Stress in Broiler Farming

Abstract

Broiler breeders’ problems arise from various factors, such as management, stress, nutrition, immunosuppression, heat and cold stress, feed restriction, stocking density, pollutants, and exposure to disease agents. Stress can have a significant impact on both performance and overall health, making individuals more vulnerable to disease. Research has shown that chickens are no exception to this, as their performance, welfare, and health can all be negatively affected by stress. This can result in a variety of issues, such as changes in behavior, decreased meat quality, damage to tissues and intestines, and even a higher risk of mortality. Managing stress is crucial for the success of breeding programs in broiler chickens. Stressors can be tackled by supplementing chicken diets with vitamins and antioxidants. Poultry birds cannot produce enough vitamins during stressful periods, and therefore, it is recommended to supplement their diets with a combination of vitamins or antioxidants. This approach is more effective than using individual vitamins to alleviate stress in chickens. This chapter discusses stress in broilers and specific causes of stress in broiler breeders. It also covers management practices and strategies to prevent and alleviate the negative effects of stress.

1. Introduction

Due to the increasing human population, there is a growing demand for animal protein. Thus, it is crucial to maximize poultry production. Successful poultry production depends on effectively managing stress in broiler chickens. Over the past decade, there has been significant interest in stress management among poultry producers and scientists. Effective stress management is crucial for managing large, intensive, and mechanized complexes of meat-type chickens. There are numerous resources available in scientific and trade journals, bulletins, seminars, and meetings to assist with this. The poultry industry must adapt to changing trends, public awareness, and legislation surrounding animal welfare. Tona [1] emphasizes the importance of applying current knowledge to stress management for modern broiler breeder operations.

Stress is a biological response to external or internal stimuli that disrupts the normal physiological balance of an organism [2]. Commercial poultry production faces a range of stresses that can negatively impact production, reproductive performance, and the overall health of poultry birds. These stresses include environmental, nutritional, and physiological factors [3]. Poultry birds are exposed to various environmental stressors, such as heat stress [4], cold stress (CS) [5], feed restriction [6], stocking density, pollutants, and others. It is important to manage these stressors to maintain optimal production and bird health.

Broiler production is severely impacted by stressors, despite advancements in environmental stress management technologies [7]. Nutritional manipulation during prolonged heat stress has been shown to be effective in alleviating stressors and maintaining the health and production performance of flocks in broiler farms, in combination with modern technology [8]. These supplements have proven to be effective due to their nutritional and pharmacological properties, minimal side effects, antioxidative and immune-boosting capabilities, ability to maintain acid-base balance, and improvement in production performance in broilers [9, 10, 11, 12, 13, 14]. This chapter aims to present scientific evidence on the environmental stress challenges faced by chickens, including heat and cold stress, internal (oxidative) stress, feed restriction, stocking density, and pollutants. It asserts the potential effectiveness of vitamin and feed supplements like lycopene, aspirin, melatonin, gamma-aminobutyric acid (GABA), and chitosan in strengthening the antioxidant system to combat these stressors. This chapter delves into the ways in which these supplements can help in protecting chickens from the aforementioned stress challenges.

2. Temperature stress

The production performance and efficiency of chickens are greatly influenced by the thermal environment in their housing [15]. Previous studies have shown that chronic exposure of chickens to high or low environmental temperatures during the production period can have negative consequences on their growth performance, meat yield, immune response, and even increase their mortality rate [9, 12, 13, 16, 17]. High ambient temperatures increase chickens’ energy requirement and reduce their efficiency of converting feed to meat or eggs, leading to losses in health and productivity. To cope with thermal stress, the body produces heat shock proteins (HSPs) that protect against tissue damage [18].

2.1 Heat stress

Heat stress (HS) is a common environmental stressor that can affect poultry performance, health, and welfare [4]. It occurs when the body cannot get rid of excess heat due to its dissipation capacity being exceeded [19], leading to various physiological interruptions. These interruptions, such as systemic immune dysregulation, endocrine disorders, respiratory alkalosis, and electrolyte imbalance [19, 20, 21], are observed and they ultimately decrease the growth performance and intestinal barrier function of chickens [22].

Impaired thermoregulation caused by heat stress (HS) can increase mortality rates [23] and alter the physiological and behavioral responses of poultry. For instance, when exposed to HS, birds tend to spend less time on activities like eating and walking, and more time on drinking, resting, and panting to cope with the heat [24]. In a recent study conducted by Branco et al. [25], birds exposed to HS were observed to be prostrate and exhibited shorter behavioral responses, such as laying down and eating. In order to deal with high temperatures, birds try to adapt by taking in more water, which helps them lose heat, and reduce feed consumption. They also spend more time lying down to reduce heat production from movement and increase panting to enhance evaporative cooling [26, 27].

Heat stress in the poultry sector causes significant economic losses amounting to an estimated $240 million per year [28]. This loss represents approximately 7% of the total heat stress-induced losses in the French livestock industry in 2003 [29]. Heat stress can be classified into different categories based on the duration and temperature of the stress treatment. Acute heat stress refers to stress lasting less than 7days, while chronic heat stress is stress that lasts for 7days or more. Additionally, it can also be classified according to the temperature of stress treatment into cyclic heat stress and sustained heat stress [30].

Research has confirmed that HS condition increases HSP expression [31], rectal temperature, heterophil: lymphocyte ratios (H/L ratios), and plasma corticosterone [32]. An increase in corticosterone triggered by heat stress leads to the breakdown of muscle protein, providing amino acids for liver gluconeogenesis, which produces energy [33]. The elevated levels of corticosterone can lead to an increase in oxidative stress, which might result in cell death in follicular cells. This can cause a reduction in follicle numbers, ultimately leading to decreased egg production [34]. It is worth noting that broilers are more susceptible to heat stress than layers [35].

Studies have shown that heat stress has a more significant negative impact on the body temperature, plasma creatine, and skeletal muscles of broilers compared to layers [36]. In broilers, heat stress can lead to oxidative stress, which weakens the antioxidant status of the body due to increased lipid peroxidation and reduced superoxide dismutase (SOD) activity [31]. It seems like some researchers have conducted studies on the effects of HS on blood biochemistry parameters in chickens. According to Gharib et al. [37], HS can significantly decrease plasma albumin, serum calcium levels, and red blood cell (RBC) count in pullets. On the other hand, Bueno et al. [38] found that cyclic HS did not affect uric acid, total protein, albumin, and globulin levels, but increased aspartate aminotransferase (AST) activity. HS can harm poultry’s gut health, affecting their intestinal morphology, microbiota, and integrity [4, 39]. During heat stress (HS), chickens try to regulate their body temperature by dissipating heat, which can cause a reduction in blood flow to their internal organs, including the gastrointestinal tract (GIT). This reduction in blood flow can result in the loosening of the tight junctions in the GIT, leading to increased intestinal permeability [40]. This, in turn, affects the intestinal barrier function [22]. HS can also hurt the small intestinal mucosa, as indicated by a reduction in villus height and crypt depth [41].

According to Santos et al. [42] and Song et al. [43], the destruction of crypt depth, reduced villus height, and decreased epithelial cell area ratio happened in heat-stressed birds. It has been found that HS can lead to an increase in the number of harmful bacteria, such as Escherichia coli and Salmonella spp., in chickens [44]. Based on Tsiouris et al. [4], chickens can suffer from necrotic enteritis as a result of heat stress. Broilers exposed to high temperatures had lower pH levels in their intestinal digesta, which was linked to changes in the protective microbiota [4]. High temperatures could potentially impact the diversity of cecal microbiota and disrupt the balance of gut bacteria in chickens. This could happen due to the increased growth of Bacteroidetes and the decreased growth of Euryarchaeota [45]. The gastrointestinal tract (GIT) of chickens is known to be very sensitive to heat stress [46]; however, the response to heat stress can vary depending on the specific segment of the intestine. Heat-stressed broilers showed significantly increased HSF3 (heat shock factor protein factor 3) messenger RNA (mRNA) levels in the jejunum and both HSF1 (heat shock factor protein factor 1) and HSF3 mRNA levels in the ileum. Varasteh et al. [47] observed that the expression of HSFs and HSPs was more pronounced in the chicken ileum compared to the jejunum following exposure to high heat [47]. HSF1 and HSF3 are the primary regulators of HSPs in chickens under heat stress. Additionally, HSPs are categorized based on their molecular weights, and 60-kilodalton (kDa) heat shock protein (HSP60), 70-kDa heat shock protein (HSP70), and 90-kDa heat shock protein (HSP90) are the most extensively studied among them [48]. Table 1 presents a summary of the impact of heat stress conditions on the populations of gut microbiota (Table 1).

				Gut microbiota population changed with heat stress
Poultry species	Heat stress condition	Experimental diets	GIT segment	Enriched	Decreased
Broilers	CT (24 to 26°C) vs. HS (34–38°C) for 28 days	Standard diets	Cecal contents	Firmicutes, Tenericutes, and Proteobacteria, genera Anaeroplasma, and Lactobacillus	Bacteroidetes and Cyanobacteria phyla; Bacteroides, Oscillospira, Faecalibacterium, and Dorea genera
Broiler chicks	CT (21 ± 1°C) vs. HS (31 ± 1°C) from 28- to 42-day-old	Standard diets	Ileal contents	Clostridium XIVb, Streptophyta, Faecalibacterium, Rothia, Alistipes, Azospirillum, and Oscillibacter	Coprococcus and Streptococcus
Ducks	CT (25°C) vs. HS (32°C) for 8 h day^−1 for 3 weeks	Standard diets	Jejunal, ileal, and cecal contents	Proteobacteria (phylum), Pseudomonadales (order), Moraxellaceae (family), Acinetobacter (genus), and Mitochondria (genus) in jejunum Rickettsiales (order) and Mitochondria (family) increased in cecum	Firmicutes (phylum), Bacilli (class), Lactobacillales (order), Lactobacillaceae (family), and Lactobacillus (genus) in jejunum Negativicutes (class) and Selenomonadales (order) in cecum
Laying hens	Minimum temperatures (25.2–29.4^−C) vs. maximum temperatures (29.6–34.1°C) at 6–8 h day^−1	Standard diets	Fecal samples	Bacteroidetes spp.	Firmicutes spp.
Broilers	CT (23± 1°C) vs. HS (34 ± 1°C) for 7 h day^−1	Normal protein (NP) diet or Low protein diet fortified with glycine	Cecal contents	Clostridia spp.	Lactobacilli spp.
Broilers	HS (30°C) for 24 h + 24 h feed withdrawal	Standard diet	Ileal contents	Salmonella enteritidis	—
Broilers	HS at 32°C for 9 h day^−1 from 15 to 42 days	Poultry Star (synbiotic supplements consisting of Bifidobacterium animalis, Enterococcus faecium, Lactobacillus reuteri, Pediococcus acidilactici, and fructooligosaccharide)	Cecal contents	Lactobacilli and Bifidobacterium spp.	Escherichia coli and Coliforms
Broilers	CT (22°C) vs. cyclic HS (33°C for 10 h, at 0800–1800 and 22°C from 1800 to 0800)	Probiotic mixture with Bacillus licheniformis, Bacillus subtilis, and Lactobacillus plantarum	Duodenal, ileal, and cecal contents	Coliforms and Clostridium spp.	Lactobacillus and Bifidobacterium spp.
Shaoxing ducks	CT (25°C) vs. cyclic HS (30–40°C) for 15 days	Standard diet	Duodenal, jejunal, and ileal contents	Phylum Firmicutes, genus Lactobacillus	—
Laying hens	CT (21 ± 1 °C), cyclic vs. HS (29–35°C)	Standard diet	Cecal contents	Phyla Bacteroidetes and Firmicutes; order Bacteroidales and Clostridiales; genera Bacieroides and Rikenellaceae	—
Laying hens	CT (26°C) vs. HS (33°C) for 20 days	Probiotic mix containing Bacillus subtilis and Enterococcus faecium	Ileal and cecal contents	Ileal and cecal Escherichia coli increased with HS	Lactobacilli in ileum and cecum

Table 1.

Effect of heat stress conditions on gut microbiota populations (Fisayo et al., 2020).

GIT: Gastrointestinal tract, CT: Control groups, HS: Heat stress groups, vs.: Versus.

Additionally, HS has the potential to reduce immune function and hinder intestinal development in birds. This can make them more vulnerable to pathogens by degrading their innate protective mechanisms. In fact, recent studies suggest that HS can exacerbate the effects of E. coli on the intestinal inflammatory injury of chickens through the upregulation of the Toll-like receptor 4-nuclear factor kappa B (TLR4-NF-κB) signaling pathway [49]. According to a study conducted by Quinteiro-Filho et al. in 2017, HS can activate the hypothalamus-pituitary-adrenal (HPA) axis, which in turn can lead to increased susceptibility to Salmonella enteritidis infection and weakened immune system in chickens. The HPA-axis is an important part of the neuroendocrine system that helps regulate body processes and manage stress responses. When the HPA-axis is activated, it triggers the release of corticosterone hormone that can reduce the levels of immunoglobulin A (IgA) and toll-like receptors (TLRs) [50].

A study conducted by Honda et al. in 2015 found that HS can affect the immune cell profile of broiler chickens. It results in a reduction of B-lymphocytes and an increase in T-cytotoxic suppressor and T-helper lymphocytes in the blood. It’s interesting to note that HS can have an impact on the weight of immune organs in chickens. According to a study by Quinteiro-Filho et al. [51], HS was found to reduce the relative weight of certain immune organs, such as the bursa, spleen, and thymus. It’s worth noting that HS in chickens can lead to a range of negative impacts, such as reduced feed intake, declined growth performance, feed efficiency, and quality, as well as increased oxidative stress in birds [52]. Table 2 provides a summary of the key features of some of the studies included on high temperature stress.

HS condition	TN condition	Duration	Age	Type	Bleed	Findings	Location	References
Cyclic HS 22 to 24, 22 to 26, 22 to 28, 22 to 30°C	22°C	10 weeks 8 h/week	11 wk	Layer-type pullets	Isa brown	30°C lowered feed intake, bacterial abundance, compared to 24°C	China	[45]
27.8°C	20°C	2 weeks (from 21 days to 35days of age)	21 d	Broilers	Mixed cobb	Decrease performance	Korea	[22]
35°C	None	8.00–13.00 h each day	3 d	Broilers	Ross 308	Increased mortality	Netherlands	[23]
32.6 °C	24 °C	9 days	28 wk	Layers	White leghorn	Altered behavioral responses, lowered egg production	United States	[24]
28.00 ± 1.0°C	20.00 ± 13°C	72 hr.	28 to 30 d	Broilers	Hubbard	Altered behavioral responses	Brazil	[25]
27.00 ± 1.1°C	19.00 ± 0.9°C		35 to 37 d
26.00 ± 0.9 °C	18.00 ± 0.7 °C		42 to 44 d
22 °C	20 °C 30 °C	90 days	40 wk	Layers	Hy-Line	Poor production performance. Decrease egg quality, egg weight	Iran
36°C
Acute HS 32°C	21°C	2 h	35 d 63 d	Broilers	Commercial breeder	Negatively affect the skeletal muscle membrane integrity	United Kingdom	[36]
40–42°C	20–26°C	5 h daily/seven consecutive days	58 wk	Pullets	Lohman leghorn chicken	Decrease blood parameters	Egypt	[37]
31°C	21°C	21 days	28 d	Broilers	Arbor Acres	Poor production performance, Altered bacterial composition	China	[54]
Cyclic acute HS 35°C	25°C	12 h (9.00–21.00)/4 days	17 d	Broilers	Ross 308	Affect gut health	Greece	[4]
38–39°C	22–23°C	8 h/day for 5 days	21 d	Broilers	Ross	Affect intestinal morphology parameters	Netherlands	[47]

Table 2.

A summary of studies on heat stress in chickens [53].

HS, Heat stress; TN, Thermoneutral.

2.2 Cold stress

Meat-type broiler chickens (Gallus gallus) that grow rapidly are prone to developing pulmonary arterial hypertension (PAH) or pulmonary hypertension syndrome, also known as ascites. The incidence of this condition is around 3% in all broiler chickens raised under conditions that encourage maximum growth. Increased oxygen demand caused by cold stress, hypobaric hypoxia at high altitudes, or sodium chloride toxicity can lead to the development of pulmonary arterial hypertension (PAH) in birds. The modern broiler industry demands rapid growth of broilers within a relatively short period of time. So growth pace necessitates keeping up with increase in the size or capacity of the cardiovascular and respiratory systems to supplying oxygen and nutrients. The development of organs responsible for maintaining the balance of body energy is slower relative to the body growth rate. As a result, their capacity to regulate body energy is weakened, particularly under extreme environmental conditions such as cold stress [55, 56]. Cold stress (CS) leads to an increase in blood triiodothyronine (T3) levels, which are necessary for generating additional metabolic heat to sustain body temperature in colder environments.

The body’s increase in basal metabolic rate can lead to an increased demand for oxygen. To meet this demand, the heart works harder to supply oxygen to the organs and muscles. However, this can result in pulmonary arterial hypertension (PAH), right ventricular hypertrophy, and ascites or water belly, and eventually lead to death.

It has been suggested by Fathi et al. [10] that the main causes of the etiology of PAH can be categorized into three classes, namely pulmonary hypertension, cardiac pathologies, and cellular damage, resulting from oxidative stress due to increased production of reactive oxygen species (ROS). In accordance with researchers, chickens suffering from ascites experience high levels of oxidative stress. This is due to an increase in mitochondrial electron leak and ROS production during periods of hypoxia and ascites [57]. It has been reported regarding high electron leakage in the mitochondria of heart of broilers developing ascites. Additionally, studies have suggested that oxidative stress can lead to severe inflammatory reactions in the affected cells, potentially resulting in more severe tissue damage and the activation of tissue apoptosis [10, 58]. Another indicator of liver cell damage is an increase in lipid peroxidation, aspartate aminotransferase, and lactate dehydrogenase (LDH) activities in serum, which can be caused by the high production of free radicals under cold stress conditions [59].

When the temperature drops below 18°C, animals may experience cold stress. This condition can make it challenging for the animal’s body to warm up, which can lead to severe cold-related illnesses, tissue damage, and even death. Adult birds are better equipped to handle cold stress because they can regulate their body temperature and produce heat through metabolism [60]. Neonatal chicks are indeed more vulnerable to cold temperatures and are unable to survive under such harsh conditions until their organs are mature enough to generate heat [61]. Mujahid and Furuse in 2009 found that exposure to CS did not affect the plasma corticosterone levels of neonatal chicks.This could suggest that the HPA-axis of such chicks was not fully developed then. Furthermore, the inactivity of the HPA-axis could be linked to hypothermia and lipid peroxidation in chicks. Neonatal chicks exposed to a cold environment (20°C for 3 hours) exhibited impaired thermoregulation even with access to food [62]. This led to behavioral changes, including a decrease in distress vocalizations. Furthermore, Mujahid and Furuse [63] reported elevated lipid peroxidation in the brain and heart tissues of chicks exposed to prolonged cold stress (20°C for 12 hours).

Mendes et al. [64] reported that cold stress (CS) at 15.5°C demonstrably increased feed intake, feed conversion ratio (FCR), and mortality of growing chickens (21–41 days old), but generally decreased body weight and overall productivity [64]. It is reported ascites and cardiomyopathy serve as the predominant pathological presentations associated with mortality in the cold environment [64]. According to a study by Zhang et al. [5], both acute (12 ± 1°C kept for 1, 3, 6, 12, 24hours) and chronic cold stress (12 ± 1°C kept for 5, 10, 20days) can cause a state of oxidative stress in the duodenum of 15-day-old broilers. This stress can also alter inducible nitric oxide synthase (iNOS), which is associated with the intestinal damage process [5]. Another study indicated exposure to CS causes oxidative stress and the production of free radicals. These free radicals trigger the upregulation of liver fatty acid-binding protein (L-FABP), which in turn acts to neutralize the free radicals and increase the uptake of fatty acids. This helps the body to manage low temperatures through lipid synthesis [65]. Furthermore, birds challenged with CS exhibited a significantly higher H/L ratio, which harmed their welfare [66]. Zhao et al. [67] found that CS-challenged birds had significantly shorter jejunum villi. They concluded that CS could damage the intestines and affect immune function in chickens. Investigation of the cold stress (CS) effects on the hypothalamic expression of messenger RNA (mRNA) for corticotropin-releasing hormone (CRH) and thyrotropin-releasing hormone (TRH) in broilers revealed a significant increase in hypothalamic TRH mRNA levels in birds exposed to acute CS [68]. Also, the negative impact of CS on meat quality has been reported in another study [69]. According to Dadgar et al. [69], the glycogen reserve in broilers’ thigh muscles was depleted, leading to a dark, firm, dry quality defect when exposed to low temperature of −8°C. It has been noted that the animal’s productive performance is weakened in the cold environmental temperature below 16°C. As it was discussed previously, cold temperature exposure has a multitude of detrimental effects on avian health and performance. Previous studies have demonstrated significant reductions in individual body weight, feed efficiency, nutrient digestibility, and egg production and increased feed intake in birds [70, 71, 72]. Additionally, Blahová et al. [73] observed significant alterations in level of physiological parameters in male broilers subjected to cold stress, including triiodothyronine, hemoglobin, hematocrit, abdominal fat content, and heart weight.

The results demonstrated that low temperatures during growth of broiler chickens have a detrimental impact on certain performance indicators and blood parameters [73]. A recent study found that under CS it may damage the intestinal barrier function, leading to bacterial translocation [74]. Chickens exposed to cold stress for 72 hours showed significant increases in blood endotoxin, aspartate aminotransferase, glucose, and low-density lipoprotein cholesterol (LDL-C) levels [74].

Qureshi et al. [75] observed a significant correlation between cold stress and increased mortality associated with ascites, a fluid accumulation condition. Additionally, their findings suggest a rise in intestinal bacterial infections in birds exposed to CS [75]. These observations collectively indicate that CS can heighten a bird’s susceptibility to infections, potentially leading to substantial economic losses in broiler production. In summary, CS enhances susceptibility to infections and accrues huge economic loss in broiler production. Studies used in the analysis of cold stress (CS) are presented in Table 3.

CS conditions	TN conditions	Duration	Age	Breed	Findings	Location	References
15°C	25°C	12 h/day for 4 days	17 d old	Ross 308 Broilers	CS predisposes broiler chicks to necrotic enteritis.	Greece	[4]
2 to 8 °C	25°C	3–4 h 8 h (third wk. to sixth wk)	3 d, 4 d, 3 wk. old	Commercial Broilers	Highest ascites-related mortality	India	[75]
16 ± 1 °C	28 ± 1°C	72 h	10-day-old male	Arbor Acres Broilers	Reduce growth performance	China	[74]
					Increase blood endotoxin
13–15 °C	29–22 °C	28 days	14 d old	Ross 308 Broilers	Decrease beneficial microbes	Iran
					Increase pathogens
					Cause oxidative cell damage
6 ± 2 °C	20 °C	12, 24, 72 h	6 wk. old	Chinese indigenous (Huainan Partridge chicken)	Cause oxidative stress	China	[65]
12 ± 1 °C	25°C	Acute CS (1, 3, 6, 12, 24 h)	15 d old	Broilers	Cause duodenum oxidative stress	China	[5]
		Chronic CS (5, 10, 20 d)
–9 to −15 °C	22 °C	24–32 h	5 wk. old, 6 wk. old	Broilers	Cause reduction in meat quality	Canada	[69]
−17.5 to 27.0 °C	7.4 to 26.5°C	21 wks	18 wk. old	Bashang Long-tail chicken	Affect egg production performance	China	[72]
				Rhode Island Red, crossbred (Layers)
12 ± 1 °C	25°C	Acute CS (1, 3, 6, 12, 24 h)	2 wkold	Broilers	Cause intestinal lesions, Change immune function of chicken intestine	China	[67]
		Chronic CS (5, 10, 20 d)

Table 3.

A summary of studies on cold stress in chickens [53].

HS, Heat stress; TN, Thermoneutral.

3. Oxidative stress

In commercial poultry farming, oxidative stress can have significant implications on meat quality due to the ongoing occurrence of oxidative reactions throughout the entire poultry meat production and processing chain, from farm practices to the final customer [76]. Driven by advancements in genetic selection methodologies, the poultry industry has witnessed significant improvements in feed efficiency and growth rate. Genetic selection might have inadvertently heightened vulnerability to environmental stressors like heat and oxidative stress [77]. This increased susceptibility could potentially compromise the birds’ thermoregulatory capacity, particularly under high ambient temperatures that could potentially become a limiting factor in broiler production [78]. Genetically selected poultry, while demonstrating improved performance traits like feed efficiency and growth rate in specific environment, exhibit limitations in adaptability to unfavorable environmental conditions (e.g., heat, oxidative stress). This reduced acclimatization capacity due to limitations in their ability to adjust internal physiological and metabolic homeostasis in response to the dynamic fluctuations observed in global environmental factors can hinder their productivity under such circumstances. Compared to other animals, birds exhibit greater sensitivity to oxidative stress and high ambient temperatures [76, 79].

Oxidative stress triggers a cascade of negative effects in poultry including biological malfunctioning, organ damage, poor performance, various health issues, and ultimately, altered meat quality. One specific consequence of oxidative damage is the disruption of normal metabolic processes, which can lead to the development of meat quality abnormalities such as wooden breast and white striping [76]. Shakeri et al. [80] reported that reduced antioxidant capacity in chickens can exacerbate oxidative reactions within muscle tissues after slaughter. This creates a pro-oxidant environment, ultimately leading to a decline in meat quality [80]. The deterioration in meat quality manifests through alterations in sensory attributes and the nutritional composition of the meat. These changes are likely due to the oxidation of various components during processing, including feed constituents, lipids, and proteins. Lipid oxidation presents a significant challenge to the poultry meat processing industry. This is driven by the inherent susceptibility of poultry meat to oxidative reactions, which by inducing lipid oxidation leads to the development of rancidity in processed poultry products [81, 82]. Protein oxidation can negatively impact both the functional properties and digestibility of these essential nutrients. Additionally, the oxidation of pigments like heme pigments can alter meat color, potentially leading to consumer rejection [81]. Moreover, oxidative stress can severely impact poultry health and productivity by disrupting the gastrointestinal tract (GIT), which is responsible for nutrient absorption and digestion. Intestinal epithelia are damaged by generating free radicals through oxidative reactions, which can ultimately impact poultry productivity [77, 83].

Oxidative stress arises from an imbalance between the production of reactive species (RS) and the cell’s antioxidant capacity [84]. Among these reactive species (RS), oxygen, chlorine, and nitrogen species are particularly reactive and pose a significant threat to living tissues and cells due to their high toxicity [84, 85]. These highly reactive species can disrupt the normal function and structure of essential cellular molecules like DNA, RNA, lipids, and proteins [84, 85]. This disruption can initiate various complications, including metabolic dysfunction and consequently, which lead to cell death [86, 87].

In the oxidative reactions, reactive species (RS) leads to the oxidation of cellular molecules. These oxidized molecules snatch electrons from neighboring molecules and trigger an unending chain reaction, which can ultimately culminate in massive tissue injury within the living organism. Oxidative stress (OS) further compromises cell defense mechanisms against oxidants by negatively affecting the expression of critical enzymes. These enzymes play essential roles in inflammatory reactions, defense mechanisms, and detoxification pathways. Antioxidant system consists of two complexes of nonenzymatic low-molecular-weight antioxidants like vitamin C, glutathione (GSH), and uric acid, and enzymatic high-molecular-weight antioxidants, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and arylesterase. These antioxidant defense mechanisms decline the rate and development of oxidation, which in turn helps to protect cells against oxidative damage [88]. Figure 1 illustrates an overview of the predisposing factors that contribute to oxidative stress during the production and processing of meat, as well as the potential consequences of oxidative stress on the quality of poultry meat.

Figure 1.

Role of oxidative stress and its predisposing factors in the deterioration of chicken meat quality from meat production to meat processing [89].

The primary cause of oxidative stress is the imbalance between free radicals and antioxidant enzymes in living cells or tissues, which result in the oxidation of lipids, proteins, and nucleic acids. The farming industry can face various sources of stress, such as environmental, nutritional, and managerial issues, which can affect poultry production’s performance and health. One important factor that can contribute to these issues is the weakening of the antioxidant mechanism within living cells. As a result, the production of free radicals may elevate during physiological oxygen metabolism [76]. Generated reactive species (ROS, reactive nitrogen species (RNS)) are necessary in cells in small quantities as they also function as signaling molecules for maintaining homeostasis. However, their overproduction can lead to oxidative stress and negative consequences. Living cells have a natural mechanism to lower the amount of oxidative species through physiological scavenging. It is important to maintain the balance between oxidant formation and elimination within cells due to the production of reactive oxygen species such as superoxide and hydrogen peroxide during oxygen metabolism. Various scavenging enzymes to neutralize ROS like superoxide dismutase (SOD), glutathione peroxidase (GPX), and catalase play specific roles in this defense system [90]. SOD tackles superoxide radicals, while catalase breaks them down into water and oxygen. GPX, on the other hand, targets lipid hydroperoxides with the help of glutathione (GSH) [91]. In contrast, reactive nitrogen species (RNS) are a byproduct of nitric oxide synthase metabolism and are primarily found in specific regions of the intestines [92]. While nitric oxide free radicals help the cell functioning in immune regulation and neurotransmission, its excessive production can prove to be fatal to the intestinal mucosa [92].

3.1 Oxidative stress due to high environmental temperature

High environmental temperature stands as one of the most extensively researched and debated environmental stressors in poultry production. It poses a significant challenge to the commercial poultry sector, particularly in warmer regions of the world. Due to the heat stress, the birds may consume less feed, grow poorly, and experience high mortality rates, which can ultimately affect the quality of the meat produced [93]. High ambient temperature is a well-established environmental stressor in poultry, known to disrupt the delicate balance between antioxidants and pro-oxidants within cells [77, 94]. This imbalance, characterized by increased pro-oxidant production, triggers a state of oxidative stress. Studies have shown that birds subjected to heat stress exhibit significant changes in the microstructure of their small intestines. Birds exposed to heat stress exhibit a decrease in villus height and crypt depth. This damage can compromise the intestine’s ability to efficiently absorb nutrients [95]. Furthermore, research suggests that chickens raised under fluctuating heat stress experience epithelial injury, apoptosis, and increased intestinal permeability. This compromised gut barrier can potentially allow bacteria from the intestinal region to translocate into the bloodstream, posing a significant health risk. Heat stress can indeed cause stimulation of the hypothalamus-pituitary-adrenal axis, leading to elevated blood serum glucocorticoid levels. This can result in reduced food intake, weight gain, and relative immune organ weight, as well as compromised natural immunity. This neuro-immune dysfunction disrupts the intestinal immune barrier, allowing harmful microorganisms to pass through the enteric mucous membrane and cause inflammation. Occurred inflammation can hamper weight gain by reducing nutrition absorption [96].

3.2 High ambient temperature is responsible for mitochondrial dysfunction

Living cells rely on mitochondria for energy production through a process called oxidative phosphorylation. This essential well-arranged function involves a series of four protein complexes within the mitochondrial membrane. However, during this normal physiological process, a small percentage (1–4%) of oxygen molecules (O2) escape complete conversion to water [97]. This “leakage” of electrons, primarily at complexes I and III of the respiratory chain, leads to the unintended production of reactive oxygen species, most notably superoxide radicals. Cells possess a sophisticated defense system to combat ROS generation. Enzymes like copper-zinc superoxide dismutase (CuZnSOD) within the intermembrane area (also present in the cytosol) and manganese superoxide dismutase (MnSOD) within the matrix efficiently convert the harmful superoxide radicals (O2•-) into hydrogen peroxide (H₂O₂). However, in the presence of ferric and cuprous ions, H2O2 can undergo a reaction known as the Fenton reaction, generating highly reactive and damaging hydroxyl radicals (OH•-). Unlike superoxide and hydroxyl radicals, hydrogen peroxide can readily diffuse across membranes to participate in further reactions beyond its initial production site. This diffusion also enables H₂O₂ to potentially reach other cells and tissues, where it can exert its effects.

Transmission of hydrogen peroxide (H₂O₂) across membranes to neighboring cells and tissues can occur very quickly, potentially reaching distances of several centimeters within a fraction of a second [97]. Under normal physiological conditions, the generation of ROS, including H₂O₂, is believed to be a controlled process by the antioxidant defense and not necessarily harmful. In fact, low levels of ROS might even be essential for proper cellular function by acting as signaling molecules. H₂O₂, with its ability to diffuse and its relative stability, is the major ROS acting as a second messenger. In this regard, it seems that 2-Cys peroxiredoxin (2-Cys PRX), an enzyme that relies on thioredoxin for regeneration, plays a critical role in regulating the basal levels of H₂O₂ in cells through its oxidase activity [94]. Research suggests that increased cellular energy demand may be the initial step in the pathological response to heat stress. A study by Mujahid et al. [94] found that exposing birds to acute heat stress for just 6hours led to a significant increase in both mitochondrial transport and beta-oxidation of fatty acids [94].

The latter was related to a rise in nonesterified fatty acids (NEFAs) in the bloodstream and is likely linked to the increased energy demands placed on cells during heat stress. To meet this heightened demand and mitochondrial biogenesis, cells ramp up the reducing equivalents and enzymatic activity of subunits’ metabolic process chain complexes. Under normal physiological conditions, the flow of electrons through the electron transport chain (ETC) is closely linked to the process of metabolism. This means electrons are only transferred to oxygen (O₂) when the cell needs to generate adenosine triphosphate (ATP) by phosphorylating ADP (adenosine diphosphate). However, during heat stress, the pH within muscles decreases, which causes the development of pale, soft, and exudative (PSE) meat through disruption in ETC and production of ROS.

Heat stress activates the enzyme phospholipase A2 (PLA2) within muscle cells, under the elevation of calcium (Ca²⁺) [98]. This activation triggers the release of arachidonic acid (AA), significantly increasing the peroxidation reactions and development of PSE meat. Studies by Soares et al. [99] support the link between PLA2 activity and PSE syndrome. Their findings showed elevated levels of lipid oxidation and arachidonic acid in PSE meat samples, further strengthening this connection [99]. In simpler terms, high ambient temperature appears to trigger the activation of pro-oxidant enzymes. These enzymes then act upon phospholipid membranes within muscle cells, liberating arachidonic acid and lysophospholipids. This disruption in membrane integrity can potentially affect the release of calcium (Ca²⁺) from the sarcoplasmic reticulum [100]. The presence of high levels of arachidonic acid in PSE meat suggests its potential contribution to cell membrane damage. Research has firmly established that phospholipase A2 (PLA2) activity, in the presence of elevated calcium (Ca²⁺) levels, plays a critical role in initiating degenerative processes in skeletal muscle cells (Figure 2) [99].

Figure 2.

Heat stress disturbs the calcium balance within muscles and enhances the production of free radicals leading to poor meat quality [89].

3.3 Oxidative stress due to toxins present in feed

In certain cases, poultry feed may contain a variety of contaminants, including dirt, chemicals, microorganisms, environmental and fungal toxins, that pose a significant threat to bird health and performance. A healthy intestine, with its intact mucosal lining and tight junctions between cells, acts as a barrier that prevents the absorption of these harmful substances. However, oxidative stress can disrupt this protective mechanism by affecting the normal cellular processes in the intestine. Mycotoxins, a diverse group of toxins produced by various fungi, known as molds, include aflatoxin, zearalenone (ZEN), deoxynivalenol (DON), fumonisin, trichothecenes, and ochratoxin (OTA). Exposure to even a single mycotoxin can trigger oxidative stress within intestinal epithelial cells. Mycotoxins’ chronic toxicity has been shown to compromise the immune system and intestinal integrity in poultry, severely. Long-term exposure to mycotoxins triggers the production of reactive oxygen species (ROS) within intestinal epithelial cells. This oxidative stress disrupts cellular signaling, weakens antioxidant defenses, and ultimately compromises intestinal integrity, leading to a hyperpermeable epithelium. These toxic fungal contaminants commonly intensify cellular death (apoptosis) and threaten poultry health and production.

Another lethal toxin is arsenic widely found in water, feed, and the environment, and has detrimental effects on digestion and nutrient absorption, leading to reduced growth performance in poultry [96]. Chronic exposure to high levels of arsenic can trigger cellular damage through lipid peroxidation. As a result, the depletion of antioxidants and subsequent activation of apoptosis by lipid peroxidation occur [96].

Additionally, the detrimental effects of arsenic could be exacerbated in combination with copper, which causes swelling and damage to the intestinal lining in poultry [101]. Inadequate air circulation in poultry houses leads to the accumulation of ammonia, a harmful gas [102]. Exposure to high ammonia levels can cause oxidative stress and respiratory problems leading to difficulty in breathing. These stressors ultimately decrease poultry production’s efficiency [102]. Chronic exposure to ammonia in poultry houses can negatively impact broiler health and welfare [103]. High ammonia concentrations damage the villus height and crypt depth in small intestine and compromise their ability to absorb nutrients and function properly. Additionally, ammonia exposure disrupts the development and function of immune organs in chickens [103].

Research by Zhang et al. [104] investigated that ammonia exposure elevates creatine kinase (CK) activity while decreasing total superoxide dismutase (T-SOD) activity. This imbalance disrupts the cellular function and creates oxidative stress. The consequence of this stress is apoptosis within the mucosal structure [104].

3.4 Oxidative stress and gut microbiota

A healthy digestive system is critical for broiler production, as it directly affects feed efficiency and optimal growth rate. The gastrointestinal tract (GIT) harbors a diverse microbiota of bacteria, protozoa, and fungi. Importantly, the composition of this microbiota varies throughout different sections of the GIT. The intestinal epithelium plays a vital role in regulating the gut environment by interacting with the microbiota. This interaction can induce the generation of ROS that function as important cellular signaling molecules at normal physiological levels. Tight junctions between epithelial cells act as a selective barrier, protecting the intestine from excessive oxidation [105]. Research has shown that interactions between the intestinal mucosal lining and the gut microbiota and their produced potential toxins can create oxidative strain within the gut. Coccidiosis, a parasitic disease caused by Eimeria, is a major health concern in poultry. This parasite disrupts the intestinal lining by damaging the tight junctions between epithelial cells. This damage leads to oxidative stress, lipid peroxidation, and a depletion of antioxidant defenses. Consequently, infested birds exhibit decreased feed consumption, impaired vitamin absorption, and stunted growth [106]. Environmental factors can further exacerbate these problems by influencing the intestinal epithelial cells and gut bacteria.

3.5 How is oxidative stress associated with poultry meat quality?

Living organisms rely on oxidative reactions for various cellular processes. An imbalance between molecules that generate free radicals (pro-oxidants) and those that neutralize them (antioxidants) within cells leads to the overproduction of reactive oxygen species (ROS). This can cause damage to cells and tissues, known as oxidative damage. Oxidation is important in the chicken production industry because it directly impacts meat quality, leading to spoilage and degradation [86, 107]. Several factors can increase a broiler’s susceptibility to oxidative stress, including high ambient temperature, exposure to toxins, and various pathological conditions [76, 108]. Chickens experience oxidative stress from two main sources. During their lifetime, a natural process in their cells called the electron transport chain can leak electrons, leading to oxidative stress [94]. Following slaughter, chicken muscle tissue undergoes increased oxidative reactions, natural antioxidant defenses suddenly fail, and pro-oxidant levels elevate in the meat. The decline in antioxidant activity after slaughter triggers various biochemical changes, including a decrease in muscle tissue pH [76]. This creates a more favorable environment for oxidation because certain pro-oxidants are naturally present in muscle tissue. These include transition metals, myoglobin, and H₂O₂. These pro-oxidants can further elevate ROS production through different mechanisms, further accelerating oxidative stress in the meat [109]. Protein breakdown by oxidation is a major challenge in meat processing. While nitrite is a natural component of muscle tissue, it can be the precursor of free radicals like RNS that contribute to both oxidation and nitration of meat proteins. In addition to nitrite, reducing sugars present in muscle tissue also play a crucial role. These sugars react and generate harmful reactive oxygen species (ROS) that can damage muscle proteins through a process known as protein glycol oxidation, ultimately affecting meat quality [110]. Various physical factors like radiation exposure can also accelerate unwanted oxidation in meat. This type of oxidation, called photo-oxidation, occurs when light triggers the formation of harmful molecules of free radicals [109].

Genetic selection for rapid growth has resulted in an unintended consequence like heightened susceptibility to environmental stressors, including oxidative stress in broiler chickens [92, 111]. Oxidative stress is particularly problematic as it can disrupt the chicken’s growth and severely lead to a decline in meat quality. Living tissues have several antioxidants to cope with oxidants. If the balance among antioxidants and oxidants is disturbed and oxidants exceed a specific limit within the body, this condition indicates oxidative stress. Most oxidants are produced during cellular metabolism in the mitochondria of living cells. Cellular metabolism is not the only source of oxidants; some external sources, including feed comprised of oxidized lipids and fats, are responsible for producing reactive oxygen species [86]. According to [94], leakage of electrons from the mitochondrial respiratory chain during oxidative phosphorylation is the primary ROS source. High ambient temperature increases ROS production by compromising the functioning of the electron transport chain, which is necessary for energy production in the muscles. Increased ROS liberation is potentially damaging as it aggravates the aging of muscles, protein degradation and inactivates the nuclear proteins, including DNA and RNA [79].