Effect on growth performance
In the present study, it has been found that, vitamin E and Se supplementation in the diet of heat stressed broilers influenced BW (P=0.002), BWG (P=0.001), FI (P=0.311) and BPFI (P=0.013) but not the FCR (Table 2). Heat stress influences the peripheral thermal receptors which transmit nerve impulses that suppresses the activity of the appetite center in the hypothalamus causing reduction in feed intake
(Marai et al., 2007) and fewer nutrients are available for enzymatic activities, hormone production and heat generation.
Geraert et al., (1996) reported that high ambient temperature reduces body weight and they found that when birds were exposed to 32°C there was 14% reduction in body weight from 2 to 4 weeks of age and a 24% reduction from 4 to 6 weeks of age. Various researchers
(Mansoub et al., 2010 and
Tawfeek et al., 2014) also reported that the hot weather conditions (35°C) significantly reduced body weight (P<0.01) and feed intake (P<0.05) in broilers and birds kept in heat stress condition. The decreased body weight was not only due to the lower feed intake, but also due to a direct effect of environmental temperature on broiler physiology and metabolism
(Ain-Baziz et al., 1996 and
Geraert et al., 1996).
The antioxidants vitamin E and Se synergistically destroys the cell damaging free radicals
(Biswas et al., 2011) leading to better utilization, digestion, absorption and metabolism of feed nutrient essential for health and body weight gain. Se has a protective role on pancreatic tissue against oxidative damage that might have helped the pancreas to function properly including secretions of digestive enzymes, thus improving digestibility of nutrients and consequently, performance
(Sahin et al., 2009). Further, Vitamin E and Se supplementation reduced stress by a reduction in lipid peroxidation that might have helped the birds in improving their performance by reducing the metabolic requirement of nutrients and diverting these nutrients for muscle growth (Rama
Rao et al., 2013).
Effect on dressing percentage
Vitamin E and Se supplementation increased the dressing percentage (Table 3) with (P<0.001) and without giblet (P<0.001) in the present study, which might be due to reduced release of cortisol resulting prevention of body catabolic activity as well as increased feed intake and consequently more nutrients were available to improve growth rate and body weight.
At high ambient temperatures, birds reduced their feed intake and consequently less nutrients were provided to the internal organs, which compromise their developments
(Ghazi et al., 2012b). High ambient temperature induced production and release of corticosteroids, which exert catabolic effects (mobilization of proteins and lipids) through muscle wasting and hence reduce growth rate
(Hayashi et al., 1994). Carcass and liver yields were adversely affected by high temperature where they represented 96% of that of normal control group
(Tawfeek et al., 2014). A strong relationship reported between live body weight and carcass yield of broiler chickens
(Fanatico et al., 2005). The effect of selenium on meat yield could be due to changes in thyroid hormone metabolism or a result of changes in broiler feathering
(El-Sheikh et al., 2006) and cellular integrity
(Peric et al., 2009). Heat stress decreased (P<0.05) dressing percentage (-2.9%) and Vitamin E (100 mg/kg diet) supplementation increased dressing percentage in broilers under heat stress
(Attia et al., 2017).
Effect on mean cut of parts (per cent of carcass weight)
Increased percentage of breast (p=0.004), Thigh (p=0.018) and Drum stick (p=0.013) in vitamin E and Se supplemented groups were observed in the present study (Table 3). It might be due to increased dressing percentage and also effective mechanism of vitamin E in its reactions with active radicals to break chains, to exert pressure, to renovate, to increase endogen defence and better protein accretion to improve breast meat characteristics
(Rokade, 2014). Carcass characteristics especially breast yield improved with an increase of both vitamin E and Selenium
(Sahin and Kucuk, 2001). Vitamin E supplementation in diet helps in fatty acid composition and oxidative stability of thigh muscle in broilers under heat stress which might have contributed to increase in the thigh yield as was found in the present experiment
(Vakili and Rashidi, 2011). The reduced breast (%) in heat stressed broilers due to abundant of white fast-contracting glycolytic fibers
(McKee, 2003), which are richer in ATP and rely on glycogen supply for its metabolism and hypertrophy. Therefore, as feed intake was limited under heat stress, glycogen supply decreased leading to decreased protein synthesis in the breast muscle
(Temim et al., 2000). Heat stress decreasing the proportion of breast muscle in the carcass were observed in several studies
(Zhang et al., 2012; Lara and Rostagno, 2013; Zeferino et al., 2016).
The abdominal fat percentage decreased significantly (p=0.004) in all the vitamin E and Se supplemented groups. Vitamin E strongly influences the lipid metabolism
(Albuquerque et al., 2017) and Se supplementation declined the activity of cytosolic malic enzyme leading to decline in abdominal fat deposition
(Vadhanavikit and Ganther, 1994).
Konca et al., (2009) reported that supplementation of Se alone decreased the abdominal fat pad. Fat deposition in the abdominal area of broilers is regarded as waste in the poultry industry and represents a loss in the market and consumer acceptability
(Toghyani et al., 2011). The increase in carcass fat of broilers raised under hot ambient temperature is a major concern since the fat content of meat products has become increasingly important to consumer perceptions of the healthfulness of meat and abdominal fat pad yield increased from 1.8 in normal control to 2.78% by more than 50% in heat stressed control group
(Tawfeek et al., 2014). Retention of more abdominal fat in chickens exposed to heat stress was credited to decreased capacity of protein synthesis and of peripheral lipolysis
(Ain-Baziz et al., 1996 and
Zhang et al., 2012).
Giblet yield (percentage of dressed weight)
The supplementation of vitamin E and Se in the diet of heat stressed broilers in the present study (Table 3) increased the giblet yield (p=0.002). At high ambient temperatures, birds reduced their feed intake and subsequently less nutrients were supplied to the internal organs leading to compromised developments
(Park et al., 2013) and resulted decrease in the giblets yield
(Attia et al., 2011). Supplementations of anti-oxidants in the diet ameliorate this effect.
Lymphoid organ weight (percentage of dressed weight)
The relative weight of spleen showed significant (p=0.026) increase in T1 group as compared to control. No variations were observed in thymus and bursa amongst experimental groups. The reason for enhanced relative weight of spleen in the present study might be due to proliferation of lymphocyte in this organ
(Akbari et al., 2018). Swain et al., (2000) reported non-significant effect of vitamin E and Se (150, 300 mg/kg vitamin E + 0.1, 0.5, 1 mg/kg Se) supplementation in broiler chicken on relative weight of thymus, while
Shaik et al., (2005) in their experiment on supplementation of 150, 300 IU Vitamin E/kg+0.15 mg Selenium / kg di
et also could not find any effect on the weight of bursa in broiler chicken.
Effect on serum HSP70 (ng/ml)
Supplementation of Vitamin E and Se significantly reduced the HSP70 level (Table 4) in all the supplemented groups (T1, T2 and T3). Dietary antioxidant supplementation suppressed the expression of HSP70 by eliminating ROS and stabilizing antioxidant status of birds in summer season
(Jang et al., 2014). It is likely that Se or Vitamin E diet supplementation may restrict the expression of HSPs by increasing the activities of superoxide dismutase and subsequent removal of ROS
(Dokladny et al., 2006). Dietary vitamin E significantly decreased the mRNA expression of HSP70
(Jang et al., 2014). Recently, it has been proposed that the lipid composition and the architecture of membranes act as membrane censors and modulate HSPs response through the HSF-1. Therefore, the up regulation of heat shock proteins in stressed birds may be due to the damages of oxidative stress in the muscle cells. Furthermore, changes in HSP70 may be an indication of cellular damage within the intestines
(Dokladny et al., 2006). Moreover, dietary antioxidants vitamin E and Se might alleviate heat stress in birds during summer conditions by down-regulation of HSP expression which might be associated with the modulation of pro-inflammatory cytokine expression
(Rettenbacher and Palme, 2009 and
Chang et al., 2010 ) and by eliminating ROS
via stabilizing antioxidant status in birds under summer conditions
(Jang et al., 2014). Combined action of Se and Vitamin E increased the antioxidant capacity and enhanced the GPx1 and GPx4 mRNA levels, which might successfully eliminate most of the ROS as the Se plus Vitamin E supplementation in broiler feed could be able to enhance the endogenous antioxidant defense system by suppressing the lipid oxidation and by regulating the heat shock proteins and thus ameliorate the negative effects of summer heat stress conditions
(Kumbhar et al., 2018).