Effects of heat stress on growth performance, selected physiological and immunological parameters, caecal microflora, and meat quality in two broiler strains

Objective This study was conducted to investigate the effects of normal and heat stress environments on growth performance and, selected physiological and immunological parameters, caecal microflora and meat quality in Cobb 500 and Ross 308 broilers. Methods One-hundred-and-twenty male broiler chicks from each strain (one-day-old) were randomly assigned in groups of 10 to 24 battery cages. Ambient temperature on day (d) 1 was set at 32°C and gradually reduced to 23°C on d 21. From d 22 to 35, equal numbers of birds from each strain were exposed to a temperature of either 23°C throughout (normal) or 34°C for 6 h (heat stress). Results From d 1 to 21, strain had no effect (p>0.05) on feed intake (FI), body weight gain (BWG), or the feed conversion ratio (FCR). Except for creatine kinase, no strain×temperature interactions were observed for all the parameters measured. Regardless of strain, heat exposure significantly (p<0.05) reduced FI and BWG (d 22 to 35 and 1 to 35), immunoglobulin Y (IgY) and IgM, while increased FCR (d 22 to 35 and 1 to 35) and serum levels of glucose and acute phase proteins (APPs). Regardless of temperature, the Ross 308 birds had significantly (p<0.05) lower IgA and higher finisher and overall BWG compared to Cobb 500. Conclusion The present study suggests that the detrimental effects of heat stress are consistent across commercial broiler strains because there were no significant strain×temperature interactions for growth performance, serum APPs and immunoglobulin responses, meat quality, and ceacal microflora population.


INTRODUCTION
Over the last few decades, considerable improvements in the growth rate of broiler chickens have been achieved. These improvements can be attributed to rapid advancements in genetic selection programs. However, heat stress remains a significant environmental challenge that can be financially costly for the poultry industry. The harmful consequences of heat stress stretch from a decreased body weight gain (BWG), feed intake (FI), and increased feed conversion ratio (FCR) of live birds [1], to substandard meat quality [2]. Furthermore, adverse effects on caecal microflora have been detected in heatstressed broilers [3].
A variety of approaches for sustaining thermal tolerance of broiler chickens and reducing the harmful effects of heat stress are elicited [4]. An elevated level of glucose in chickens subjected to thermal challenges may enhance their survivability of chicken and promote maximal brain function. Creatine kinase (CK) is an enzyme that plays a role in the reversible transphosphorylation of adenosine diphosphate and creatine.
In an earlier study [5], it was revealed that thermal stress in creased the activity of blood CK. In related studies [68], it was found that heat stress can raise the level of serum in a group of proteins termed acute phase proteins (APPs). As such, these APPs, which include α1acid glycoprotein (AGP), ovotrans ferrin (OVT), and ceruloplasmin (CPN), can be employed as indicators for assessing the physiological condition of chickens [9].
Immunoglobulin, which is generated by lymphocytes, rep resents a defence mechanism against the intrusion of foreign substances into the living body. Immunoglobulin adheres to the antigen (pathogen), which is subsequently swallowed up and digested by macrophages to shield the host from harm [10]. The three major immunoglobulin classes in chicken are immunoglobulin Y (IgY), IgM, and IgA. According to the results derived from previous studies, heat stress may decrease the relative weights of lymphoid organs in both laying hens [11], and broilers [12]. In an earlier investigation [13], it was revealed that broilers exposed to heat stress were negatively affected by a reduced level of total circulating antibodies.
The commensal intestinal microbiota significantly influ ences the digestion and absorption of nutrients, as well as the development of immunity [14]. The activities of com mensal intestinal bacterial populations, also has a notable impact on the physiological and pathological conditions of the host. The adverse effects of heat stress on the intestinal mucosa and microbiota of broiler chickens and pigs have been verified through several investigations [1,15,16].
The influence of preslaughter stress reactions on ante and postmortem muscle metabolism can lead to i) an increase in the speed and degree of glycogen breakdown, ii) a drop in pH, and iii) drip loss [17]. These effects of preslaughter stress are mainly attributed to alterations in the activities of ade nosine triphosphate (ATP) and muscle glycogen reserves. Exposure to heat can have direct effects on organ and muscle metabolism [18]. For instance, the threats of palesoftexu dative meat in turkeys, and heat shortening in broilers, can be promoted by heat stress. Also, operational alterations executed to counter the threats arising from heat exposure, can indirectly affect meat quality negatively [18]. Broilers that experienced an acute or shortterm heat stress just be fore slaughter, displayed pale, soft, and exudative meat changes in the quality of their meat [19].
Cobb 500 and Ross 308 are the two most popular commer cial broiler strains. The former is an early developing strain while the latter is considered a late developing strain [20]. The resistance to heat stress in relation to different breeds has been thoroughly investigated [21]. A study on heat stress resistance involving jungle fowl, village fowl and commercial broiler chickens, revealed that domestication and selective breeding were resulting in birds that are more susceptible, rather than resistant, to heat stress [22]. However, information on the response of commercial broiler chicken strains towards a hot environment is still limited. As such, our investigation aims to determine the impact of normal and high tempera tures on the growth performance, caecal microflora and immune response in two broiler strains.

Ethical note
The protocol used in this experiment was carried out in full compliance with the Research Policy and Code of Practice for the Care and Use of Animal for Scientific Purposes of Universiti Putra Malaysia.

Birds, housing and husbandry
Altogether, 120 Cobb 500 and 120 Ross 308 onedayold male broiler chicks were obtained from a commercial hatchery. Subsequent to their weighing and wing banding, the chicks were randomly assigned by strain in groups of 10, in 24 bat tery cages. These cages, which come with wire floors, were arranged in 6 rooms. Each of these rooms, in which the en vironment is controllable, contained two cages of Cobb 500 and two cages of Ross 308 birds. The cages measured 122 cm in length, 91 cm in width and 61 cm in height. The initial room temperature was fixed at 32°C on d 1, and gradually lowered until it reached 23°C on d 21. Relative humidity ex tended from 70% to 80%. On d 7 and d 21, live Newcastle disease vaccine was administered to the birds.

Diets
Both strains received the same commercial broiler starter (d 1 to 21) and finisher (d 22 to 35) diets. The feed was sup plied from a local feed mill. The nutrient specifications of the diets are presented in Table 1. The starter (crumble form) and finisher (pellet form) diets were provided ad libitum. Metabolisable energies were 12.56 and 12.98 MJ/kg in the starter and finisher diets, respectively. Crude protein (CP) was 21.0% CP in the starter diet and 19.0% CP in the finisher diet.

Heat challenge
In order to create a heat stress environment, 6 cages of birds from each strain (three rooms) were subjected to a tempera ture of 34°C from d 22 to d 35. This heat stress environment was prolonged for 6 hours each day. It is well documented that the heat challenge regimen [7,23,24], as measured by plasma corticosterone concentration, heterophil:lymphocyte ratio and heat shock protein 70 density, was stressful to broiler chickens. All throughout the heat challenge period, the reg ular temperature of 23°C was retained for the other rooms. This resulted in 4 subgroups from d 22 to d 35 (2 strains×2 temperatures).

Growth rate parameters and sampling
On d 7, 14, 21, 28, and 35, the weights of the birds were re corded individually. The FIs were recorded on a weekly basis for a period of 5 weeks. The FI data was adjusted for mortality. FCR was calculated as feed/ body weight; and the mortality rate was registered upon its occurrence. On d 35, two birds from every cage (12 birds for each straintemperature sub group) were picked at random for slaughter, which was conducted in compliance with halal procedures [25], which was performed humanely by severing the jugular veins, ca rotid arteries, trachea and oesophagus with a sharp knife by a single swipe in order to incur less pain. The exsangui nation blood samples of these randomly selected birds were gathered and stored in ice. Soon after, these blood samples were centrifuged at 4,000×g for 20 minutes at 4°C. Serum samples were garnered, kept in storage at -80°C, and later tested for their concentrations of glucose, CK, APPs, IgY, IgA, and IgM. Subsequent to exsanguination, the caecal contents were immediately frozen in liquid nitrogen, and then stored at -80°C. Bacterial quantification analysis was later conducted on these caecal contents. The slaughtered birds' entire pectoralis major breast meat was used for tests to determine the levels of pH, drip loss, and shear force.

Glucose and creatine kinase
An automated chemistry analyser (Hitachi 902 Automatic Analyser; Hitachi, Tokyo, Japan) was harnessed to determine the concentration levels of serum CK and glucose. Cat. No.: 11447513 216 (glucose) and Cat. No.: 12132524 216 (CK) com mercial kits (Roche Diagnostics, Basel, Switzerland) were utilized for this purpose.

Acute phase proteins
The CPN concentration was determined by establishing the formation rate of a coloured product from CPN and the sub strate, 1,4phenylenediamine dihydrochloride, while OVT concentration was determined using a radial immune dif fusion procedure, as described previously in detail [26]. A commercial enzymelinked immunosorbent assay (ELISA) kit exclusively designed for chickenrelated research (Cat. No. NBE60049, Life Diagnostic Inc., West Chester, PA, USA) was used for measuring the serum AGP concentration.

Bacterial quantification
A quantitative realtime polymerase chain reaction (qPCR) [27] was used to determine the Lactobacilli, Escherichia coli (E. coli) and Clostridia populations. The extraction of deoxy ribonucleic acid (DNA) from the caecal content samples was done by a QIAamp Fast DNA Stool Mini Kit (Qiagen Inc., Valencia, CA, USA). While the purity and concentration of these DNA samples were measured with the use of a Nano drop ND1000 spectrophotometer, their purification was done by MEGA quickspinTM (Intron Biotechnology, Inc., Seongnam, Korea). The total count of DNA replications for each mL of the elution buffer was appropriately computed. The structuring of standard curves was achieved through serial dilution of the PCR products from pure cultures of Lactoba cilli, E. coli, and Clostridia. The primers utilized for bacterial quantification were as follows: Lactobacilli (F5 CATCCAG TGCAAACCTAAGAG3 and R5 GATCCGCTTGCCTT CGCA3), E. coli (F50GTGTGATATCTACCCGCTTCGC3 and R50AGAACGCTTTGTGGTTAATCAGGA3), and Clostridia (F5 GAGTTTGATCMTGGCTCAG3 and R5 CCCTTTACACCCAGTAA3). Optical grade plates were used to execute the qPCR with BioRad CFX96 Touch (BioRad, Hercules, CA, USA). The PCR was performed in 25 μL of total volume. Each reaction involved 12.5 μL Maxima SYBER Green qPCR Master Mix (Thermo Fisher Scientific, Waltham, MA, USA), 1 μL of each primer, 1 μL of the DNA samples, and 9.5 μL of nucleasefree H 2 O. The amplification reaction stipulations of the DNA were 94°C for a period of 5 min, then 40 cycles of 94°C for a period of 20 s, 58°C (Lactobacilli), or 60°C (Clostridia and E. coli) for a period of 30 s, and 72°C for a period of 20 s. Subsequent to every final amplification cycle, a melting curve analysis was conducted to verify the particularity of the amplification.

Meat quality
Muscle pH: The samples to determine muscle pH were taken from the right pectoralis major. The initial pH (pH i ) was re corded 45 minutes after slaughter, while the ultimate pH (pH u ) was recorded after 24 hours of chilling. This procedure is in accordance with that modified by Abdulla et al [28]. Roughly 0.5 g of meat sample was first homogenized in 10 mL of 4°C distilled water. Subsequently, a pH meter furnished with an electrode was used for the measurement of pH (Mettler Toledo, Columbus, OH, USA). Drip loss: Samples for the determination of drip loss were obtained from the left pectoralis major following the proce dure recommended by Honikel [29]. Directly after slaughter, the weights of the separately weighed meat samples were registered as the initial weight (W1). The samples were then consigned to a sealed, vacuumed polyethylene bag and stored at 4°C. After 24 hours, the samples were removed from the bags, blot dried with the use of soft tissue paper, and reweighed (W2). The equation below was used to calculate the drip loss percentage of muscle chilled over 24 hours: Shear force: Samples from the drip loss determination were used to measure the texture of the sample via shear force anal ysis. Following the determination of the drip loss percentage, these samples were shifted into polyethylene bags, and im mersed in a water bath set at a temperature of 80°C. After a period of 30 minutes, the samples were taken out of the water bath and left to cool down to room temperature. Upon ar rival at room temperature, the samples were sliced into three subsamples measuring 10 cm in width, 1 cm in height and 2 cm in length. In order to ascertain the shear force value of each subsample, a Volodkevitch bite jaw, fastened to a TA.HD plus texture analyser (Stable Micro System, Surrey, UK) equipped with a 5 kg load cell, was brought into play. The average of the three subsample values was calculated, and registered as the shear force values.

Statistical analysis
The data were subjected to analysis of variance by way of the general linear model procedure of SAS software (SAS Insti tute Inc. Cary, NC, USA). The FI, BWG, and FCR data from d 1 to 21 were analysed with the strain as the only main effect. The FI, BWG, and FCR data from d 22 to 35 and d 1 to 35, glucose, CK, APPs, immune response, caecal microflora, and meat quality data were analysed using strain, tempera ture, and their interactions as main effects. Comparisons were made within each experimental variable, whenever the in teractions between the effects were observed to be significant. Duncan's multiple range test was employed to determine the differences between means. Chisquare analysis was applied for the mortality data, and the statistical significance was considered at p<0.05.

Growth rate and mortality
Throughout the starter phase, strain did not have any impact on FI, BWG, or FCR (Table 1). Throughout the heat exposure period, no notable interactions were perceived between strain and temperature for FI, BWG, or FCR during the finisher and overall (d 1 to 35) phases (Table 2). During the finisher and overall phases, the heat exposure significantly (p<0.01) decreased FI, BWG, and FCR. Irrespective of temperature, the Ross 308 birds displayed a significantly (p<0.05) higher BWG than Cobb 500 during the finisher and overall phases. Throughout the finisher and overall phases, FI and FCR were not influenced by strain. All through the heat exposure phase, the mortality rate was not affected by strain, but it was con siderably (p<0.05) higher in heat challenged birds than in their nonheat challenged counterparts.

Physiological stress indicators
Significant interactions were noted between strain and tem perature for serum concentration of CK, but not for glucose, AGP, CPN, or OVT (Table 3). At 35 d of age, heat stress re sulted in significantly (p<0.05) higher CK within the Cobb 500 broiler chickens, but not within Ross 308 (Table 4). Re gardless of temperature, strain had no significant effect on glucose, AGP, CPN, and OVT concentrations (Table 3). Se rum concentrations of glucose, AGP, CPN, and OVT were greater (p<0.01) in heat stressed birds than in their nonheat stressed counterparts (Table 3).

Immune response
No interactions were noticeable between strain and temper ature for IgY, IgM, or IgA (

Caecal microbial population
No interactions were detected between strain and tempera ture for caecal populations of Lactobacilli, E. coli or Clostridia (Table 6). Both strain and temperature did not have any im pact (p>0.05) on the quantified caecal microbial populations.

Meat quality measurements
No significant interactions were observed between strain and temperature for breast muscle pH i , pH u , drip loss or shear force in broiler chickens at 35 d of age (Table 7). Strain had no effect on breast muscle pH i , pH u or drip loss. However, strain significantly (p<0.05) affected the shear force value as Ross 308 broilers showed higher meat shear force than Cobb 1) The heat stressed birds were exposed to 34°C for 6 h daily. a,b Means within a row-subgroup with no common superscripts are significantly different at p < 0.05.

DISCUSSION
In the present study we evaluated the growth performance, gut microbiota and immune response of two common com mercial broiler strains, Cobb 500 and Ross 308 under heat stress conditions. The current results indicated that there were no differences in FI, BWG, or FCR between the two strains before the heat stress exposure (1 to 21 d of age). However, during the heat stress period (22 to 35 d of age), the Ross 308 birds showed significantly higher BWG compared to Cobb 500 during both the finisher and overall phases without af fecting the FCR or FI. The noted improved BWG in the former during the finisher and overall phases could also be attributed to nutritional factor. Ross 308 and Cobb 500 broilers have different metabolizable energy and CP requirements [30,31]. However, in this study, to mimic the commercial production setting, both Ross 308 and Cobb 500 broilers were fed similar commercial starter and finisher diets. Tona et al [32] who found that at 6 and 60 h post hatch, heat production was higher in Cobb than in Ross. The authors concluded that Cobb and Ross embryoschicks had different growth trajec tories leading to different patterns of growth resulting from physiological variations. However, our current results suggest that there were no genetic differences between Ross 308 and Cobb 500 in response to heat stress conditions. Working with various levels of dietary lysine, Sterling et al [33] report ed that Cobb broiler chicks gained more weight, consumed higher feed, and had a better FCR when compared with Ross 308 broiler chicks. In the present study, in terms of BWG, there were no interactions between strain and temperature during both the finisher and overall phases. This suggests that irrespective of temperature, the growth rate of Ross 308 broilers is superior to that of Cobb 500 broilers. The discrep ancies between the present findings and those of Sterling et al [33] could be attributed to the genetic disparities between the present day Ross 308 strain, and those of 2006. Unsurprisingly, heat stress adversely affects the FI, body weight, and FCR of broiler chickens. The issues related to re duced feed consumption and weight gain, in terms of broilers reared in a stifling environment, are well documented in rel evant literature [34]. The inferior growth performance of broilers subjected to heat stress conditions can be due to the behavioural, metabolic, and physiological alterations in re action to a hot environment. Other than the decline in FI, respiratory alkalosis can also be held accountable for the drop in growth rate under heat stress [35]. Moreover, the greater dispersal of energy by birds subjected to heat stress can also lead to their unsatisfactory growth rate [36]. Mor tality rate during the heat treatment was not affected by strain, but the heat challenge was detrimental to the survivability rate. The higher mortality rate during heat stress is expected, and confirmed the association between the high environ mental temperatures and the increase in mortality rate in broilers [34].
Stress triggers the liberation of catecholamines and gluco corticoids in birds. This brings about alterations in biochemical variables (such as glucose) aimed at maintaining homeostasis SEM, standard error of the mean for main effects (n = 24); Temp, temperature. 1) The heat stressed birds were exposed to 34°C for 6 h daily. 1) The heat stressed birds were exposed to 34oC for 6 h daily. a,b Means within a row-subgroup with no common superscripts are significantly different at p < 0.05. [37]. During our investigation, it was observed that heat stress caused an increase in the levels of blood glucose. This obser vation is in agreement with that of Akşit et al [38]. A rise in the level of glucocorticoids contributes directly towards an elevation in the concentration of blood glucose [39]. Gluco corticoids significantly influence metabolism by inciting gluconeogenesis from muscle tissue proteins, lymphoid and connective tissues. They also dictate many aspects of glucose homeostasis. The main role of glucocorticoids, in the con text of glucose homeostasis, is the conservation of plasma glucose for the brain during a stressful situation. This is im portant as transitorily higher blood pressure is essential for the achievement of optimal brain function [40]. The CK is a fundamental enzyme for the continued supply of intracellular energy through the spatiotemporal buffering of ATP con centrations [41]. An increase in plasma CK is indicative of skeletal muscle damage or myopathy, the consequence of disruptions in muscle cell membrane (sarcolemma) function and permeability [42]. In our study, we detected noteworthy interactions between strain and temperature for CK. These interactions became evident when the heat stressed Cobb 500 birds exhibited substantially greater CK than their non heat stressed counterparts. On the contrary, temperature did not significantly influence CK in Ross 308 broilers. These re sults suggest that, Cobb 500 birds were more susceptible to heat stress than Ross 308. According to the present results, there was a significant elevation in serum concentrations of AGP, CPN, and OVT following heat exposure. These findings are in accord with recent studies indicating that heat stress elevates serum levels of APPs in broilers [68], and thus APPs are reliable stress biomarkers in broilers. The goal of the APPs is to reestablish homeostasis. While AGP is an established immunoregulator that influences Tcell performance and conveys negative feedback on the acute phase response, CPN portrays a more shielding role by its removal of oxygen radicals, its antihistamine activity, and its reversal of the hypoferrae mic state of the APR [43]. The OVT, an iron binding protein, can impart antimicrobial properties by requisitioning iron. This protein also comes with the capacity to adjust the hetero phil and macrophage roles in chickens. Thus, the reaction of APPs can be deemed part of the overall physiological stress response, which involves the hypothalamicpituitary adrenal axis and the sympathetic system. Leshchinsky and Klasing [44] compared APPs response to disease challenge in broiler and layer chickens and concluded that the latter showed a better reaction. During our investigation, it was established that the serum levels of AGP, CPN, and OVT are identical for the Cobb 500 and Ross 308 strains. The prominent disparities between broilers and layers, as identi fied by Leshchinsky and Klasing [44], imply that discrepancy in the rate of growth may influence the response of APPs in poultry.
Under commercial broiler management conditions, fast growing broilers exhibited high mortality from commonly encountered infectious or metabolic diseases when compared with slower growing groups of birds [45]. The present study indicated that Cobb 500 chickens had a higher serum level of IgA than Ross 308 while strain had a negligible effect on concentrations of IgM and IgY on d 35. This suggests that IgA appears to be less sensitive to the heat challenge than IgM and IgY. According to Cheema et al [46], genetic selection for a better broiler performance, resulted in an unfavourable effect on the adaptive element of the immune response (antibody generation). Our current results further support the study by Cheema et al [46] who reported higher weight gains in Ross 308 was associated with lower serum IgA levels than those of Cobb 500. According to our results, the heat challenge sig nificantly reduced the levels of IgY and IgM, but not IgA on d 35. These results confirm the claim from previous studies [4749] that heat stress damages the immune response in poultry.
The commensal intestinal bacterial populations also play an important pathological effect on the host [14]. This in tricate ecosystem achieves this by i) contending for epithelial binding locations and nutrients, ii) reinforcing the intestinal immune response, and iii) generating antimicrobial bacterio cins [15]. Song et al [3] reported lower counts of Lactobacillus spp. and higher counts of Clostridium spp. in heatstressed broilers. Contrastingly, our results indicated that the effect of temperature on caecal populations of Lactobacilli, E. coli or Clostridia, is minimal. This contradiction may be due to differences in the harshness and time span of these heat en durance exercises. While Song et al [3] exposed broilers to a temperature of 33°C for 10 h each day from d 22 to d 42, our study involved a temperature of 34°C for 6 h each day from d 22 to d 35. The genetic alterations related to enhanced weight gain and FI, also led to transformations in the birds' gut physiology and microbial population makeup [50]. Gut microflora profoundly improves the nutrition, health and growth performance of its host [51]. Our investigations re vealed that the caecal populations of Lactobacilli, E. coli, and Clostridia, were the same for Ross 308 and Cobb 500 birds.
Thus, it appears that the greater weight gain in Ross 308 birds when compared to their Cobb 500 counterparts was not associated with intestinal microflora.
Heat stress, feed retraction, conveyance and liquid depri vation, are among preslaughter stressors that can affect meat quality [52]. Preslaughter heat stress can quicken the develop ment of rigor mortis, which encourages a rapid pH decline, a lower pH u , and an increased lightness value in bird meat. The ultimate result is chicken/turkey meat with pale, exuda tive features [19,53]. Generally, muscle pH is roughly 7, and descends quickly subsequent to slaughter. In our study, the meat pH 24 hours postslaughter arrived at an average of 5.90 www.ajas.info

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Awad et al (2020) Asian-Australas J Anim Sci 33:778-787 to 6.09. According to our results, while strain had no distinct effect on meat pH values, heat exposure drastically decreased pH i and pH u values. This could be linked to disparities in the glycogen stores of the muscle, or disparities in the glycolytic possibilities between heat enduring and nonheat enduring situations. It appears that the greater pH fall was caused by the higher rate of post mortem glycolysis which led to the higher production of lactate, and the lowered ultimate pH in the meat of broilers. Breast meat pH u measurements 24 hours after chilling at 4°C revealed an overall decline in pH. The more pronounced reduction in pH u in comparison to pH i , could be due to the further extent of post mortem glycolytic metabolism in more aged muscles [54]. The retention and gains or losses of water, need to be seriously considered as they affect the weight of the chicken, and consequently, its economic value. Also, the amount and manner of water dis semination in muscles play a prominent role in determining the meat's appearance, tenderness and succulence. Previous studies have indicated that severe and persistent heat stresses can lead to mediocre waterholding properties [53,55]. The elevated metabolic rate of rigor mortis associated with heat stress, causes marked protein denaturation in muscles [56,57]. This condition reduces the protein's capacity for waterbind ing, which consequently culminates in unsatisfactory water holding properties [58]. Lu et al [59] stated that drip loss is obvious in the muscles of heatstressed Arbor Acres broilers. In our investigation, however, both Cobb 500 and Ross 308 strains showed a negligible effect on drip loss during heat endurance. These conflicting outcomes could be due to dis parities in the intensity of the heat challenge. While the birds in our study were subjected to a temperature of 34°C for 6 hours daily from d 21 to d 35, the birds in the experiment by Lu et al [59] were exposed to constant heat stress condi tions. In our present results, it was observed that shear force (a measure of meat tenderness) is substantially greater in the meat of heat stressed birds, than in their nonheat stressed counterparts. This outcome is in harmony with that of Dai et al [60] who found that shear force is greater in the breast meat of heat stressed birds, than in the breast meat of control broilers raised in a normal temperature. Heat stress can in duce liquid outflow from muscle, loss of soluble nutrients and flavour. This is likely to render the muscle dry, hard and tasteless [60].

CONCLUSION
In conclusion, considering the lack of significant strain× temperature interactions for all the parameters measured, except for CK, the Cobb 500 and Ross 308 strains did not differ in any major way in response to heat stress. However, irrespective of temperature, the Ross 308 showed higher BWG during the finisher phase and higher serum IgA concentration on day 35 than their Cobb 500 counterparts.

CONFLICT OF INTEREST
We certify that there is no conflict of interest with any financial organization regarding the material discussed in the manu script.