Climate change has negatively impacted poultry production in tropical regions, primarily due to increases in temperature, which cause heat stress. Poultry exhibit a rapid metabolic rate and limited capacity to dissipate excess body heat due to the lack of sweat glands, which makes them highly sensitive to heat stress (Nawaz et al., 2021). Economically, climate change has significant impacts, as heat stress can compromise growth rates and reduce feed efficiency. In this case, energy derived from feed, normally used to support growth, is diverted to restoring and maintaining homeostasis under stressful conditions (Vandana et al., 2020).

Heat stress in poultry can trigger an imbalance between heat production and the body’s ability to dissipate heat. Under these conditions, chickens demonstrate various adaptive physiological and hematological changes, including increased rectal temperature, respiratory and heart rates, blood glucose levels, and changes in their erythrocyte, hemoglobin, hematocrit, and differential leukocyte count (Brugaletta et al., 2022). When these changes exceed the physiological range, chickens can be highly susceptible to health problems, weakened immune function, decreased productivity, and compromised welfare (Oluwagbenga & Fraley, 2023).

Indonesia is a country with abundant and diverse genetic resources, one of which is the native chicken. The Kedu is a type of native chicken with potential as a meat producer (Setiaji et al., 2025). In the current poultry industry, feed efficiency and environmental sustainability have recently become important trends and are a shared concern for farmers. Although the growth rate of the Kedu chicken is relatively slower than that of modern broiler chickens, the advantage of this local chicken breed is its better adaptability to local environmental conditions (a hot and humid tropical climate). This ability is invaluable in ensuring sustainable production in the face of climate change and the increasingly serious threat of disease in commercial breeds (Maharani et al., 2021); (Kpomasse et al., 2023).

To mitigate the adverse effects of heat stress on the growth performance and health of modern broiler chickens, producers typically construct chicken houses with controlled internal environmental conditions. However, the cost of constructing these is very high. This situation makes it unaffordable for indigenous chicken farmers, who typically operate on a small scale with very minimal capital. With specific regard to Kedu chickens, (Mushawwir et al., 2024) note that they demonstrate excellent tolerance to high tropical temperatures. Owing to this characteristic, they are likely to require different environmental conditions than modern broiler strains. Chicken body weight is one factor that determines the severity of heat stress during rearing. Under typical conditions, higher body weight increases the risk of heat stress. (Gogoi et al., 2021) document that commercial broiler chickens with higher body weights are more vulnerable to heat stress compared to those with lower weights. In line with this, native chickens with higher body weights have also been shown to be more vulnerable to heat stress (Boonkum et al., 2024). More specifically, (Malila et al., 2022) found that chickens approaching market age (harvesting age) were more prone to heat stress than younger chickens.

A number of studies have been conducted to compare the resilience to heat stress and physiological changes between native chicken breeds from tropical countries and modern broiler chickens (Tirawattanawanich et al., 2011); (Malila et al., 2022); (Malila et al., 2023). However, to date, little research has focused on the comparative effect of heat stress on Kedu and modern demonstrate various adaptive physiological and hematological changes, including increased rectal temperature, respiratory and heart rates, blood glucose levels, and changes in their erythrocyte, hemoglobin, hematocrit, and differential leukocyte count (Brugaletta et al., 2022). When these changes exceed the physiological range, chickens can be highly susceptible to health problems, weakened immune function, decreased productivity, and compromised welfare (Oluwagbenga & Fraley, 2023).

Indonesia is a country with abundant and diverse genetic resources, one of which is the native chicken. The Kedu is a type of native chicken with potential as a meat producer (Setiaji et al., 2025). In the current poultry industry, feed efficiency and environmental sustainability have recently become important trends and are a shared concern for farmers. Although the broiler chickens. Therefore, a study comparing the impact of heat stress on the two strains as a basis for considering whether or not it is necessary to build environmentally controlled broiler houses for Kedu chickens is vital. Taking all these facts into consideration, this study was conducted to investigate the effects of thermal stress during the two weeks prior to market age on the physiological and hematological responses of Kedu and broiler chickens.

The study was approved by the Animal Ethics Committee of the Faculty of Animal and Agricultural Sciences, Universitas Diponegoro, Indonesia (Approval No. 61-08/A-17/KEP-FPP).

Body weight, feed consumption, and the feed conversion ratio (FCR) were recorded throughout the study. At the end of the period (when the Kedu chickens were 12 weeks old and the broiler chickens 5 weeks old), two chickens in each replication or pen with body weights close to the average pen weight were selected. The rectal temperature of these chickens was measured using a digital rectal thermometer inserted into the chicken’s cloaca. The respiratory rate of the selected chickens was also measured by counting chest movements over a minute, while heart rate was measured directly using a stethoscope placed on the left-hand side chest of the birds. Subsequently, the same chickens had blood drawn from the brachial vein on the wings. The sample collections were conducted at around 1:00 p.m.

A 1 mL blood sample was placed in a blood tube containing ethylenediaminetetraacetic acid (EDTA) for complete blood count analysis, and 2 mL of blood was placed in a non-EDTA blood tube for serum glucose and uric acid analysis. Complete blood counts were analyzed using a Prima fully automated hematology analyzer, manufactured by PT. Prima Alkesindo Nusantara, Jakarta, Indonesia. The non-EDTA blood was centrifuged at 500 × g for 10 minutes to produce serum. The levels of glucose were determined using spectrophotometric techniques, while uric acid analysis was conducted based on enzyme-based colorimetric techniques according to the manufacturer’s protocols (DiaSys Diagnostic System GmbH, Holzheim, Germany).

After the final body weight was measured, the chicken from which blood had been drawn was slaughtered following halal convention. The blood was collected in a special container and weighed to determine the percentage of blood in the chicken’s body. The feathers were then plucked and their weight measured to determine their percentage to live weight. The chicken was then dissected, and the internal organs were removed and weighed to determine their relative weight to the chicken’s live weight.

Data were treated according to a 2 (Kedu and broiler chickens) × 2 (23.83 ± 0.23 °C and 35.33 ± 1.78 °C) factorial arrangement on the basis of the general linear models procedure in SAS (SAS Inst. Inc., Cary, NC, USA). Significant interactions between the main factors were included in the statistical model, whereas any insignificant interaction effect was eliminated. The results are presented as least-squares means (LSMEANS) and standard error (SE) for the treatment groups. Duncan’s Multiple Range Test was conducted to show the differences in LSMEANS (p≤0.05).

The data on the growth performance of the Kedu and broiler chickens raised in temperatures of 23.83 ± 0.23 °C and 35.33 ± 1.78 °C are presented in Table 2. There was a significant interaction (p≤0.05) between chicken type and temperature for the final BW, weight gain, feed intake, and water intake, while no interaction was observed between the two factors concerning the FCR of the broiler chickens. In addition, there was no significant difference in weight gain between the Kedu chickens raised at the two temperatures, while the weight gain in the broiler chickens was significantly lower (p≤0.05) at 35.33 ± 1.78 °C compared to 23.83 ± 0.23 °C. At 35.33 ± 1.78 °C, feed consumption decreased (p≤0.05) and water intake increased (p≤0.05) for both types of chicken. The broiler chickens had a lower FCR (p≤0.05) than the Kedu ones, while the temperature of 35.33 ± 1.78 °C significantly increased (p≤0.05) FCR values for both.

Data on the physiological variables of the chickens are presented in Table 3. Significant interactions (p≤0.05) were observed for rectal temperature, respiratory rate, heart rate, and glucose levels. Rectal temperature and heart rate increased significantly (p≤0.05) in both chicken species due to exposure to the temperature of 35.33 ± 1.78 °C for two weeks of rearing. The respiratory rate increased (p≤0.05) by approximately 2.68 times in the Kedu chickens and 4.40 times in the broiler chickens following exposure to 35.33 ± 1.78 °C. No significant interaction (p>0.05) was observed between the two factors in terms of the H/L ratio. However, the ratio of the Kedu chickens was higher (p≤0.05) than that of broiler chickens, irrespective of rearing temperature. Rearing at 35.33 ± 1.78 °C increased (p≤0.05) glucose levels in the broiler chickens, but these levels did not change in the Kedu chickens. Regardless of chicken type, uric acid levels increased (p≤0.05) due to the high temperatures.

The hematological profiles of the Kedu and broiler chickens raised under the two temperatures are presented in Table 4. A significant interaction (p≤0.05) was observed for PDW, but other parameters showed no interaction between the two factors. Heterophils, mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) levels were higher (p≤0.05) in the Kedu chickens compared to the modern broilers, while platelet counts were higher (p≤0.05) in the broilers than in the Kedu chickens. Irrespective of chicken type, erythrocytes, hemoglobin, and hematocrit decreased (p≤0.05) with exposure to 35.33 ± 1.78 °C. Platelet distribution width (PDW) in the broiler chickens increased (p≤0.05) at 35.33 ± 1.78 °C, but not in the Kedu chickens.

The proportions of blood and feathers, and the relative weight of the chickens subjected to the two temperatures are presented in Table 5. A significant interaction (p≤0.05) between chicken type and temperature was observed for the proportions of feathers and liver relative weight, while no interaction was observed for the other parameters. The proportion of blood was higher (p≤0.05) in the Kedu chickens than in the broilers, while rearing at 35.33 ± 1.78 °C significantly decreased (p≤0.05) the proportion of blood in the chickens’ bodies. Rearing at 35.33 ± 1.78 °C caused a decrease (p≤0.05) in the proportion of feathers in the broilers, but this did not occur in the Kedu chickens. The higher rearing temperature reduced (p≤0.05) the relative weight of the liver in the Kedu chickens, but not in the modern broilers. Irrespective of rearing temperature, the Kedu chickens had a higher relative gizzard weight (p≤0.05) compared to the broilers. The high temperature decreased (p≤0.05) the relative weight of the thymus and Bursa of Fabricius in both chicken species. Compared to broiler chickens, the Kedu type had higher relative spleen and Bursa of Fabricius weights (p≤0.05).  

In general, poultry are highly susceptible to heat stress due to their limited ability to dissipate excess body heat (Nawaz et al., 2021); (Oluwagbenga & Fraley, 2023). However, unlike modern broiler strains, native chickens have better tolerance to high temperatures (Mushawwir et al., 2024). The study found no significant difference in weight gain between the Kedu chickens raised at 23.83 ± 0.23 °C and 35.33 ± 1.78 °C. Unlike the Kedu chickens, weight gain in the broilers decreased with increased rearing temperature. The study data further indicate that Kedu chickens, a native Indonesian breed, have a higher tolerance to heat stress due to their innate genetic adaptation to hot tropical environmental conditions, as evidenced by their more effective and efficient physiological stress responses and physical structures that facilitate heat loss (Tirawattanawanich et al., 2011). In contrast, modern broiler chickens lack a thermoregulatory system that aligns with their rapid growth. As a result, as they age and gain weight, broiler chickens become increasingly sensitive and vulnerable to heat because their heat dissipation area decreases relative to their body mass (Malila et al., 2022). Heat stress is a factor that can alter homeostasis in poultry. Loss of body fluids is a consequence of heat stress due to increased panting frequency (Sugiharto, 2020). To compensate for this fluid loss, both chicken species studied increased their water intake at high rearing temperatures. Interestingly, broiler chickens had a higher increase in drinking water consumption (20.5%) compared to the Kedu type (13.6%). Reduced feed consumption during heat stress is a way for chickens to reduce their metabolic heat production (Nawaz et al., 2021). Consistent with Nawaz et al., both chicken species in our study reduced their feed consumption under high temperature conditions. One interesting finding was that the broiler chickens displayed a greater decrease in feed consumption (11.17%) than the Kedu ones (8.92%). Although increased water consumption and reduced feed intake can be indicators of stress in poultry, it was difficult to establish which type of chicken was more stressed at temperatures of 35.33 ± 1.78 °C, as the Kedu type exhibited a higher water-to-feed ratio (4.00) than the broiler chickens (3.46). The FCR value in this study increased in reaction to high temperatures, indicating that feed efficiency decreased due to heat stress. The increase in the FCR value in the Kedu chickens was significantly greater (23.09%) than that of the broiler chickens (21.90%). This seems to be associated with the relatively smaller decrease in feed intake in the Kedu chickens caused by heat stress compared to the broilers. Overall, the conditions discussed above suggest that although heat stress did not notably affect weight gain, it compromised feed efficiency in the Kedu chickens.

Many parameters are commonly used to indicate heat stress in chickens, one of which is rectal temperature. In this study, rectal temperature in both chicken breeds increased in response to high temperatures during two weeks of rearing. However, the increase was greater in the broilers (1.3 °C increase) compared to the Kedu chickens (0.3 °C increase). This suggests that modern broiler strains are more prone to heat stress. Indeed, increased body temperature in chickens can indicate an imbalance between metabolic heat production and heat dissipation (Oke et al., 2021)(Rostagno, 2020)(Sugiharto, 2020). Respiration rates also increased significantly in response to heat stress, especially in the broilers. This suggests that these chickens are more responsive to heat stress than native ones. In line with our study, (He et al., 2019) noticed an elevation in respiratory rate from 47.8 breaths/min at 23 °C to 127 breaths/min at 32 °C. In addition, (Kim et al., 2025) found an increase from 59.1 breaths/min at 21 °C to 104.0 breaths/min at 25 °C, and even up to 162.7 breaths/min at 33 °C. In poultry, an increased respiration rate (panting) is the principal evaporative mechanism for cooling the body (Oluwagbenga & Fraley, 2023). In line with this rate, the heart rate also increased in both chicken types following the increase in environmental temperature, with a greater increase found in the modern broilers compared to the Kedu chickens. (Xu et al., 2018) demonstrated that heat-stressed chickens had respiratory rates of 120-160 breaths/min, in conjunction with elevated heart rates. (Kim et al., 2021) also observed that the heart rate rose from 253-287 beats/min at 22 °C to 327-352 beats/min at 32 °C. Such an increase is a physiological response by chickens to increased oxygen demand during stress and their effort to maintain homeostasis conditions (Gogoi et al., 2021). In addition to the effects of high temperatures during two weeks of rearing, increases in rectal temperature, respiratory rate, and heart rate in chickens may also indicate physiological responses to elevated body temperatures caused by high protein levels in feed, potentially exacerbating heat stress. (Furlan et al., 2004) reported that high-protein feeds are often attributed to increased heat gain (heat increments) and metabolic heat production, suggesting that such diets may lead to higher body heat production. In our study, the crude protein content in the broiler feed was 2% higher than that in the Kedu feed. Considering all of these facts, the observed increases in rectal temperature, respiratory rate, and heart rate in the broilers at a rearing temperature of 35.33 ± 1.78 °C were likely to have been influenced not only by the hot ambient temperature but also by the increased crude protein content in their feed.

As a physiological response to high temperatures, blood glucose levels increased significantly in the broiler chickens, although these remained unchanged in the Kedu chickens after exposure to high temperatures. Increased blood glucose levels indicate activation of the sympathetic nervous system and the release of glucocorticoids, which stimulate glycogenolysis and gluconeogenesis activities to supply more energy during stressful conditions (Siddiqui et al., 2021)(Zmrhal et al., 2023). In line with our findings, (Mohammadizad et al., 2025) reported an increase in glucose levels from 253.2 mg/dL at 21-22 °C to 261.0 mg/dL after three days at 33-34 °C, whereas (Abuajamieh et al., 2025) noted an increase from 254.0 mg/dL at 22.6 °C to 292 mg/dL after seven days’ rearing at 33.7 °C. When considering the potential effects of differing crude protein levels in diets on the blood glucose levels of broiler and Kedu chickens, it seems quite likely that variations in protein content may not have had a major impact. Supporting this view, (Son et al., 2024) demonstrated that reducing the dietary crude protein levels (by up to 4%) in broilers did not significantly alter their blood glucose levels. Uric acid levels also increased under heat stress conditions in both chicken types. This was closely related to increased protein catabolism to supply amino acids for gluconeogenesis (energy production) in the liver during thermal stress. Indeed, protein catabolism during heat stress may cause higher levels of uric acid as a byproduct of protein breakdown (Zeng et al., 2024)(Gharib et al., 2025).

Heat stress is typically associated with an increased H/L ratio in poultry (Sugiharto et al., 2017). However, data from our study did not show such an impact on either chicken species during rearing. The exact explanation for this condition is unknown, but it is likely that the two-week heat stress treatment accustomed the chickens to high temperature conditions, rendering the impact of heat stress on the H/L ratio insignificant. Consistent with this inference, Ogundeji and (Ogundeji & Ayo, 2025) reported that heat-habituated chickens showed no change in their H/L ratio during rearing at high temperatures. Irrespective of heat stress treatment, Kedu chickens had a higher H/L ratio compared to broiler chickens. Typically, the H/L ratio in chickens is strongly influenced by genetics (species/ strain) and the environment. In addition to being an indicator of stress, the ratio also indicates the capacity of the chicken’s immune system, as this is defined by the proportion of heterophils and lymphocytes in the blood (Stefanetti et al., 2023). As an indicator of stress, the proportion of heterophils will increase and that of lymphocytes will decrease in line with the stress experienced by chickens (Sugiharto et al., 2017)(Stefanetti et al., 2023). In this study, the impact of heat stress was not seen on the H/L ratio, so it can be inferred that the high H/L ratio in the Kedu chickens was due to a higher proportion of heterophils (irrespective of stress treatment, there was no difference in lymphocytes between the two types of chicken). Consequently, it had an impact on improving the innate immune system (first line of defense) in the chickens.

Heat stress showed varying effects on the hematological profiles of the Kedu and broiler chickens. Leukocyte counts did not change for either under high temperature conditions. Likewise, the lymphocyte percentage in both types of chicken were unaffected by thermal stress. These results are different from those reported by (Sugiharto, 2020), which indicated that heat stress was associated with the lower counts of leukocytes and lymphocytes. In this context,(Molnár et al., 2021) revealed that the leukocyte and lymphocyte count of chickens may vary in response to heat stress, depending on its length and extent. Regardless of the heat stress response, the heterophil proportion was significantly different between the two chicken types, with Kedu chickens having higher values than those of the broilers. As a critical part of the innate immune system, high heterophil implies that Kedu chickens exhibit stronger immune competence than broilers. Similar findings were also made by (Malila et al., 2023), who found that Thai native chickens exhibited a better innate immune system than modern broiler types. However, in contrast to our study, (Anoh et al., 2025) noted no difference between Philippine native chickens and broilers with respect to the heterophil count in the blood. In another study, (Duangjinda et al., 2017) reported that native chickens had lower heterophil counts than modern broilers. Overall, the above variations seemed to be closely related to the genetic potential of each indigenous chicken (Duangjinda et al., 2017)(Anoh et al., 2025).

Irrespective of chicken type, the values of erythrocytes, hemoglobin, and hematocrit decreased in response to heat stress. In such cases, heat stress inhibits the production of the hormone erythropoietin, thereby inhibiting erythrocyte production (Ayo & Ogbuagu, 2021). This causes a decrease in hematocrit and hemoglobin in chickens exposed to heat stress. Regardless of heat stress treatment, MCV and MCH levels were higher in the Kedu chickens than in the modern broilers. Similarly, the levels of MCV and MCH were found to be higher in indigenous Vanaraja chickens than in broiler chickens (Mohanty & Acharya, 2020). In contrast to our findings, (Baudouin et al., 2021) reported that indigenous chickens had lower MCV and MCH values than broilers. MCV is a parameter used to indicate the size of erythrocytes, while MCH is used to measure the amount of hemoglobin per blood cell. Although the values of both parameters vary between chicken species, they remain within the normal physiological range (Odunitan-Wayas et al., 2018). Irrespective of high temperature treatment, platelet counts were higher in the modern broilers than in the Kedu chickens. In agreement with our findings, (Mabelebele et al., 2017) reported that Ross 308 broilers had higher platelet counts than indigenous Venda chickens. Platelets can indicate potential inflammation in chickens. This is because they release inflammatory mediators and are involved in the immune response, with levels increasing under conditions of stress and infection (Ferdous & Scott, 2023). Based on this, it can be assumed that broiler chickens have a higher potential for stress than native types. In our study, PDW increased significantly in the broiler chickens due to heat stress, but this was not observed in the Kedu chickens. PDW shows heterogeneity of platelet morphology due to the presence of large platelets alongside normal-sized ones. Under conditions of heat stress, (Farran et al., 2025) also reported an increase in PDW in laying hens. This is because heat stress can induce inflammation and platelet activation, encouraging the chicken’s body to produce younger and activated platelets, which vary more in size, thereby increasing PDW (Farran et al., 2025).

The blood proportion (relative to live body weight) was higher in the Kedu chickens than in the modern broilers. Generally, blood volume in chickens varies, largely determined by their body weight. Chickens with smaller weights typically have a higher blood percentage than those with larger body weights. Despite the Kedu chicken’s smaller body weight, their higher blood percentage indicates that this type of chicken has better oxygen transport and greater heat dissipation capacity, thus significantly supporting its resilience to heat stress (Beckford et al., 2020). In both types of chicken, heat stress caused a decrease in the blood percentage relative to the chicken’s body weight. Consistent with our study, (Kim et al., 2025) noted a decrease in whole blood volume in broiler chickens reared at 33 °C for 31 days, compared to broiler chickens reared at 21 °C for the same period. In this context, heat stress can cause significant loss of body fluids, primarily through increased panting frequency, which releases water vapor. Unlike Kedu chickens, high temperatures during rearing reduced the proportion of feathers in the broiler chickens. (Wasti et al., 2020) reported that heat stress causes broiler chickens to lose their feathers because, when heat-stressed, they frequently flap their wings to cool their bodies. Over time, this activity can potentially damage the chicken’s feathers. In contrast to studies reporting an increase in relative liver weight in broiler chickens due to fat accumulation under heat stress (Ma et al., 2022), the relative liver weight of Kedu chickens in fact decreased due to heat stress over the two-week rearing period. The precise reason for this decrease remains unclear, but it is highly likely that the heat stress treatment did not cause fat accumulation in the liver of Kedu chickens. Furthermore, the reduction in final body weight (although not statistically significant) in the Kedu chickens may further explain the decrease in relative liver weight, as the live body weight of the Kedu chickens serves as the denominator during calculations. This condition causes a decrease in the relative liver weight in these chickens exposed to heat stress. Although heat stress did not affect relative gizzard weight, the Kedu chickens exhibited a higher relative weight than the broilers. The higher crude fiber content in Kedu chicken feed compared to that in broiler feed was likely responsible for the greater relative gizzard weight in the Kedu chickens. (Jha & Mishra, 2021) explain that crude fiber in feed can stimulate the physical grinding activity of the gizzard, thereby stimulating more intensive muscle layer development and ultimately resulting in increased gizzard weight. Our study shows that Kedu chickens have a higher relative weight of immune organs, especially the spleen and bursa of Fabricius, compared to broilers, indicating that they have superior immune competence. Although no research has directly compared the relative weight of immune organs in native and broiler chickens, the higher relative weight of the spleen and bursa of Fabricius in the Kedu chickens indicates that native chickens, especially the Kedu type, have stronger immune competence than broilers (Lola et al., 2016). Specifically related to heat stress treatment, high temperatures during the two-week rearing period significantly reduced the relative weight of the immune organs, especially the thymus and bursa of Fabricius, in both chicken types. The relative weight reduction of the two organs at high temperatures represents a thermal stress response of the immune system in both chicken species. These primary lymphoid organs are susceptible to stress, and their relative weight reduction is indicative of immunosuppression or a decrease in the proliferative capacity of T and B lymphocytes. This is consistent with the literature, which shows that heat stress can trigger the release of corticosterone, which induces atrophy in the thymus and bursa of Fabricius, potentially reducing the chicken’s immune capacity against pathogens(Huang et al., 2024) ;(Kim et al., 2025).

Heat stress for two weeks prior to market age impaired the physiological and hematological variables of the chickens, as indicated by the increases in rectal temperature, respiratory rate, heart rate, and blood levels of glucose, uric acid levels and MCHC, alongside reductions in erythrocytes, hemoglobin and hematocrit, the proportions of blood and feathers to live body weight, and the relative weights of the thymus and Bursa of Fabricius. Compared to broilers, Kedu chickens showed more tolerance to heat stress, as indicated by the absence of an effect on their weight gain, blood glucose levels, PDW, and feather proportion. Therefore, farmers do not need to build environmentally controlled broiler houses for Kedu chickens, as they do not require the same environmental conditions as commercial broiler types.