Heat Stress Management via Nutritional Strategies for Broilers

Abstract

Over the last decades, the rate of broiler production has been increased to encounter the increase growing demand and to assure the food security among the human. Heat stress is kind of a leading concern in the broiler productiveness because it directly has effects on the profit of the industry. Poultry birds can standardize their body temperature with a much tapered range of environmental temperatures. High ambient temperature unpleasantly effect on the enactment of broiler birds rather than other livestock species. The unfavorable effects of increased heat include reduce growth performances, deprived meat quality, impairing metabolic activities, destructive morphology of gastrointestinal tract, wearying immune functions, prejudicing hematological parameters and endocrine systems, etc. Henceforth, observance in the opinion the present condition, it is essential to recognize the depth of the combative consequences of high environmental temperature on broilers and look advancing to diminish the influence wherever. Thus, introducing upgraded genes, modified housing condition, altering management practices, novel feeding techniques and nutrition management via changing compositions of the diet, acquaint with innovative feed additives are some preferences which can be adhered to. The emphasis of this article is to unfold the evidence on disparaging influences and open the eye of stake holders to take corrective measures while disseminating the findings of scholars.

1. Introduction

The intensive commercial production of poultry faces a myriad of challenges and stressors that impact the physiological well-being of the birds. These challenges include heat stress, high stocking densities, nutritional imbalances, and disease outbreaks. One major concern for broiler producers worldwide is the effect of high environmental temperatures on their birds. Broilers lack sweat glands and are fully covered in feathers, making it difficult for them to regulate their body temperature during hot weather conditions. The optimal temperature range for poultry, referred to as the comfort zone, typically falls between 16°C and 25°C [1]. Temperatures that exceed this range can result in heat stress. In tropical regions where temperatures fluctuate significantly throughout the year, poultry are particularly vulnerable to heat stress.

When chickens are exposed to heat stress, they employ various strategies to regulate their body temperature and maintain normal bodily functions. These include reducing feed intake; limiting physical activity such as walking and standing; and increasing resting time, drinking, and panting [2]. The continuous panting in response to heat stress (HS) can disrupt the blood pH balance, leading to respiratory alkalosis. This can impair the immune system’s function and disrupt hormonal activities in the body [3].

Heat stress also has negative effects on production performance in broilers. It is associated with poor fertility, dehydration, low survival rates, and increased morbidity and mortality rates. Additionally, it alters meat quality by increasing adiposity (fat deposition) while reducing skeletal muscle mass [4, 5, 6].

Collectively, these impacts of HS result in significant economic loss for the poultry industry. Indeed, addressing heat stress in poultry flocks is crucial during hot weather periods. Improving the housing environment is one approach, and nutritional manipulation can also play a significant role in mitigating the negative effects of heat stress on poultry birds. In this chapter, we will discuss the impact of heat on broiler performances and explore various strategies to minimize heat stress. Additionally, we will delve into nutritional interventions that can help alleviate heat stress and examine both conventional and modern techniques of nutritional management, which can be employed in broiler farming to effectively combat the HS.

2. The effect of heat on broiler production

The increasing global demand for animal protein, coupled with the rise in human population, has led to a significant growth in poultry production, particularly broiler production. This increase is necessary to meet the food and livelihood needs of consumers. In the recent decades, there has been a steady increase in the consumption of poultry meat worldwide.

According to the Food and Agriculture Organization (FAO), global poultry meat production reached an estimated 133 million tons in 2020, accounting for 40% of total meat production (excluding fish). Poultry farming is considered to be one of the most efficient animal protein production systems with a relatively small carbon footprint per unit of product.

Poultry industry plays a prominent role in agricultural industries across many parts of the world. It is increasingly seen as a healthier alternative to red meat and other sources of protein. Chicken meat, in particular, has gained popularity due to its lower cost and high nutritive value. As demand continues to grow, there is a need for fast-growing broiler chickens that can meet market demands. Therefore, constant genetic improvement is being carried out in broilers to enhance their growth rate and improve the quantity and quality of chicken meat produced. This ensures that the industry can keep up with consumer demands while providing nutritious and affordable poultry products.

The modern hybrid poultry strains are growing faster than the old strains and their genotypes are advanced to have greater metabolic activity which causes them to become more delicate to thermal relevant ecological changes. Thus, the modern broiler chickens reach a marketable weight of 2 kg within 35 days due to their rapid growth rates with high production efficiency. However, ideal growth of the broiler bird can be reached by rearing them under thermoneutral zone of 18–24°C [7].

At present, global warming leads to increase in the environmental heat and is reasoned to make a huge stress on broiler farming and their production efficiency. Therefore, it is required to define the stress derived from the heat on broiler production and the effect of heat cope up with losing the efficiency of the broiler industry.

Heat stress means the inequality between the heat energy production in the animal and the heat energy that dissipates from the body of the animal to the environment. According to the strength of heat derived from the body and incorporation of humidity and thermal irradiation like environmental factors with age, thermoregulatory ability, and metabolic rate of animal, HS can be classified into three dimensions such as mild, moderate, and severe. According to Ref. [8], two concepts were revealed on HS such as exposure of high temperature for short period as acute heat and disclosure of increased temperature for prolonged period mentioned as chronic heat stress. Because of the absence of sweat glands and plumage coverage in poultry birds at high temperatures, broilers struggle to maintain their body temperature within the normal range and various stress reactions are elucidated to guarantee the body functions, which happen at an average rate. Due to the strain, feathering rate, nutrient availability, and the management system, the intensity of HS may vary in broiler industry.

In the case of broilers, numerous physiological responses can be identified in different systems such as neuroendocrine, immune, digestive, and circulatory. Further, HS affects feeding process, nutrition digestion on enzymatic activities, nutrition absorption via altering histology of absorptive surfaces, metabolic path ways, hematological parameters, etc. There are solemn negative effects which caused to physiological malfunctions of broilers while reducing the sustainable productivity with increased temperature on broiler production systems were recorded in tropical countries.

2.1 Symptoms of heat-stressed broiler birds

Raising wings away from body, gasping with open mouth, fatigued, deliberateness, weary eyes, high water intake, lying down without active movement, diminished appetite, descent body weight, augmented cannibalism are some of the investigated signs of HS in broiler birds [9, 10]. Further, Ref. [11] observed that broiler birds are trying to lose heat by panting with open mouth which helps to cool the surface of lungs while heat-stressed birds devote more time for drinking water rather than for feeding. In addition, researchers were revealed that increased glucocorticoids and catecholamine production, lower metabolism of thyroid hormones, reduced immune responses from weaken lymphoid organs, elevating lipid peroxidation while damaging the health of survival birds are identified internal signs of HS [12, 13, 14, 15].

2.2 Reactions of heat-stressed broilers

In high temperature when HS occurs, neurogenic system gets activated and accordingly blood pressure, muscle degeneration, nerve sensitivity, blood sugar level, and rate of respiration are increased as a first reaction. When the first reaction doesn’t work, the hypothalamic-pituitary-adrenal cortical system come into forefront and inducing of hypothalamus releases corticotrophin-releasing factors and raises the activity of pituitary gland which releases corticotropin hormone simultaneously [16]. The proliferation of adrenal cortical tissues start to release corticosteroids and Ref. [17] investigated that high level of blood corticosterone concentrations while Ref. [18] recorded the decreased level of thyroxine and adrenaline hormone levels in heat-stressed broilers.

2.3 Effect of heat stress on broiler production performances

Further researchers have investigated few of the detrimental effects such as low feed intake, decreased weight gain, reduction of meat quality, increased mortality rate, high morbidity, lower feed conversion efficiency, etc. from heat-stressed broiler birds [19, 20, 21, 22, 23, 24, 25]. In addition, Table 1 explains the effect of heat stress on different functions of broilers.

Heat stress condition	Type of effect on broilers	Reference
32°C	High metabolizable energy intake (20.3%); Increased heat production (35.5%), lower energy retention (20.9%), decreased energy efficiency (32.4%)	[26]
34°C of temperature for 6 h daily from 22 to 35 days	Low feed intake (FI) around 8–9% and dropped down of body weight gain (BWG) by 17%	[27]
32°C from 17th to 42nd of age	5–10% higher intake of water	[28]
34–36°C-chronic heat stress	Decrease in breast muscle mass (31.53%) and thigh muscle (11.17%)	[22]
(33°C for 9 h, 25°C for 15 h 1–42 days) cyclical HS	Reduced breast muscle weight by 16% (42)	[29]
(32°C for 14 days)	Increased intramuscular fat, raised activity of pyruvate kinase and lactate dehydrogenase in pectoral muscles	[30]
32.2–37.8°C	Deduction in feed intake of 9.9%/bird/day	[31]
	High mitochondrial ROS production	[32]
Acute heat stress	Increment of oxidative stress, mitochondrial ROS generation in the skeletal muscle	[33, 34]
	Decreased growth rate and feed intake, and increasing morbidity	[35]
35–45°C	Reduction of growth performance in poultry	[36]
	Increased concentration of uric acid	[37]
34 ± 2°C for 8 h/day, 08:00–17:00 h	Reduction in serum, liver, and breast muscle Cr concentrations	[38]
36 ± 2°C	Reduced dressing percentage, lower gaining of body weight, decreased protein digestibility, low feed intake, reduced relative weight of breast muscles, reduction in PCV, monocyte, basophil, total protein, albumin and glucose and increment of heterophil/lymphocyte ratio, cholesterol, AST	[39]
33 ± 0.5°C for 3 h	Low growth performance (average daily gain, average daily feed intake, and feed conversion ratio)	[40]

Table 1.

Effect of heat stress on different functions of broilers.

Moreover numerous physiological turbulences such as hyperpnoea, electrolyte inequity, fast pulse rate, vascular dilatation, endocrine disarrays, irregularities in immune system, muscle shocks and faintness, and collapse occur in heat-stressed birds that prejudice the welfare and production efficiency were observed in heat-stressed broilers [41, 42].

Moreover the cyclic heat stress caused to reduced feed intake by 15% and constant heat effect further reduced feed intake by 25–45% due to the findings of Ref. [43]. Same authors confirmed that 1.5% decline in feed intake °C⁻¹ in cyclic heat stress and 3.5% lessening in feed intake °C⁻¹ in constant heat stress were recorded during their researches.

Consequently body weight gains of heat-stressed broilers were declined due to the effect of low feed intake, nutrient digestion and assimilation which could be ascribed to a decrease in intestinal weight, length, and altered villus structure in broiler chicken affected by HS. Further Ref. [44] suggested that inflection in nutrient transport genes can be affected by HS and caused to reduce the ability of nutrient assimilation process. Correspondingly dry matter digestibility of broilers in high temperature has decreased by 1.6–3.9% and lower crude protein or nitrogen digestibility were recorded due to the lower activity rates of digestive enzymes such as trypsin, chymotrypsin, lipase, amylase, and maltase [45, 46, 47, 48].

Besides the performance parameters Ref. [30] elaborated poor meat quality characters, depression of chemical composition of chicken meat, reduced carcass weight, reduced breast and thigh meat weight, low breast meat yield, increased shear force values, and high level of abdominal fat percentages were elucidated because of exposure of heat stress during rearing period [21, 22, 49, 50]. Moreover, declining pH, increased oxidation, high malonaldehyde concentration in liver and breast meat, incidence of pale, soft, exudative in broiler chicken were reported by Ref. [51].

Further, Ref. [52] was found that lower antioxidant defense ability, increased lipid peroxidation levels, reduction of protein synthesis while increase in catabolic rate of proteins, lower RNA concentration in pectoralis muscle tissue in chicks exposed to high temperature [44, 53].

2.4 Effect of heat stress on reproduction performances of broiler birds

Different studies were conducted to evaluate the influence of heat stress on reproductive activities of broiler birds and investigated that reducing luteinizing hormone (LH) levels and hypothalamic gonadotropin-releasing hormone-I (GnRH), decrease size of reproductive organs, delaying ovulation process, low quality of semen in broiler breeders [54, 55, 56, 57].

2.5 Effect of heat stress on immunological responses of broiler birds

Immune responses of heat-stressed broiler chicken were identified by several researches and revealed that reduced the size and weight of lymphoid organs, reduced intestinal barrier functions whereas disturbing the blood flow from the gastrointestinal tract (GIT) to the skin, consequently damages the mucosal tight junction barrier in the GIT and it caused to escalation the permeability to transmit luminal endotoxins to the birds’ body [58, 59]. Therefore the increment of penetration of pathogens such as Salmonella sp., Clostridium sp., and Escherichia coli reasoned to decline the immune performances like increase colonization of destructive bacteria, high invasion of inflammatory microorganisms in heat-stressed broiler birds [60].

Further Ref. [61] confirmed that the reduction of T and B lymphocytes production, lower phagocytic activity of blood leucocytes, decline of total antibodies, fall of IgM and IgG levels, drop down of white blood cells and activities of leukocytes in heat-stressed broilers. As to support the aforesaid statement, Ref. [62] investigated accent of the immune responses by central nervous system (CNS), damaged lymphoid tissues, suppressed humoral and cell mediated immunity, lower lymphocytes to heterophil ratio, reduction of phagocytic activity of macrophages, macrophage basal and oxidative burst were observed from heat-stressed broilers by Refs. [63, 64].

2.6 Effect of heat stress on morphology of intestinal tract of broiler chicken

High temperature effects negatively on morphology of absorptive surface area of small intestine in broiler chicken by reducing relative weight of jejunum, villus height, crypt depth, cell proliferation and appearing of heat shock proteins (HSP70) in liver, heart, duodenum, jejunum, ileum were detected by Refs. [45, 65, 66, 67, 68]. In addition, Ref. [67] corroborated that HS destructively effect on gut integrity composed of enterocytes and tight junctions of small intestine in broiler chicken.

2.7 Effect of heat stress on hematological parameters of broiler chicken

Alteration of multiple blood parameters of broiler chickens which are exposed to high temperature were reported by different authors such as increment of heterophil to lymphocyte (H:L) ratio, decline in thiobarbituric acid reactive substances, movement in blood minerals and glucose concentration, changes in serum concentrations of uric acid, aspartate aminotransferase (AST), creatine kinase, total protein, albumin, and globulin [69, 70, 71, 72]. Recently, Ref. [73] investigated that reduction in plasma CO₂, HCO₃, and potassium and an increase in pH caused respiratory alkalosis and increase of glucose levels once the broilers were opened to over 33°C temperature at 5 h/day. Further, aforesaid author confirmed that reduction of K and Na levels of plasma which caused to increase water consumption and reasoned to occur hemodilution and Ref. [74] elaborated all those negative effects may finally have influence on plasma hematocrit, uric acid, total protein, and globulin levels in heat-stressed broilers. Moreover, Japanese quails which exposed to high temperature had lower blood vitamin E level by 29% and ascorbic acid level by 40% elucidated by Ref. [75].

2.8 Effect of heat stress on endocrine system of broilers

Plasma triiodothyronine (T3), thyroxine (T4), and adrenaline levels were declined and high corticosterone levels of quail broilers and chicken broilers were reported by Ref. [4]. Further, Ref. [76] reported that stress indicators such as corticosterone, ceruloplasmin, and ovo transferrin were detected in heat-stressed broiler chicken.

3. Different approaches can be used to diminish the effect of heat stress

Considering the destructive effects of high temperature and its influence on broiler industry, researchers and the stakeholders of the poultry industry advocated numerous findings which can be applied in the ongoing process of broiler husbandry. Extenuating the adversative effects caused by increased temperature in broiler birds entails holistic and multi-factorial processes. Different authors have focused on modified housing facilities [77], altering routine management practices, selection of heat tolerant broiler strains [78], novel feeding techniques and nutrition management [79, 80], etc.

3.1 Modification of poultry housing conditions

Open housed poultry cages are needed to be established toward the east to west directions with the roof angled at 45 degrees, some of modified farms have designed with sprinkle watering system to the rooves. Exhaust fans, cooling pads (wet pads), air conditioners, misting systems, and automated curtains were occupied with closed housed poultry cages which were designed with modern technology. In addition, altering the environmental conditions of poultry cages by increasing in air velocity to 2–2.5 ms⁻¹ and introducing fresh air to the rate of 6–7 m³/kg of body weight/h in to the cages were suggested by Refs. [81, 82]. Moreover, introducing novel techniques such as use heat insulating and reflective materials for rooves, spray cold water to the roof, reducing the thickness of bedding materials less than 5 cm are some of the counteractive measures that can be implemented to mitigate the HS on broilers [83, 84, 85, 86].

3.2 Altering management practices

3.2.1 Expose to heat shock treatment

Refs. [87, 88] revealed that heat resistance and weight gain can be improved by exposing eggs to heat treatment at the pre stage of hatching (39.5°C/12 h/day, from days 7 to 16 of incubation) and after hatching of chicks (35–37.8°C, 24 h, on day 5 after hatching).

3.3 Selection of heat tolerant broiler strains

The introduction of heat tolerant indigenous broiler breeds/strains such as Naked neck (Na), Frizzle (F), and Dwarf (Dw) are commended by Ref. [89] and Naked neck breed having 40% lower feather coverage and increased immunity under high temperature conditions compared to normal broiler chicken as Ref. [90] elaborated.

Other than genetic selection multiple nutritional strategies have been suggested to alleviate destructive effects of HS in the poultry industry.

3.4 Alleviation of heat stress via feeding management

Ref. [91] suggested several management strategies relevant to feeding such as allocating different forms of feed and coarser diets, practice wet feeding, introduce free choice and diurnal feeding patterns can be implemented to alleviate the influence of high temperature for broiler birds.

3.4.1 Change the form of feed

Mash, crumble, and pellets are the three different forms of feed available for broiler birds and Ref. [91] reported that broilers rear in hot climate conditions, prefer to intake feed with larger particle size in which reduce the heat generation due to prolong feeding. Similarly Ref. [92] confirmed that broilers fed with pelleted diet in high temperature conditions showed an increment of feed intake, body weight gain, and crude protein digestibility by 10%, 8.3%, and 2.3%, respectively and increased energy utilization, lower relative weight of digestive tract which cause to reduce the maintenance energy requirement for intestinal tract, increment of villus length and villus to crypt depth ratio in the jejunum, improved carcass weight and yield were observed by Ref. [93].

Nonetheless, Ref. [94] analyzed the use of coarse particles (2280 μm) of corn increased panting compared to finer particles (605 μm) in broilers fed a mash diet under natural HS conditions.

3.4.2 Practice special feeding regime

Some of the authors have recommended to practice feeding 4–6 h prior to the reaching high temperature levels such as early morning and late evening or night feeding in order to reduce the heat load of broilers [82, 95].

3.4.3 Feeding time and space

Some of studies revealed that animals kept under fasting condition, produce low heat compared to the full fed animals and aforesaid authors suggest to have feed withdrawal during the high temperature conditions while allocating feed for the duration of low temperature conditions and they observed improvements in feed consumption among heat-stressed broilers [96, 97].

3.4.4 Feed restriction and feed withdrawal

Heat production has been reduced by 23% with having feed restriction from 44 to 48 weeks of broiler breeders and reduction of rectal temperature, mortality and feed conversion ratio, heterophil-to lymphocyte ratio while having 75–50% feed restriction compared to full fed broilers was reported by Refs. [96, 98]. Ref. [99] confirmed that he received the same results such as minimum negative effects of HS by applying feed restrictions and feed restriction at early stages caused to encourage heat tolerance at latter part of the growing period of broilers.

3.4.5 Practice dual feeding

Broiler birds affected with HS are directed to the two different consecutive diets such as protein and energy concentrated feeds for a specific period achieved lower body temperature, mortality and recommended to give protein concentrated diet during low temperature and energy rich diet at high temperature time [100]. In addition, the study conducted with high energy low protein diet and low energy with high protein diet proved same performances in heat-stressed broilers.

3.4.6 Facilitate for choice feeding

By allowing for choice feeding in HS broilers, some of authors investigated that those broilers had improved the heat tolerance and during the high temperatures broilers preferred to intake less protein and high energy content feed ingredients to reduce the heat production [101, 102].

3.4.7 Increase the access for wet feeding

Water is one of the most chief nutrients in broiler nutrition, and plays a vital role for thermoregulation under hot temperatures, and promoting wet feeding is an attempt to maximize water intake and utilization. Several studies have investigated that increased body weight gain, feed intake, dry matter intake, carcass weight, protein content in chicken, abdominal fat and lipid content, and low dry matter conversion efficiency can be achieved from HS broilers by mixing feed with water in 2:1 or 1.5:1 ratio [103, 104].

However, Ref. [93] observed wet feeding increased abdominal fat, abate breast HSP70 mRNA expression, breast creatine kinase protein level, low FCR and heterophil-to-lymphocyte ratio, and high passage rate of digesta in HS broilers [105].

3.5 Increase the availability of water

According to NRC [19], water intake of chicken escalations by about 7% for each 1°C increase above 21°C. Reduction of water intake directly related to lower the production because at high temperatures broilers consume more water than their diet. Temperature of water, type, shape and height of drinker and some authors investigated that intake of water reduces, when use nipple lines rather bell drinkers in high temperature [106]. Aforesaid authors reported that the height of drinker also affected to heat production in broilers due to stretching of neck muscles and broiler birds may get high opportunity to immerse their whole face in to water to relief heat stress, when use wider and deep drinkers rather narrowed drinkers during high temperature. Researches were found that during the lower temperature broilers consume 2–3 mL water for 1 g of feed while at high temperatures it increases to 4–5 mL. In addition, assigning enough number of drinkers, increase watering space for birds, placing water storage tank in the shade, ensuring sufficient water flow (>70 mL/min/nipple), withdrawing chlorination process during high temperature are some of strategies can be applied to mitigate the heat stress in broilers.

3.5.1 Increase the access to cold water

Some of researches have investigated that giving cold water to heat-stressed broiler birds caused to ameliorate the negative effects and improved feed intake, body weight gain, increased performance, dressing percentage, relative weight of gizzard, liver, enhance meat juiciness, tenderness and reduced body temperature, tonic immobility, and total antioxidant capacity and lower malonaldehyde concentration in broiler chicken [107, 108]. Further it was recommended to supply ice added water to keep body temperature in stable less than 25°C.

3.6 Nutritional management to attenuate the heat stress

Nutritional management has become effective to minimize the deleterious effects of heat in poultry production. Supplementation with some feed additives, minerals, and vitamins into broiler diets has shown to partially mitigate the negative effects of HS including mortality. Out of different nutritional methods that are more frequently adopted are: vitamin and mineral handling, modification of the electrolyte balance of the feed among others. Multiple studies have shown supplementation of vitamin E and vitamin C reduced the adverse effects of chronic HS on performance. Ref. [109] suggested the use of non-chloride salts, such as sodium bicarbonate, more effective to reduce the negative effects of heat stress through an improved balance of electrolytes.

3.6.1 Supply dietary electrolytes and minerals

Sodium (Na), potassium (K) and chlorine (Cl) are key mineral elements involved in the maintenance of the acid-base balance of body fluids. The beneficial effects of supplementing the feed or drinking water of heat stressed poultry with compounds of these minerals such as ammonium chloride (NH₄Cl), sodium bicarbonate (NaHCO₃), sodium chloride (NaCl), potassium chloride (KCl), and potassium sulfate (K₂SO₄) are well documented such as enhancement of immune responses, nutrient digestibility [34, 79, 110]. In addition, Table 2 explains effect of dietary electrolytes and minerals to overcome the heat stress in broilers.

Type of mineral	Concentration	Effect on heat-stressed broilers	Reference
NaHCO3	5 g/kg	Increased body weight gain of 9%	[111]
Na, K, and Cl salts		Increased water consumption facilitates heat dissipation normalization of blood electrolyte balance	[111]
KCl	0.2–0.5% in water	Maintain osmotic and acid base balance increased water consumption	[112]
NaHCO3	4–10 kg/t feed	Restore the acid-base balance lost in alkalosis	[111]
KСl	0.25–0.5% in drinking water or 0.5–1.0% in feed	Restores the electrolyte balance; Retain more electrolytes (Na, K, Cl); Maintain the acid-base balance	[82]
NH4Cl; KCl; KCl; NaHCO3	0.2% or; 0.15%,; 0.6%,; 0.2% and carbonated water in drinking water	Enhanced growth performance	[34]
Zn	40 mg Zn/kg of feed	Improved growth rate, FCR, and antioxidant enzymes and minimized lipid peroxidation in broilers raised in heat stress conditions	[113]
	4.5 mg/kg of feed	Improved the performance of the heat-stressed broiler	[114]
Organic Se	At 0.3 mg/kg diet	Enhanced live body weight and feed conversion ratio	[115]
Magnesium and Zn	600 mg magnesium and 30 mg zinc/kg diet	Increased live BW, FI, and dressing percentage in Japanese quail under heat stress conditions.	[116]
KCl, NaCl	0.4% KCl or 0.4% sodium chloride (NaCl) to drinking water	Improved BW, FI, and FCR	[117]
Chromium	120 ppb chromium	Improved the performance of the heat-stressed broiler	[118]
	600 mg/kg or (400 mg/kg) with 250 mg/kg vitamin C	Increases the bodyweight of broilers
Selenium	0.4–1 mg/kg	Enhanced growth performance of broilers, improved DNA synthesis, enhanced cellular antioxidant levels and immune system responses	[119, 120]

Table 2.

Effect of dietary electrolytes and minerals to overcome the heat stress in broilers.

In addition, Refs. [113, 121] elaborated that the significant effects of minerals such as selenium, chromium, zinc, etc. on the improvement of performance parameters and reduction in heat shock proteins, the requirement of phosphorous and calcium increases during HS in broiler chicken reared in tropical climates.

The dietary supplementation of selenium increased growth performance, immune responsiveness and provision of organic selenium enhanced antioxidant capability, productive and reproductive performances in Japanese quails and broiler roosters [120, 122]. Moreover, incorporating organic zinc to the heat-stressed broiler diet, investigated that enhancement of growth, digestibility, antioxidative status, humoral immune responsiveness and decreased lipid oxidation in meat extracted from heat-stressed broilers.

Ref. [123] observed that the feeding of dietary amino acid-chelated minerals such as Fe, Mn, Co, Zn, Cu, Mg, K, and Ca decreased serum and intestinal HSP70 and gene expression of pro inflammatory cytokines in heat-stressed broilers. Ref. [124] were revealed that the dietary requirement of potassium (0.6–0.7%), sodium (0.20–0.25%), chloride (0.30%) levels were increased with increase in temperature and the precise electrolyte balance and electrolyte ratio (EB of 350 mEq/kg and an ER of 3) improved broiler performance, livability and prevented the negative effects of HS.

3.6.2 Supplementation of dietary vitamins to the heat-stressed broilers

The supplementation of Vitamin A, B, D, C, and E reasoned to attenuate the effect of HS in broilers and improved the performance, feed intake, immune functions, reduced respiratory quotient, body temperature, carcass weight, nutrient utilization, and protein content of meat during the high temperature [39, 103, 125, 126, 127, 128].

Ref. [114] explored that the broiler diets which contained vitamin A protected the tissues against damages due to oxidation, increased weight gain and feed efficiency.

Ref. [129] recommended adding (vitamin C) VC at 1 g/L of drinking water throughout heat periods and addition of VC to the drinking water declined corticosterone. Further, VC declined metabolic rates, enhanced antioxidant activity, reduced lipid oxidation, increased performance in heat stressed Japanese quails and Ref. [130] ensured that the addition of VC 250 mg/kg of feed increased feed conversion efficiency, high growth performances, survival rate, and carcass quality of broiler chicken effected from HS.

An experiment based on vitamin E (VE) fortification in the rate of 200 mg/kg for heat-stressed broilers confirmed that increased phagocytic activities of macrophages, enhanced serum antibodies (IgM and IgG) levels, growth performances, increment of carcass and cuts yield, lower lipid peroxidation in chicken semen, increased antioxidant capacity, increased activity of the glutathione peroxidase, catalase, superoxide dismutase and glutathione reductase in erythrocytes [119, 122, 131]. Further Ref. [132] elaborated that aforesaid VE level helped to reduce the effect of incidences of Pale, Soft, Exudative meat from HS broilers.

Ref. [133] confirmed that supplementation of VE 100 IU/kg feed, enhanced number of T-lymphocytes and SRBC (sheep red blood cells) antibodies, spleen weight, and bursa of Fabricius, reduced the activity of stress hormones and it helped to safeguard lymphocytes, macrophages and plasma cells contrary to oxidative impairment, enhanced immune responses, etc. in heat-stressed birds [134, 135]. Further, provision of 250 mg VE/kg caused to increase the action of GSH-Px and declined the expression of heat shock protein-60 (HSP60) and improved meat quality in heat-stressed broilers [128].

Correspondingly, Ref. [136] investigated that the combination of 100 mg/kg of vitamin E + 50 mg/kg zinc is a strategy for broilers reared under HS, as it favors the growth performance. Moreover addition of electrolytes such as sodium bicarbonate with vitamins E and C in to the diet of broilers reared under heat stress caused to increase feed conversion ratio, plasma calcium level from 7 to 35 days. Synergistic effects of vitamins made higher efficiency rather than single form of vitamin such as VC at 200 mg/kg or VE at 150 mg/kg reasoned to enhance semen quality and fertility of broiler roosters was identified by Ref. [137]

3.6.3 Alter the composition of diet changing energy and protein concentration

Poultry kept under HS, make effort to regulate body temperature by lowering intake of energy. Ref. [79] suggested that feeding of diet which contained increased energy level would be beneficial for the heat-stressed broilers. Further some of authors recommended to feed high proteins with moderate energy levels also a good solution and contradictorily, Refs. [138, 139, 140] advocated that inclusion of increased level of fat by incorporating coconut oil or beef tallow caused to enhance the heat tolerability, body weight gain, and feed conversion efficiency, of broilers. Similarly, in high temperatures (32–39°С) feeding broilers with low crude protein but rich with essential amino acids and high metabolizable energy containing diet showed higher efficacy rather than high crude protein and same level of energy fortified diet was investigated by Ref. [141]

3.6.4 Regulate the amino acid composition in the broiler diet

National Research council confirmed that broiler feed must be articulated with 100% digestible amino acids which directly target to the requirement of broiler production. There was a suggestion to adjust the crude protein levels according to the requirement of essential amino acids of broilers can be reduced the heat production during the amino acid oxidation. Different researchers had conducted numerous researches based on reduced crude protein levels for broilers in constant heat stress [43], cyclic heat stress [43, 71, 142], and hot climates [137, 141].

In addition, Attia et al. [137] defined that supplementation of Lysine with Methionine in a diet which has lower crude protein level could be able to balance the same performances and carcass characters of the heat-stressed broilers as the diet of high crude protein level. Similarly Ref. [143] verified that enhanced feed conversion efficiency and some of wellbeing signs from Pekin ducks exposed to HS by supplementation of arginine. Further, Ref. [144] investigated that Trypsin supplementation decreased rectal temperature, corticosterone responses, and some other authors confirmed that by balancing the essential amino acids levels in a ration could reduce the energy cost need to evacuate the additional nitrogen and it would support the chicken to cope with HS.

Moreover, Ref. [145] investigated that adding 1% of Glutamine to the diet of heat-stressed broilers caused to increased growth performances, development of intestinal environment and humoral immunity. Similarly Ref. [44] confirmed that adding Glutamine to the broiler diet reduced the effect of HS by regulating the intestinal barrier functions such as cell proliferation in intestine, secretion of cytokines, amending the tight junction proteins. Subsequently, Cystein and Methinine incorporated broiler diet could be enhanced the growth performance, lower rectal temperature in heat-stressed broilers.

3.6.5 Adding fat in to the diet

Fat generates lower amount of heat rather than protein and carbohydrate at metabolism is a standard statement. Thus some researchers suggested increasing the fat level up to 5% reasoned to decline stress made by the heat on broilers. Incorporation of dietary omega-3 fatty acids ameliorated the effect of HS by increasing antioxidant status and antibody responsiveness, reducing lipid peroxidation in Japanese quails reared in high temperature and stated that it caused to enhance growth and performance index of heat-stressed broiler chicken [146].

3.6.6 Use of feed additives to decline the effect of heat stress

Some of researchers have identified that the value of incorporating Betaine enhanced ion balance, immunity, Methionine sparing, fat circulation, and developed a resistance against heat stress [135, 147].

3.6.6.1 Feeding probiotics for heat-stressed broilers

Probiotics enhanced the serum T3 and T4 levels, functions of immune system, responses of antibody levels, leucocytes count, decline corticosterone, increment of intestinal morphology in heat-stressed broilers [17, 148]. Further, probiotic fortified diet could increase the growth performance, feed conversion ratio and immune responses in heat-stressed broilers. In addition the protection of skeletal muscle quality and increased breast meat weight were experienced by giving probiotic incorporated diet to heat-stressed broilers. Provision of Bacillus subtilis, Lactobacillus, L. pentosus, L. acidophilus like probiotics encouraged the protection of lymphoid organs from heat damages, increased production performance, behaviors and immunity, augmenting the multiplicity of flora in the jejumum and cecum, enhance the growth performances, ameliorate the manifestation of four sugar transporter genes (GLUT2, GLUT5, SGLT1, and SGLT4), increment of fatty acid profile in meat in heat-stressed broilers [149].

The mixture of mannan-oilgosachcharides, prebiotics and probiotics influenced on increment of performance, welfare aspects, morphology of intestine, immune responses of heat-stressed broilers and enhanced aforesaid parameters were detected by researchers.

3.6.6.2 Incorporation of exogenous enzymes in to the diet

The dietary exogenous enzyme mixtures assorted broiler feed caused to increase palatability of feed, digestibility of nutrients, reduction of feed cost, enhance the growth performance of heat-stressed broilers [34, 82]. In addition, provision of bile acids enhanced the weight gain, feed conversion efficiency, antioxidant capability, and upheld homeostasis in the gut-liver axis [150].

3.6.6.3 Supplementation of ordinary antioxidants

The incorporation of natural antioxidants to the diet of heat-stressed broilers is an efficient strategy to overcome the adverse effects of high temperature during the rearing period and scientists have proved that this is due to discharging action on free radicals derived from heat stress. Different authors have explained about their findings related to the numerous antioxidant types such as tocopherols, ascorbic acid, carotenoids, flavonoids, non-flavonic phenols, Se, Zn, Cu, and Mn like minerals, etc. [5, 79].

3.6.6.4 Incorporate photobiotic to the broiler diet

Plant originated naturally presented bio active compounds are useful as the form of phytogenic feed additives and they efficiently perform as accepted antioxidants when integrate in to broiler diet such as Curcumin, Ginger, Epigallocatechin Gallate, Resveratrol and Lycopene, etc. [151, 152]. Moreover, Table 3 elaborates, benefits of natural herbs to mitigate the heat stress in broilers.

Type of herb	Inclusion level and available compounds	Effect on heat-stressed broilers	References
Ginger	2%	Increased biochemical blood parameters Enhanced growth performance High antibacterial potential Increment of the digestibility, palatability, metabolism, and health status of broilers	[134, 153]
Fermented dried ginger	1%	Lower abdominal fat Enhance the health	[154]
Garlic	2 and 4 g/kg	Increase intestinal villi Reduce crypt depth	[155]
Garlic powder incorporated probiotic and citric acid		Increased feed utilization Ameliorate antioxidant status and humoral immunity	[156]
Nigella sativa (black seed)	Thymoquinone, nigellone, and thymohydroquinone; 2%	Increase antitoxic and antimicrobial properties Higher defense mechanisms against infectious diseases Weight gain in pigeon and body weight enhancement Protect from formation of hepatic lesion, mild vascular congestion and vacuolization of the hepatocyte	[157]; [158]
Black seed	1%	High the feed intake Increased dressing percentage High body gain Decline the panting behavior Declined water to feed ratio, corticosterone, and T3 levels	[159]
Black seed	5.6%	Increased BW, BWG, FI and FCR	[160]
Anthocyanins		Antioxidant, anti-inflammatory Immunomodulatory Antidiabetic Anti-obesity Neuroprotective and anticancer biochemical agents	[147]
Lycopene	200 or 400 mg lycopene/kg	Powerful antioxidant Increase cell growth and the immune response Natural anti-stressor Lessen adverse impacts of heat stress Anti-inflammatory, immune-booster effect Better performance and enhanced meat quality parameters Augment antioxidant enzymes such as SOD and GSH-Px Dropping MDA level Boosted FI, BW and FCR Upgraded meat quality characterizes	[161, 162, 163]; [164]; [164]; [165]; [162]; [162]; [165]
Curcumin derived from turmeric	100 mg curcumin/kg; 200 or 400 mg/kg diet; Japanese quail broilers	Antioxidative and anti-inflammatory properties Amplified growth performance Superior antioxidant biomarkers Immune-related gene expressions Higher final BW Diminished the mitochondrial MDA concentration Lessened ROS production through activating GSH-Px, SOD and glutathione S-transferase and enhancing gene expression of peroxiredoxin-3 and thioredoxin-2 Amplified BW, FI and Declined serum lipid peroxidation and HSP70	[166]; [152]; [152]; [167]
Resveratrol (trans-3,5,4′-trihidroxiestilbeno)	300–500 mg/kg of feed	Improvement in FI and BWG in chickens under chronic heat stress (37°C for 15 days) Lower HSP27, HSP70, and HSP90 mRNA in spleen and bursa of Fabricius Advancement in the low mRNA expression of HSP27 and HSP90 in thymus Upgrading of antioxidant enzymes SOD, CAT, and GSH-Px Diminished MDA Renovating the dislocated villus-crypt structure due to heat stress Repair of the normal intestinal microflora (amplified Lactobacillus and Bifidobacterium and lessened Escherichia coli) Enhanced the intestinal epithelial barrier function and tight junction via modulating the mRNA expression of related genes	[168]; [68]; [112]
Epigallocatechin-3-gallate (EGCG)from green tea	300 or 600 mg EGCG/kg diet	Upgraded growth performance Enhanced antioxidant enzymes (CAT, SOD, and GSH-Px) in the liver and plasma Reduced oxidative stress	[151]; [169]
Rosemary; Purified rosemary extract	0.4%	Improved growth performance Amplified cecal microbiological populations and fat accumulation Motivation of crystallin alpha B and HSP70	[170]
Cinnamon powder	0.5%	Enhanced daily gain and antioxidative status (SOD,CAT, and total antioxidant capacity) Decline the concentrations of MDA	[171]
Thyme essential oil	150 and 200 mg/kg	Improved growth performance, Augmented humoral immune response, and relative weight of lymphoid organs (spleen, thymus, and bursa of Fabricius)
Onion; Quercetin; Allicin	2.5 onion kg/t as a powder +2.5% onion extract in drinking water	BWG and FCR Encouraged heat tolerance Eradicate the free radicals, Preserve and regenerate vitamin E and prevent the deleterious effects of chelate metal ions Improved antimicrobial activity Upgraded growth performances	[172]
Essential oils; Curcuma xanthorrhiza Oreganum compactum Orange peel extract		Decline the oxidative stress in broilers under cyclic and chronic heat stress	[173]
Dried plum (Prunus domestica L.)	2.5%	Enhanced the growth performance Expression of heat shock protein-related, antioxidant related, immune related, ileum histomorphology and tight-junction related genes Enhanced intestinal integrity Amplified butyrate along with total VFAs concentration in the cecum Improved the relative abundance of beneficial bacteria in the cecum	[25]

Table 3.

Benefits of natural herbs to mitigate the heat stress in broilers.

4. Conclusion

Considering the increment of environmental temperature, it is essential to identify the adverse effects of high ambient temperature on broilers. Various up-to-date studies are being published in relation to heat stress in broilers, effect of heat stress on different roles of broiler bird and the way of influence to the broiler industry and numerous mitigation strategies to overcome the major issues related to the heat stress. Among the discussed evidences, nutritional management represents higher efficiency compared to the other alleviation techniques of HS. Thus it is a promising topic that needs to focus in broiler industry while introducing updated knowledge to the stakeholders of the industry.

Conflict of interest

The authors declare no conflict of interest.

Appendices and nomenclature

AST	aspartate aminotransferase
BWG	body weight gain
CNS	central nervous system
Cr	chromium
FI	feed intake
FAO	Food and Agriculture Organization
GnRH	hypothalamic gonadotropin-releasing hormone
GIT	gastrointestinal tract
H:L	heterophil to lymphocyte ratio
LH	luteinizing hormone
HSP	heat shock protein
HS	heat stress
PCV	packed cell volume