arginine is available for protein synthesis and biological functions that rely on arginine. Also, excess lysine decreased the arginine:glycine amidinotransferase activity (Jones et al., 1967), which catalyzes the first step of creatine synthesis, depressing muscle creatine concentration (Chamruspollert et al., 2002). The antagonism between arginine and lysine is more pronounced in birds that are not able to synthesize arginine endogenously due to the lack of carbamoyl phosphate synthetase I and a low renal activity of argininosuccinate synthetase, argininosuccinate lyase, and ornithine transcarbamylase (Ball et al., 2007). Therefore, arginine is an essential amino acid, and its requirement must be supplied by diet to support broiler growth.
The effect of heat and cold stress on arginine requirements
The environmental conditions in arginine studies are important to be considered to avoid any interference in the estimated SID arginine levels. Blood flow is redirected to peripheral organs involved in heat dissipation, when broilers are exposed to a warm environment. Therefore, dietary arginine requirement may increase in high temperature as this amino acid is a substrate for nitric oxide synthesis, which plays an important role in vasodilation and blood flow (Perez Laspiur et al., 2006). Likewise, high environment temperature demonstrated to decrease arginine absorption in the intestinal epithelium increasing their dietary needs (Brake et al., 1998). Besides alleviating the adverse effect of heat stress, arginine supplementation has shown to recover broiler performance of birds submitted to cold stress (Saki et al., 2013; Kodambashi Emami et al., 2017). In birds subjected to low-temperature stress, the sympathetic nervous activity is stimulated to produce heat (thermogenesis) by increasing the bird cellular metabolic rate and cell oxygen intake (Estler and Ammon, 1969). The relative underdeveloped cardio-respiratory system of modern broilers fails to fulfill the required oxygen demand, resulting in hypoxemia (Aftab and Khan, 2005). Thus, the bird increases heart rate and blood flow to compensate for the oxygenation deficit that may result in pulmonary hypertension and ascites (Olkowski et al., 1999). A previous study reported that arginine supplementation in broilers exposed to cool environmental temperature attenuated the effects of pulmonary hypertension (Wideman et al., 1995) by increasing the endogenous pulmonary vasodilator nitric oxide. Therefore, either cold or heat stress increase arginine needs being an important amino acid to attenuate the negative effects on performance of broilers submitted to suboptimal temperatures. In addition, the reactive oxygen species production is higher in birds raised in high- or low-temperature environments, increasing the oxide nitric production for its well-documented potent cellular antioxidant function. Therefore, arginine levels could be insufficient to support broiler growth or meet their metabolic requirements during stress periods.