Usage of outdoor
The data about using open area are given in Table 2. As seen in that Table, the proportion of birds outside at all three times (morning, noon, and afternoon) was significantly (P < 0.01 for all) affected by the seasonal factor, and the data for the spring is higher. Naturally, it is expected number of birds in the open area will be lower in the summer when a very high ambient temperature is experienced, and that was the case in this study as well. Singh and Cowieson (2013) determined that the ambient temperature affects the proportion of chickens outside; the ratio, which was around 37% when the ambient temperature was 17°C on average, decreased to 15% when the temperature increased to around 40°C.
Table 2
Effect of season and applications on the proportion of birds outside (%)
| Morning | Noon | Afternoon |
Season | | | |
Spring | 53.46 | 41.26 | 44.44 |
Summer | 24.74 | 19.30 | 22.88 |
Applications | | | |
FR-FG | 37.53b | 27.66b | 31.68b |
FR-SG | 43.24a | 38.49a | 41.13a |
TFR-SG | 36.53b | 24.69b | 28.16b |
SEM | 1.029 | 1.074 | 0.957 |
P values |
Season (S) | 0.000 | 0.000 | 0.000 |
Application (A) | 0.013 | 0.000 | 0.000 |
S x A | 0.168 | 0.638 | 0.922 |
FR free-range; TFR traditional free-range; FG fast-growing; SG slow-growing, SEM standard errors of mean; a,b Values within a row with different superscripts differ significantly at p < 0.05 |
For three times of day, the fast- in the FR system used a lower rate of open area than the slow- (P < 0.05 for all, Table 2). Nielsen et al. (2003), in line with our findings, concluded that slow- birds used more of the outdoor area than fast-. According to our findings, in the case of slow-, those in the TFR system showed a lower (P < 0.05 for all, Table 2) open area usage rate than those in the FR system. TFR paddocks had more open area per bird (2 vs. 1 m2) than FR paddocks, and we observed that the grass at the edge of the open area reached a higher height in TFR outdoor areas. Could the tall grass have frightened the chickens? However, we think there is one more thing to consider, Nielsen et al. (2003) fed fast- and slow- chickens with moderate and low energy feeds and observed that those fed lower energy feed used less open area than those fed moderate energy ones. We would like to remind that the TFR group in our study was fed a diet with a lower energy value than the FR group (Table 1). In addition, it should be noted that consumption of pasture causes dilution of energy and protein intake (Singh and Cowieson, 2013). The use of lower energy feed may have decreased the activity of TFR birds.
Fear and stress response
The data on fear and stress responses are presented in Table 3. As seen in that table, RT was affected by season and application factors at a statistically significant (P < 0.01, P < 0.01) level. We detected higher rectal body temperature in the summer trial (42.49 vs. 41.96) according to spring. Altan et al. (2000) also reported that applying heat stress caused an increase in RT (P < 0.05) in broilers. Fast- has higher (P < 0.05, Table 3) RT averages than slow- in our study. Some authors (Deeb and Cahaner, 1999; N’dri et al., 2007) concluded that internal heat production in hot climate in fast- is known to be higher than slow-. While fast- exhibited similar RT in closed and grazing systems, pasturing caused lower RT (P < 0.05, Table 3) in slow-. Using fast-, Oke et al. (2021) reported the mean RT in the closed and FR system at 56 d as 41.63 and 41.67°C, respectively (P > 0.05).
Table 3
Effects of seasons and applications on fear and stress parameters (n = 60/season, 12/application)
| RT | Corticosterone (ng/ml) | H/L | TI Duration (second) |
Season | | | | |
Spring | 41.96 | 8.26 | 1.79 | 216.53 |
Summer | 42.49 | 9.58 | 1.42 | 156.23 |
Applications | | | | |
EI-FG | 42.58a | 12.05a | 1.82 | 179.73 |
EI-SG | 42.17b | 7.56c | 1.40 | 179.25 |
FR-FG | 42.38ab | 9.98ab | 1.42 | 186.25 |
FR-SG | 41.87c | 7.13c | 1.71 | 192.58 |
TFR-SG | 42.12bc | 7.90bc | 1.67 | 194.08 |
SEM | 0.04 | 0.34 | 0.15 | 8.34 |
| P values |
Season (S) | 0.000 | 0.155 | 0.411 | 0.001 |
Application (A) | 0.000 | 0.000 | 0.735 | 0.975 |
S x A | 0.000 | 0.022 | 0.342 | 0.854 |
EI Extensive indoor; FR free-range; TFR traditional free-range; FG fast-growing; SG slow-growing, SEM standard errors of mean; a-c Values within a row with different superscripts differ significantly at p < 0.05 |
Serum corticosterone levels were determined as 8.26 and 9.58 ng/ml for spring and summer, respectively (P > 0.05, Table 3). But Farahani and Hosseinian (2022) concluded that broilers subjected to chronic heat stress exhibit higher (P < 0.05) serum corticosterone levels. In his review, Scanes (2016) concluded that many, but not all, stresses in chickens increase circulating concentrations of corticosterone. On the other hand, serum corticosterone level was significantly (P < 0.01, Table 3) affected by the application factor, the highest average was found in indoor raised fast-, and the lowest average in slow- raised in FR system. The serum corticosterone level was lower in the slow- than fast- and this feature was not affected by grazing. In line with our findings, Sanchez-Casanova et al. (2019) found similar serum corticosterone levels for broilers in FR and deep litter systems.
All three factors (season, system, and genotype) considered in this study were not found to be effective on the H/L (Table 3). Although we found the H/L ratio to be similar in summer and spring, Habibian et al. (2014) and Gogoi et al. (2021) concluded that the H/L was significantly (P < 0.05) higher in the high temperature group compared to the thermoneutral group. Diktaş et al. (2015), in parallel with our findings, found the H/L ratios of the deep-litter and the FR systems to be similar. But Sanchez-Casanova et al. (2019) concluded that outdoor access reduced H/L, indicating a lower stress response. On the other hand, Mancinelli et al. (2018) found a higher H/L in slow- than fast- ones, under FR system.
TI duration determined in the summer season was significantly (P < 0.01) lower than the spring. Ghayas et al. (2021) concluded that a shorter TI duration means that birds are more alert and respond more quickly to perceived danger. According to this assumption, chickens were more alert during the summer period in our study, because we found the TI time to be lower in the summer trial compared to the spring. Campo et al. (2007), on the other hand, suggested that those who showed longer TI duration were more cowardly. Thus, contrary to our findings, Yalçın et al. (2003) concluded that high ambient temperatures between 21 and 42 d of age resulted in longer TI. But Akşit et al. (2006) stated that duration of TI was not influenced by neither acute nor chronic heat stress. It is understood that the findings on this subject are contradictory with respect to each other. In our study, grazing broilers exhibited a slightly (P > 0.05) higher TI duration. Like our findings, numerous research (Diktaş et al., 2015; Stadig et al., 2017; Çavuşoğlu and Petek, 2019) found no statistically significant difference between the FR system and those grown indoors in terms of TI duration. But Oke at al. (2021) reported that 8-week-old FR broilers showed a much longer TI duration than indoor-raised chickens. On the other hand, according to Ghayas et al. (2021) and Mutibvu et al. (2017) fear response was less (TI duration was shorter) in FR group than intensive system. We can say that the findings are contradictory again. In this study, no difference was also found between genotypes in terms of TI. But, according to Mancinelli et al. (2018), compared to the slow-growing naked neck genotype, fast- broilers showed longer (P < 0.05) TI.
External quality and leg health
Table 4 refers to the effects of seasons on external quality of birds and leg health. As can be seen from the related table, the effect of the season on breast feather dirtiness is significant (P < 0.01). The rate of those who had no spots on their breast feathers (“0” point) was determined as 64.5% and 94.0% in spring and summer seasons respectively. As can be understood from these averages, the rate of dirty plumage is lower in summer. There is a strong possibility that there is a relationship between litter moisture and the cleanliness of the breast feathers, naturally the litter is expected to be drier in summer. The breast scratches and injuries were similar across seasons (P = 0.057) but thigh scratches and injuries were affected by the season factor. These last defects were more common in the summer (P < 0.05, Table 4). We did not find any research on the effect of heat stress on carcass damage. In our work, could the drier litter in the summer have caused leg scratches and bruises?
Table 4
Effects of seasons on external quality and leg health (n = 180, 90/season)
| | | Spring | Summer |
| | | % | % |
Dirtiness | | 0 | 64.50 | 94.00 |
| | 1 | 31.40 | 6.00 |
| | 2 | 4.10 | 0.00 |
P Value | 0.000 | | a | b |
Breast scratches and injuries | | 0 | 63.80 | 58.90 |
| | 1 | 36.20 | 36.30 |
| | 2 | 0.00 | 4.50 |
| | 3 | 0.00 | 0.30 |
P Value | 0.057 | | | |
Thigh scratches and injuries | | 0 | 94.20 | 80.90 |
| | 1 | 5.80 | 18.10 |
| | 2 | 0.00 | 1.00 |
P Value | 0.000 | | b | a |
FPD | | 0 | 43.20 | 44.80 |
| | 1 | 47.60 | 54.20 |
| | 2 | 9.20 | 1.00 |
P Value | 0.090 | | | |
HB | | 0 | 57.00 | 47.40 |
| | 1 | 37.00 | 46.30 |
| | 2 | 6.00 | 6.30 |
P Value | 0.011 | | a | b |
GS | | 0 | 98.60 | 87.90 |
| | 1 | 1.40 | 10.10 |
| | 2 | 0.00 | 2.00 |
P Value | 0.000 | | b | a |
FPD food pad dermatitis, HB hock burn; GS gait score; a,b Values within a line with different superscripts differ significantly at p < 0.05 |
As seen in Table 4, foot pad dermatitis (FPD) was not significantly affected by the season factor. While the rate of those with the most severe defect (2 points) in terms of FPD is only 1% in the summer season, it is as high as 9.20% in the spring. Litter moisture is likely to be higher in the spring than in the summer. Thus, Rauch et al. (2017) observed an apparent correlation between litter moisture and the prevalence of foot pad alterations. On the other hand, the hock burn (HB) rates were significantly affected (P < 0.05) by season. While the rate of those in the worst situation in terms of HB (2 points) is similar in spring and summer seasons at 6.00% and 6.30%, respectively, the rate of those who get 1 point is higher in summer (46.30% vs. 37.00%). In addition, GS is also significantly (P < 0.01) affected by the season, the proportion of birds walking perfectly is 98.60% and 87.90% in spring and summer, respectively. In a field study of broiler chickens (Cordeiro et al., 2009), it was found that GS decreased with average ambient temperature and relative humidity. Li et al. (2015) concluded that heat stress significantly affected the daily behaviour of broilers; compared with the control group, the duration and frequency of lying-down behaviour of the heat stressed birds increased (P < 0.01), whereas the duration of standing behaviour significantly (P < 0.01) decreased. It is expected that broilers who lie more and stand less in the summer will demonstrate worse GS. Perhaps the increase in the incidence of HB in the summer may be due to the same reason – that is, less movement of birds.
Table 5 summarizes the effects of different applications on external quality and leg health. As seen in that table, this factor was found to be significant in terms of breast feather dirtiness (P < 0.01). We observed that the rate of those with no spots on their breast feathers was higher (P < 0.05) in slow- than fast-. Working with four different slow-growing broiler genotypes, Louton et al. (2019) observed that there were more dirty chickens in heavier genotypes compared to lighter genotypes, and we can say that our result is in line with this report. Thus, Westermaier (2015, cited by Louton et al., 2019) concluded that more of the fast- Ross 308 showed soiling of plumage compared with the slow- strain Cobb Sasso. As seen in Table 5, using to open areas resulted in an increase in those who scored "0" in terms of dirtiness in fast- (P > 0.05) and slow- (P < 0.05). The TFR birds were the group with the highest percentage of those evaluated with "0" points in terms of this feature. The breast feathers of chickens housed on high humidity litter were found to be dirtier than the control group on dry litter (de Jong et al. 2021). We can say that the use of grazing area helped maintain the quality of the indoor litter, especially for slow-.
Table 5
Effects of different applications on external quality and leg health (n = 180, 36/application)
| | | EI-FG | EI-SG | FR-FG | FR-SG | TFR-SG |
| | | % | % | % | % | % |
Dirtiness | | 0 | 50.80 | 72.00 | 62.20 | 90.60 | 94.80 |
| | 1 | 47.60 | 28.00 | 27.60 | 8.80 | 4.80 |
| | 2 | 1.60 | 0.00 | 10.20 | 0.60 | 0.40 |
P Value | 0.000 | | a | b | a | c | c |
Breast scratches and injuries | | 0 | 38.90 | 80.40 | 13.40 | 72.40 | 79.60 |
| | 1 | 60.30 | 19.60 | 78.00 | 27.60 | 17.80 |
| | 2 | 0.80 | 0.00 | 7.90 | 0.00 | 2.60 |
| | 3 | 0 | 0 | 0.8 | 0 | 0 |
P Value | 0.000 | | a | b | a | c | c |
Thigh scratches and injuries | | 0 | 71.40 | 91.60 | 81.10 | 93.40 | 93.00 |
| | 1 | 26.20 | 8.40 | 18.10 | 6.60 | 7.00 |
| | 2 | 2.40 | 0.00 | 0.80 | 0.00 | 0.00 |
P Value | 0.000 | | a | b | a | b | b |
FPD | | 0 | 23.80 | 59.80 | 35.40 | 45.30 | 50.40 |
| | 1 | 73.80 | 40.20 | 55.10 | 53.00 | 40.70 |
| | 2 | 2.40 | 0.00 | 9.40 | 1.70 | 8.90 |
P Value | 0.000 | | a | a | c | b | b |
HB | | 0 | 10.30 | 83.20 | 11.80 | 72.90 | 64.80 |
| | 1 | 69.00 | 16.80 | 72.40 | 27.10 | 33.70 |
| | 2 | 20.60 | 0.00 | 15.70 | 0.00 | 1.50 |
P Value | 0.000 | | a | c | a | b | b |
GS | | 0 | 87.30 | 99.10 | 81.90 | 100.00 | 94.80 |
| | 1 | 9.50 | 0.90 | 15.00 | 0.00 | 5.20 |
| | 2 | 3.20 | 0.00 | 3.10 | 0.00 | 0.00 |
P Value | 0.000 | | a | bc | a | c | b |
EI Extensive indoor; FR free-range; TFR traditional free-range; FG fast-growing; SG slow-growing; FPD food pad dermatitis, HB hock burn; GS gait score; a-c Averages within a line with different superscripts differ significantly at p < 0.05 |
The application factor has led to significant differences in terms of breast and thigh scratches and injuries (P < 0.01 for two, Table 5), these flaws are higher in the fast- than in the slow-. The fact that fast- spend more of their time lying down (Li et al., 2015) may have contributed to this result. EI and FR groups were found to be similar in both genotypes in terms of tight scratches and injuries. Despite that, outsiders in both genotypes exhibited more (P < 0.05 and P < 0.05) breast scratches and injuries than those grown indoors. Undoubtedly, the floor of the open area contained damaging elements such as small stones and irrigation pipes.
As seen in Table 5, FPD and HB were significantly affected by the application factor (P < 0.01 and P < 0.01). FPD defect was higher in the fast- than slow- (P > 0.05 and P < 0.05, for EI and FR groups, respectively). The frequency of HB was very distinctly higher in the fast- than slow- for FR group (P < 0.05). Comparing different genotypes with each other, Louton et al. (2019) found that there is a correlation between body weight and HB. On the other hand, outdoor use resulted in improvement in fast- and worsening in slow- in terms of FPD (P < 0.05 for two, Table 5). Slow-birds used the open space more intensively than fast- (Table 2), staying outside for longer may have increased FPD in slow-. The TFR group and the slow- in the FR group were found to be similar for FPD and HB.
Nielsen et al. (2003) stated that litter quality was worse, more dermal lesions were observed, and mobility worsened in fast-growing ROSS chickens compared to slower growing (Label Rouge) ones. We also measured worse walking ability in fast- than slow- in EI and FR systems (P < 0.05, P < 0.05). We observed that going out to the pasture did not improve walking ability (Table 5). Fanatico et al. (2008) concluded that slow- had better GS, but according to them, outdoor access resulted in better GS. On the other hand, TFR chickens benefiting from grazing for longer periods exhibited worse (P < 0.05) GS than FR slow- birds grazing for a shorter period (Table 5). Prolongation of the time in the open area appears to worsen the ability to walk.
Blood Parameters
The effects of season and applications on the haematological findings are illustrated in Table 6. As seen in that table, the factors considered by us in this work, were not found to influence the WBC. Ding et al. (2020) reported that, in parallel with our findings, the application of heat stress did not cause any difference in terms of WBC. But Altan et al. (2000) concluded that broilers exposed to acute heat stress had a decrease in monocyte and lymphocyte ratios. In line with our findings, Diktaş et al. (2015) found no difference in the proportion of WBC cells between broilers reared in a deep litter system and those reared on pasture.
Table 6
Effects of seasons and applications on haematological parameters (n = 60/season, 12/application)
| WBC (103/µL) | RBC (106/µL) | HGB (g/dL) | HCT (%) | MCV | MCH (pg)* | MCHC |
Season | | | | | | | |
Spring | 305.74 | 2.58 | 11.19 | 33.07 | 127.17 | 42.36 | 33.94 |
Summer | 301.17 | 2.45 | 10.73 | 31.73 | 129.79 | 44.57 | 34.54 |
Applications | | | | | | | |
EI-FG | 298.78 | 2.44b | 10.64c | 32.73ab | 131.98a | 42.84b | 32.71c |
EI-SG | 301.04 | 2.50b | 10.73bc | 31.04b | 124.39b | 43.01b | 34.67b |
FR-FG | 316.05 | 2.66a | 11.29a | 34.38a | 129.25a | 42.43b | 32.88c |
FR-SG | 306.36 | 2.53ab | 11.01abc | 31.64b | 124.84b | 41.73b | 34.84b |
TFR-SG | 295.04 | 2.45b | 11.15ab | 32.21b | 131.94a | 47.30a | 36.10a |
SEM | 2.77 | 0.02 | 0.07 | 0.32 | 0.72 | 0.56 | 0.43 |
| P values |
Season (S) | 0.613 | 0.002 | 0.002 | 0.018 | 0.531 | 0.085 | 0.821 |
Application (A) | 0.211 | 0.024 | 0.017 | 0.001 | 0.005 | 0.000 | 0.000 |
S x A | 0.256 | 0.245 | 0.158 | 0.412 | 0.116 | 0.834 | 0.912 |
EI Extensive indoor; FR free-range; TFR traditional free-range; FG fast-growing; SG slow-growing, SEM standard errors of mean; WBC white blood cell; RBC red blood cell; HGB haemoglobin; HCT haematocrit; MCV mean corpuscular volume; MCH mean corpuscular haemoglobin and MCHC mean corpuscular haemoglobin, *) pg = 10− 12g; a-c Values within a row with different superscripts differ significantly at p < 0.05 |
Season and application factors significantly (P < 0.05, in all) affected RBC, HGB and HCT, the averages for the summer season were lower for three blood parameters (Table 6). Yahav et al. (1997), in line with our findings, concluded that as the ambient temperature increases, the HCT ratio and the amount of HGB decrease. Ding et al. (2020) concluded that heat-stressed broilers had a numerical decrease in RBC and significant (P < 0.05) decrease in HGB compared to control ones. In the same study, heat-stressed chickens exhibited a lower (P < 0.05) HCT ratio, as parallel our findings. Altan et al. (2000) reported that the decrease in HCT due to high temperature application is at only a numerical level. In our study, pasturing did not cause a difference in terms of HCT ratio. Diktaş et al. (2015) reported that broilers in the deep litter system and those raised in the FR system, like us, showed similar averages in terms of HCT. In addition, fast- broilers showed higher HCT than slow- (Table 6, P > 0.05 and P < 0.05 for EI and FR, respectively). Cömert et al. (2016), who raised fast- and slow- broilers in conventional and organic systems, concluded that the mean of WBC, RBC, HCT and HGB were not affected by either genotype or method. According to Taha and Aljumaily (2021), the total number of red and white blood cells was not affected by the genotype factor. In our study, MCV (mean corpuscular volume), MCH (mean corpuscular haemoglobin) and MCHC (mean corpuscular haemoglobin concentration) levels were found to be similar between seasons (Table 6). Because of heat stress, Ding et al. (2020) did not detect any difference in terms of all three features.
Table 7 belongs to serum indices. As can be seen, seasonal factor was not influenced serum glucose levels. In fact, stress was expected to accelerate gluconeogenesis, as some researchers (Attia and Hassan 2017; Law et al. 2019; Awad et al. 2020, Ding et al. 2020) concluded that heat stress increases serum glucose levels. The application factor did also not significantly affect the serum glucose in our work, that is, neither the genotype nor the rearing system was found to be effective on the serum glucose. Diktaş et al. (2015) and Eleroğlu et al. (2015) found that glucose levels were similar between those grown indoors and those raised on pasture, like us. But Jin et al. (2019) concluded that grazing resulted in reduced glucose levels in the local slow- genotype.
Table 7
Effects of seasons and applications on serum biochemical parameters (n = 60/season, 12/application)
| Glucose (mmol/l) | TP (g/l) | Albumin (g/l) | TG (mmol/l) | TC (mmol/l) | VLDL (mmol/l) | Creatinine (µmol/l) | CK (U/ml) | AST (µkat/l) | ALT (µkat/l) | ALP (µkat/l) | LDH (µkat/l) |
Season | | | | | | | | | | | | |
Spring | 13.29 | 34.68 | 15.09 | 0.75 | 2.77 | 0.15 | 3.16 | 6.25 | 5.12 | 0.08 | 58.63 | 28.25 |
Summer | 13.20 | 33.06 | 15.59 | 0.73 | 2.97 | 0.15 | 4.09 | 5.71 | 6.00 | 0.05 | 56.79 | 31.37 |
Applications | | | | | | | | | | | | |
EI-FG | 13.81 | 33.50b | 14.61c | 0.74b | 2.95ab | 0.15b | 4.79ab | 10.71a | 6.81a | 0.08 | 45.89c | 34.21a |
EI-SG | 13.22 | 33.33b | 15.93b | 0.65b | 2.79bc | 0.13b | 3.09bc | 3.26c | 4.88c | 0.06 | 66.92ab | 26.89b |
FR-FG | 13.00 | 31.77b | 14.31c | 0.63b | 2.80bc | 0.13b | 3.29abc | 8.16b | 6.23b | 0.07 | 56.53bc | 32.66a |
FR-SG | 13.01 | 33.63b | 14.75bc | 0.74b | 2.72c | 0.15b | 1.92c | 3.14c | 4.63c | 0.06 | 72.95a | 26.65b |
TFR-SG | 13.19 | 37.13a | 17.08a | 0.92a | 3.09a | 0.19a | 5.03a | 4.62c | 5.25c | 0.05 | 46.28c | 28.65b |
SEM | 0.09 | 0.29 | 0.17 | 0.02 | 0.03 | 0.00 | 0.27 | 0.27 | 0.10 | 0.00 | 2.00 | 0.60 |
| P values |
Season (S) | 0.626 | 0.021 | 0.752 | 0.272 | 0.004 | 0.274 | 0.108 | 0.330 | 0.000 | 0.000 | 0.373 | 0.011 |
Application (A) | 0.053 | 0.000 | 0.000 | 0.004 | 0.003 | 0.003 | 0.005 | 0.000 | 0.000 | 0.068 | 0.000 | 0.000 |
S x A | 0.120 | 0.813 | 0.003 | 0.315 | 0.011 | 0.370 | 0.000 | 0.001 | 0.028 | 0.149 | 0.051 | 0.013 |
EI Extensive indoor; FR free-range; TFR traditional free-range; FG fast-growing; SG slow-growing, SEM standard errors of mean; TP total protein; TG triglyceride; TC total cholesterol; VLDL very low-density lipoprotein; CK creatine kinase; AST aspartate amino transferase; ALT alanine amino transaminase; ALP alkaline phosphatase; LDH lactate dehydrogenase |
a-c Values within a row with different superscripts differ significantly at p < 0.05 |
A lower serum TP level was detected in the summer (Table 7, P < 0.05). Some researchers (Ding et al., 2020; Attia and Hassan, 2017) concluded that heat stress did not affect TP levels. The older TFR birds had the highest TP and albumin levels, and the differences with other groups were significant (P < 0.05 in all). Filipović et al. (2007) determined that the serum TP increased with age in broiler chickens (P < 0.01), while the albumin level did not change. We found EI and FR groups to be similar in terms of TP, but Jin et al. (2019) is of the opinion that as the grazing period gets longer, the serum TP level decreases. Indeed, Singh and Cowieson (2013) suggested that grass consumption would dilute energy and protein intake, implying that this would have some physiological consequences. But Diktaş et al. (2015) found no difference between different growing systems (indoor and FR) in terms of serum TP.
Serum TC level was statistically (P < 0.01, Table 7) higher in summer, but serum TG and VLDL levels were not affected by the season. But Khan et al. (2021) argue that heat stress leads to higher TG levels and higher temperatures result in higher TG mobilization. Indeed, Habibian et al. (2014) reported that heat stress causes an increase in TG and TC levels in broilers. Serum TG, TC and VLDL were significantly affected (all P < 0.05, Table 7) by the application factor. In terms of TG and VLDL, the highest mean was found in TFR birds, the differences from others were significant (P < 0.05 for all, Table 7). In terms of TC, the highest average was also found in TFR chickens and fast- in EI, the birds in these two groups had higher body weight. Diktaş et al. (2015) found no difference between different growing systems in terms of serum TG, and TC.
Serum creatinine concentration, which is a kind of renal function indices, was numerically higher (P > 0.05, Table 7) in summer than spring. In parallel with our findings, Attia and Hassan (2017) found similar creatinine concentrations in heat stressed broilers and in the control group. On the other hand, the treatment factor significantly (P < 0.05, Table 7) affected the mean of the mentioned criterion. The highest creatinine level was found for the slow- in TFR group and the lowest for slow- in FR (P < 0.05). TFR birds had a significantly higher body weight than slow- in the FR system (Aksoy et al. 2021a), there seems to be a relationship between high body weight and creatinine levels in slow-.
As for enzymes, CK levels were similar for each season (Table 7), but Xie et al. (2015) observed that CK were lower in heat-stressed bird. It is obvious that the genotype factor influences the CK level, fast- showed higher mean (P < 0.05) than slow-. Pasturing resulted in a reduced (P < 0.05) CK concentration in fast-. AST and ALT enzyme levels, which are a kind of liver function index, were affected statistically (P < 0.01, P < 0.01, Table 7) by the seasonal factor. AST is higher and ALT is lower in summer. But according to Yahav et al. (1997) high environmental temperature increases serum ALT and AST enzymes in serum. Attia and Hassan (2017) reported that AST was higher in heat stress group compared to the control, but ALT was similar. In our study, ALP was not affected by the seasonal factor, but LDH, which is an indicator of tissue damage and cell destruction, was affected, and higher LDH levels were detected in the summer season. A higher (P < 0.05, Table 7) level of LDH was detected in fast- compared to slow-. To our knowledge, there has been no research on ALP and LDH levels during heat stress and for different genotypes.