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Poult Sci
Poult Sci
Poultry Science
0032-5791
1525-3171
Elsevier

S0032-5791(24)00833-2
10.1016/j.psj.2024.104254
104254
MANAGEMENT AND PRODUCTION
Early-age heat exposure improved growth performance and heat tolerance in broilers
Liu Y.S. *†
Lv T.J. *
Lin H. *
Jiao H.C. *
Wang X.J. *
Liu L. leiliu@sdau.edu.cn
*1
⁎ Department of Animal Science and Technology, Key Laboratory of Efficient Utilization of Non-grain Feed Resources (Co-construction by Ministry and Province), Ministry of Agriculture and Rural Affairs, Shandong Provincial Key Laboratory of Animal Nutrition and Efficient Feeding, Shandong Agricultural University, Tai'an City, Shandong Province, 271018, China
† Department of Technology, Shandong Haiding Agriculture and Animal Husbandry Co. Ltd., Jinan City, Shandong Province, 250113, China
1 Corresponding author: leiliu@sdau.edu.cn
29 8 2024
12 2024
29 8 2024
103 12 10425418 6 2024
20 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
A total of 440 one-day-old healthy male Arbor Acres broilers were equally assigned to a control group (CTL) and an early-age high-temperature exposure (EHT) group (4 replicates per group, 55 chickens per replicate). At d 3, the broilers in CTL group were reared in the normal temperature 33 ± 1°C, while the broilers in EHT group were exposed to 36 ± 1°C for 24 h. At d 43, all broilers were treated with an acute high temperature 35 ± 1°C for 5 h. The results showed that average daily gain in EHT group was decreased at d 3, but average daily gain in EHT group was increased at d 36 to 42 (P < 0.05). Plasma GLU level in EHT group was lower in broilers at d 7 or facing subsequently high temperature for 5 h (P < 0.05). The relative expression of myogenic differentiation (MyoD) gene in pectoralis major and myogenic factor 5 (Myf5) gene in biceps femoris were significantly improved at d 42 after early-age heat exposure (P < 0.05). Broilers in EHT group have a higher temperature tolerance with a lower mortality than control broilers (P < 0.05). Broilers in EHT group have a lower rectal temperature and a higher comb and ear temperature when facing subsequently acute high temperature than control broilers (P < 0.05). In addition, our study demonstrated that early-age heat exposure significantly decreased the mortality and increased the heat tolerance of broilers when facing an acute short-term heat exposures. Early-age heat exposure increased the process of myogenesis via up-regulating the MyoD and Myf5 gene expression in skeletal muscle, which accelerated average daily gain.

Key words

early-age heat exposure
production performance
heat tolerance
broiler
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pmcINTRODUCTION

Poultry performance and economic profitability would be largely affected due to the remarkable escalation in global warming. High temperature-induced stress is becoming an inevitable “new normal” in the tropical and subtropical regions (Nawaz et al., 2021). Broilers are susceptible to high temperature due to the high basal body temperature, thick feather cover and lack of sweat glands to dissipate heat (Qaid and Al-Garadi, 2021.). Broilers with heat stress show panting and dehydration, anorexia, lower weight gain and feed conversion ratio, and higher mortality (Vandana et al., 2021). Improving housing condition and husbandry management or regulating dietary nutrient level may alleviate heat stress. Early-age heat exposure may help broilers to adapt to subsequent heat stress. Early-age heat exposure reduced the mortality rate and stimulated myogenesis in broilers raised in high-temperature environment (Kang et al., 2019). Early-age heat exposure may be a easy and effective way to improve heat tolerance in broilers. The present study explores the beneficial effects of early-age heat exposure for broilers in a summer heat wave.

MATERIALS AND METHODS

Ethics Statement

All experimental procedures were approved by the Animal Care and Use Committee of Shandong Agricultural University (No. 2001002), and the “Guidelines for Experimental Animals” of the Ministry of Science and Technology (Beijing, China).

Animals and Treatments

A total of 440 male Arbor Acres broilers (1 day old) with similar body weight were reared in pens and fed with commercial diets. At 3 d of age, broilers were divided into 2 groups (4 replicates per group, 55 broilers per replicate): control group (CTL, temperature: 33 ± 1°C and relative humidity: 75% ± 1%) and early high-temperature exposure group (EHT, temperature: 36 ± 1°C and relative humidity 83% ± 1% for 24 h). Growth performances were recorded during the high-temperature exposure and thereafter weekly. At 7 d, 14 d, 28 d, and 42 d of age, 8 broilers (2 broilers/replace/group) were randomly selected from each group for blood collection from wing vein, and were slaughtered by cervical dislocation and exsanguination for the collection of muscle samples. Pectoralis major and biceps femoris samples of broilers at d 7 and d 42 were isolated and stored at −80°C for subsequent mRNA level analysis. At 43 d of age, all broilers were treated with a high temperature 35 ± 1°C (relative humidity: 80% ± 1%) for 5 h and withdrawal from feed.

Two broilers were randomly selected from each replace of group for blood collection from wing vein at 0 h, 3 h and 5 h during acute heat treatment. Rectal temperatures and body surface temperatures of comb, wattle and ear were measured with a clinical thermometer (Model CA291, Suzhou, China) and an infrared thermometer (Model TP 160, Suzhou, China), respectively. Broiler was placed on a table and held, and a thermometer was inserted approximately 2 cm into the cloaca and was read after temperature stabilization, typically 45 to 60 s. The temperature was collected from 2 broilers randomly selected from each replicate.

Growth Performances and Mortality

Body weight and feed consumption were recorded weekly during the trial. Average daily gain (ADG), average daily feed intake (ADFI), and average feed intake/ average body weight gain (F/G) were calculated on a replicate. The number of dead broilers were recorded daily after early-age heat exposure until d 42 and hourly during the high temperature treatment at d 43, respectively. Cumulative 42-d mortality was calculated on a replicate from the total number of dead broilers until d 42 divided by the total number of broilers present at d 3. Cumulative 43-d mortality was calculated on a replicate from the total number of dead broilers divided by the total number of broilers present acute high temperature treatment for 5 h.

Blood Sampling and Plasma Biochemical Analysis

Plasma samples were collected by centrifugation of blood samples at 3,000 g for 10 min at 4°C and stored at -20°C. Plasma glucose (GLU), triglyceride (TG) and uric acid (UA) concentrations were measured using an Automatic Biochemical Analyzer (Hitachi 7170A, Japan) with corresponding kits commercially available from Hitachi (Japan).

RNA Extraction and Real-Time PCR Analysis

Total RNA was isolated with Trizol (Invitrogen, San Diego, CA). The quality and quantity of RNA were determined by agarose-gel electrophoresis and using a biophotometer (Eppendorf, Germany). Reverse transcription and quantitative real-time PCR were performed as described previously with corresponding kits (TaKaRa, Japan) (Liu et al., 2015). Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) primer was used as internal control, and 2−△△Ct method was used to quantify gene expression. Gene expression in CTL was deemed to be 1.

Statistical Analysis

All the data were analyzed with a T test by using the Statistical Analysis Software (SAS version 8e, Cary, NC). For production performances, n = 4; for gene expression, blood biochemical parameters or body temperatures, n = 8. All the values were expressed as mean ± SEM. Differences were considered significant at P < 0.05.

RESULTS AND DISCUSSION

Effect of Early-Age Heat Exposure on Production Performances, Heat Tolerance and Body Temperatures of Broilers Subsequently Raised Under an Acute Heat Stress

Compared with control group, ADG in EHT group was decreased at d 3 (P < 0.05, Table 1), but ADG in EHT group was increased at d 35-42. However, the ADFI, F/G, BW and mortality were not statistically different between 2 groups until d 42 (P > 0.05).Table 1 Effects of early high-temperature exposure on growth performance and mortality of broilers.

Table 1Age	Item	Treatment	P-value	
CTL	EHT	
3 d	ADG (g/d)	13.4 ± 0.1a	12.8 ± 0.2b	0.047	
ADFI (g/d)	13.4 ± 0.3	12.7 ± 0.2	0.101	
0–7 d	ADG (g/d)	19.1 ± 0.4	18.7 ± 0.4	0.526	
ADFI (g/d)	20.8 ± 0.5	20.7 ± 0.4	0.826	
F/G	1.09 ± 0.01	1.11 ± 0.013	0.189	
7 d	BW	180.3 ± 3.2	177.7 ± 2.7	0.561	
8–14 d	ADG (g/d)	29.2 ± 0.5	29.7 ± 0.3	0.340	
ADFI (g/d)	47.1 ± 1.1	47.7 ± 1.1	0.738	
F/G	1.57 ± 0.01	1.55 ± 0.02	0.438	
14 d	BW	384.1 ± 6.0	385.7 ± 3.3	0.822	
15–21 d	ADG (g/d)	36.1 ± 0.5	36.3 ± 1.0	0.886	
ADFI (g/d)	68.0 ± 0.7	69.7 ± 1.0	0.188	
F/G	1.89 ± 0.04	1.91 ± 0.04	0.631	
21 d	BW	638.3 ± 7.3	642.4 ± 10.0	0.750	
22–28 d	ADG (g/d)	55.8 ± 1.2	57.8 ± 1.9	0.420	
ADFI (g/d)	103.5 ± 1.6	103.8 ± 2.7	0.936	
F/G	1.71 ± 0.06	1.62 ± 0.02	0.210	
28 d	BW	1029.2 ± 1.1	1047.2 ± 1.9	0.448	
29–35 d	ADG (g/d)	38.7 ± 3.7	40.9 ± 4.2	0.706	
ADFI (g/d)	106.0 ± 4.5	113.5 ± 4.6	0.284	
F/G	2.88 ± 0.19	2.81 ± 0.17	0.792	
35 d	BW	1311.0 ± 3.6	1357.8 ± 4.6	0.452	
36–2 d	ADG (g/d)	55.5 ± 1.7b	60.7 ± 1.2a	0.045	
ADFI (g/d)	127.1 ± 6.6	137.0 ± 4.0	0.249	
F/G	2.55 ± 0.24	2.39 ± 0.08	0.560	
42 d	BW	1699.2 ± 4.1	1782.5 ± 5.0	0.246	
3–42 d	Mortality (%)	17.0 ± 4.4	18.5 ± 2.2	0.769	
Abbreviations: ADFI, Average daily feed intake; ADG, average daily gain; BW, body weight; CTL, control; EHT, early high-temperature exposure; F/G, Feed intake/body weight gain.

a,b Means within the same row with different superscript differ significant, P < 0.05, n = 4.

Broilers in EHT group have a higher temperature tolerance with a lower mortality than control broilers (P < 0.05, Figure 1A). Broilers in EHT group have a lower rectal temperature facing subsequently acute high temperature for 5 h than control broilers (P < 0.05, Figure 1B). Broilers in EHT group have a higher comb temperature facing subsequently acute high temperature for 1 to 3 h than control broilers (P < 0.05, Figure 1C). Broilers in EHT group have a higher ear temperature facing subsequently acute high temperature for 3 h than control broilers (P < 0.05, Figure 1D). Early high-temperature exposure did not significantly affect the wattle temperature of broilers subsequently raised under an acute heat stress (P > 0.05, Figure 1E).Figure 1 Effects of early high-temperature exposure on heat tolerance, body temperatures, blood biochemical parameters and gene expression of broilers. (A–E) Heat tolerance and body temperatures of broilers subsequently raised under an acute heat stress; (F–H) blood biochemical parameters of broilers at d 7 to 42; (I–K) blood biochemical parameters of broilers subsequently raised under an acute heat stress; (L–M) gene expression involved in muscle development of broilers at d 7 and d 42. CTL, Control; EHT, Early high-temperature exposure; GLU, glucose; TG, triglyceride; UA, uric acid; DNMT1, DNA methyltransferase 1; DNMT3a, DNA methyltransferase 3a; Myf5, Myogenic factor 5; MyoD, Myogenic determining factor. Means with different superscripts differ significantly, P < 0.05, n = 8.

Figure 1

Broilers are very susceptible to high temperature. Broilers will pant fast, spend less time walking and standing, spread their wings, consume less feed and drink more water under the thermal stress. These behavioral signs are closely associated with their physiological and neuroendocrine changes, and lead to higher mortality and lower body weight gain. Broilers with the early-age heat exposure experiences could efficiently dissipate body heat and be less “nervous” when raised under a high-temperature environment (Hassan and Gopal, 2012). Lower mortality and higher feed efficiency would be achieved when broilers have early-age heat exposure and are raised under a high-temperature environment (Hassan and Gopal, 2012). In the present study, early-age heat exposure improved the ADG at d 35 to 42. Meanwhile, the mortality rate was even decreased when the broilers with early-age heat exposure experiences were further exposed to an high-temperature environment at marketing ages. The present results suggest the broilers with early-age heat exposure have a higher heat tolerance, indicating that early-age heat exposure could improve thermoregulation ability of broilers. Comb, wattle and ear are important heat dissipation sites. Broilers with early-age heat exposure have a lower rectal temperature and a higher comb and ear temperature when facing subsequently high-temperature environment, indicating that early-age heat exposure promotes heat dissipation from body surface which decreases body core temperature. Therefore, early heat exposure emerged as an economically viable method to improve heat acclimation and decrease mortality of boilers in hot summer seasons.

Effect of Early-Age Heat Exposure and Subsequently Acute Heat Stress on Hematological Parameters

Plasma GLU concentration in EHT group was lower than CTL group at d 7 (P < 0.05, Figure 1F). plasma GLU concentration was not significant different between CTL and EHT groups from d 14 to d 42 (P > 0.05). Broilers in EHT group have a lower plasma GLU concentration when facing subsequently high temperature for 5 h than control broilers (P < 0.05, Figure 1Z). TG and UA concentrations were not statically different between 2 groups throughout the trial (P > 0.05, Figure 1G, H, J and K).

Blood metabolites are valuable predictors for evaluating metabolic status. Acute heat stress could increase the plasma GLU level in broilers (Beckford et al., 2020). But the previous study also found that heat treatment did not significantly change plasma GLU concentration (Song et al., 2012). In line with the previous study indicating that heat stress increased whole-body insulin-stimulated glucose uptake (Sanz et al., 2015), plasma GLU level in EHT group was lower in broilers at 7 d or facing subsequently high temperature for 5 h. The duration and severity of heat stress programs should be considered to account for the apparent discrepancies between these studies. UA is a major end-product of amino acid catabolism, and its blood level is controlled by the balance between its synthesis and excretion. TG is an important index of lipid metabolism. Early-age heat exposure experiences or subsequently acute high-temperature environment did not significantly affect the plasma UA and TG levels, indicating that early-age heat exposure or late acute thermal stress did not affect the process of amino acid metabolism and lipid metabolism.

Effect of Early-Age Heat Exposure on Gene Expression Involved in Muscle Development

The gene expression of myogenic determining factor (MyoD) in pectoralis major and myogenic factor 5 (Myf5) in biceps femoris in EHT group at d 42 were increased compared with CTL group (P < 0.05, Figure 1M). The gene expression of MyoD, Myf5, DNA methyltransferase 1 (DNMT1) and DNA methyltransferase 3a (DNMT3a) in pectoralis major and biceps femoris at d 7 and DNMT1 and DNMT3 in pectoralis major and biceps femoris at d 42 were not significant different between 2 groups (P > 0.05, Figure 1L–M).

As the early-age heat exposure improved the ADG of broilers at d 36 to 42, we further explored the gene expression involved in muscle development. MyoD and Myf5, positively regulating myogenesis, are 2 important transcription factors of MyoD family. Previous research demonstrated that early-age high temperature exposure accelerated the proliferation and differentiation of myogenic cells as well as the expression of MyoD and Myf5 in broiler muscle (Nawaz et al., 2021). In agreement with these results, the relative expression of MyoD gene in pectoralis major and Myf5 gene in biceps femoris were significantly improved at d 42 after early-age heat exposure, demonstrating early-age heat exposure induced an adaptive improvement of myogenesis process in broilers.

In conclusion, our study demonstrated that early-age heat exposure significantly decreased the mortality and increased the heat tolerance of broilers when facing an acute short-term heat exposures. Early-age heat exposure increased the process of myogenesis via up-regulating the MyoD and Myf5 gene expression in skeletal muscle, which accelerated average daily gain.

DISCLOSURES

The authors declare no conflicts of interest.

ACKNOWLEDGMENTS

This research was supported by the Key Research and Development Program of Shandong province (2022TZXD0017-02 , 2021LZGC002 ) and National Natural Science Foundation of China (32372937 and 32172787 ).
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