The effects of dietary Se on broilers performance have been reported differently by various researchers. Some studies have shown positive effects (Briens et al., 2014; Boostani et al., 2015; Chadio et al., 2015), while others reported no significant effects on growth performance parameters (Zhou and Wang, 2011; Marković et al., 2014).
In a study conducted by El-Deep et al. (2016), it was found that high ambient temperature negatively impacted broiler chickens’ performance. However, when animals were fed diets containing nano-selenium (Nano-Se), the negative effects of HS were reduced. In the current study, the inclusion of 1.30 mg of Zn-L-SeMet resulted in lower FI and FCR without any change in BWG. This result suggests that Se was efficient alleviating HS when included in high level in the diets, improving energy utilization. Similarly, Shabani et al. (2019) observed reduced FI and FCR in their study, but they also observed increased BWG.
Selenium plays vital functions in the organism, primarily due to its role as an integral part of selenoproteins (Wang et al., 2021). Selenoproteins are involved in thyroid hormone metabolism, including the conversion of T4 to its active form triiodothyronine (T3), which regulates energy and protein assimilation in the body (Schomburg, 2012). Additionally, Se is recognized for its dual role as an antioxidant and a key component of enzymes such as thioredoxin reductase (TrxR) and glutathione peroxidase (GPx) (Sözcü and İpek, 2018). These enzymes are involved in pancreatic functions, including the secretion of digestive enzymes (Combs Junior and Combs, 1986). Although there was no increase in BWG in the present study, it is possible that Se improved the body's antioxidant capacity, action of digestive enzymes, and energy and protein utilization. This in turn, may have contributed to a reduction in FI and FCR.
During the current study, the animals were exposed to an average temperature of 31.1°C from 22 to 42 days of age, which falls outside the thermoneutral (TN) zone of 23–27°C and 18–24°C for four and five-week-old poultry, respectively (Dei and Bumbie, 2011; Scheele et al., 2014). The recorded Tc values ranged from 41.8 to 42°C, which is within the typical temperature range for broilers, varying between 41 to 42°C (Wilson, 1948; Welker et al., 2008). Giloh et al. (2012), rearing broilers under TN conditions, found an average Tc value of 41.3°C, while Chang et al. (2018) observed 42.4°C when exposing broilers to 35°C. These findings suggest that the conditions applied in the current study may not have been sufficient to induce a significant physiological response in which the animals would change their body temperature. Furthermore, while dietary Se may have positive effects on broiler health and growth (Saad et al., 2009), it is unlikely to have a direct impact on their cloacal temperature, as suggested by Raduta et al. (2017).
The mortality rate of broilers that received 0.15 mg Zn-L-SeMet (4.6%) was lower than those fed with the control diet (8.4%). Additionally, the inclusion of 0.71 mg/kg of Zn-L-SeMet was predicted to reduce broiler mortality. This finding is consistent with earlier studies conducted by Tayeb and Qader, (2012), Albuquerque et al. (2017), and Wang et al. (2021), which have also reported reduced mortality rates with the inclusion of Se in various forms and levels. Selenium has been shown to enhance the activity of IgM and IgG antibodies, which improve the immune system, and could potentially help mitigate the negative effects of HS (Niu et al., 2009; Habibian et al., 2016). This could potentially explain the reduced mortality observed in the current study.
The evaluation of the biochemical markers in the blood can provide valuable information about the physiological status of the body (Hrabčáková et al., 2014). In the current study, plasma TRG levels were found to be similar in animals that received 0.47 mg Zn-L-SeMet and those in the control group. However, they were significantly higher compared to the other Se treatments. These findings differ from those reported by Kang et al. (2000), who suggested that Se has a hypocholesterolemic effect, reducing CHO and TRG levels. This is supported by Beer-Ljubić et al. (2012), who found reduced TRG levels in broilers fed diets containing 0.30 mg/kg of SeMet compared to the control treatment. Furthermore, Amer et al. (2019) reported that regardless of the source (organic or inorganic), Se inclusions reduced TRG levels in rabbits.
Some studies involving humans have shown a positive correlation between Se and serum/plasma lipid concentrations (Bleys et al., 2007; Zhou et al., 2013). Selenium is involved in lipid metabolism (Zhang et al., 2018) and has been found to be inversely correlated with TRG levels (Hasani et al., 2018). However, some studies have indicated a concerning correlation between elevated Se intake and/or levels of Se in the body, and an increased risk of hyperglycemia, hyperlipidemia, or type 2 diabetes (Bleys et al., 2007; Stranges et al., 2010; Zhao et al., 2016; Nawab et al., 2018). Zhao et al. (Zhao et al., 2016, 2020) have confirmed this in pigs and mice, respectively, by showing that Se levels above those considered necessary for optimal nutrition can increase blood CHO and TRG levels. In the present study, it was possible to observed an increase in plasma TRG levels in broilers fed 0.47 mg of Se, but not in those fed 1.30 mg, making it difficult to interpret the result. Further investigation is needed to better understand the effects of Se on the blood profile of broilers.
Heat stress can lead to metabolic changes in broilers, causing an elevation in oxidative stress (Lin et al., 2006). The enzyme GGT is involved in releasing reactive oxygen species from redox reactions, and its levels increase in response to oxidative stress (Lim et al., 2004). Higher levels of GGT have been associated with lower levels of GPx and overall antioxidant status, indicating a negative correlation between these two blood markers (Gunawan et al., 2011).
In the current study, GGT levels were found to be affected quadratically by Se, with an estimated concentration of 14.14 IU/L when 1.19 mg of Se was included in the diet. This decrease in GGT is consistent with previous studies by Placha et al. (2009) and Wang et al. (2019). However, the GGT levels reported in their studies were significantly higher than those observed in the current study. This discrepancy could be attributed to liver damage caused by the exposure of birds to deoxynivalenol or aflatoxins. The hepatobiliary system is the primary source of GGT found in the bloodstream (Kunutsor, 2016). If there is significant damage to hepatocytes, GGT can be released into the bloodstream from the smooth endoplasmic reticulum (Wang et al., 2019). Therefore, in the present study, it appears that the animals did not experience liver damage, and the decrease in GGT concentrations may be attributed to the protective effects of Se, which improved the organism’s antioxidant status.
The hormones IGF-1 and GHR play a significant role in regulating muscle growth as anabolic factors (Florini et al., 1996). Increased levels of IGF-1 are associated with high growth rates in broilers, while disruptions in growth hormone pathways can lead to growth retardation and metabolic disorders (Brown-Borg, 2009). In conditions of HS, decreased expression of these growth-related hormones can negatively impact the muscle development (Fink et al., 2018; Nawaz et al., 2021). Selenium deficiency has also been linked to growth retardation and reduced plasma GHR and IGF-I levels (Arthur et al., 1990). This reduction may be attributed to the decrease in the activity of selenoenzymes, which impairs the conversion of T4 to T3, a hormone necessary for normal GH synthesis (Thompson and Sunde, 1995).
Selenium is known to influence the growth hormone-IGF axis (Moreno-Reyes et al., 2001), and the addition of Se to the diet has been found to increase the expression of IGF-I and insulin receptor genes (Saleh and Ebeid, 2019). In humans, there is a positive correlation between Se and IGF-1 levels (Maggio et al., 2010). In a study with broilers, the inclusion of Se, with or without vitamin E, upregulated both IGF-1 and GHR levels (Khalifa et al., 2021).
Selenium has been suggested to have a regulatory role in growth-related hormones through its participation in the GPx enzyme, which aids in mitigating oxidative stress by facilitating the reduction of hydrogen peroxide and organic hydroperoxides (Frank and Ilene, 1987). Oxidative stress can damage key molecules involved in hormone signaling pathways, impairing their activity (Higashi et al., 2010, 2013). Higashi et al. (2010;2013) also stated a relationship between IGF-1 and GPx, supporting the potential influence of Se on growth-related hormones. However, in the current study, the graded levels of Zn-L-SeMet in the diets did not affect the gene expression of IGF-1 and GHR. Despite the possible beneficial effects of Se on growth-relate hormones, further research is needed to better understand their metabolism and the factors influencing their expression.