Deleterious Effects of Heat Stress on Poultry Production

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1		Majid Shakeri	--	2208	2022-08-30 14:57:36	\|
2	Reference format revised.	Lindsay Dong	-67 word(s)	2141	2022-08-31 02:46:59	\|

This entry is adapted from the peer-reviewed paper 10.3390/poultry1030013

High environmental temperature is one of the significant factors challenging poultry production during hot seasons or in tropical areas causing heat stress (HS). The detrimental effects of HS on broilers range from reduced growth performance to impaired poultry meat quality. HS impairs physiological responses caused by alteration in blood parameters, which could lead to impaired product quality by reducing moisture content and altering the production of antioxidant enzymes resulting in increased oxidative stress. There has been a focus on the use of nutritional supplements as a cost effective HS amelioration strategy, such as betaine and polyphenols. Supplementing broiler chicken’s diets with polyphenols aims to enhance growth performance via reduced levels of oxidative stress in tissues under HS conditions. Furthermore, using betaine as an osmolyte aims to protect tissues during osmotic stress conditions.

broiler chickens heat stress betaine polyphenols antioxidants oxidative stress growth performance

1. High Stocking Density and Heat Stress

Stocking density for poultry is defined as the number of birds in a standard area such as birds per square meter. The ultimate goal of having more birds per unit of area is to maximize the production of chicken meat. In many cases, commercial farms settle for slightly reduced growth performance to achieve a satisfactory economic return. However, having more birds per unit of area is not without consequences. High stocking density impacts animal welfare by reducing the quality of the environment and increasing competition for available resources such as feed ^[1]. Generally, overcrowding has adverse effects on performance, livability, litter moisture, feed efficiency ^[2], etc., and may decrease the amount of production resulting in economic losses ^[1]^[3]. High stocking density is considered to directly or indirectly cause HS, for a variety of reasons such as increased litter moisture during warm seasons ^[1]^[4].

Heat stress is a response to high temperature and humidity. It happens when birds are outside of their comfort temperature zone and struggle to regulate their body temperature. When birds are unable to dissipate their body’s heat, physiological and biological disorders appear following multi organ dysfunction and often result in death ^[5]. Furthermore, depressed feed intake and minimized activity lessen the heat burden ^[6]. Additionally, HS alters the functions of hypothalamo-pituitary axis and orthosympathic nervous system resulting in altered thyroid hormonal activity that is involved in metabolism ^[7]. Thyroid hormones are necessary for skeletal development, growth and body temperature regulation that help chickens adapt their body’s temperature to cope with the environment ^[8]. Therefore, any disruption of thyroid activity can impair performance ^[6]. In fact, thyroid hormones are considered to have major roles in metabolic processes (catabolism and anabolism), thereby influencing nutritional efficiency, catabolism, anabolic synthesis, and thermogenesis ^[9].

Heat stress challenges management decision making in the poultry industry by causing significant reduction in meat production, and can become one of the most damaging factors in the industry.

2. Thermoregulatory Responses

Thermoregulation in broilers is mediated by the hypothalamus in conjunction with central and peripheral thermoreceptors. Thermoreceptors are found throughout the skin. They are specialized nerve cells located in the preoptic anterior hypothalamus that detect environmental temperature. Thermoreceptors allow sensory reception throughout the body to evoke adequate thermoregulatory responses via the control of physiological, endocrinological, and behavioral responses to adjust the rates of heat produced by metabolism, dissipated to and absorbed from the environment ^[8].

Chickens are homeothermic and capable of regulating their body heat balance and saving most of their energy obtained from feed for growth ^[10]. Broiler chickens can control and maintain their body’s temperature when the environmental temperature is not higher than the upper limit of the critical temperature zone according to their thermoregulation mechanisms ^[5]. Thermoregulation is the ability to keep the body temperature within optimum temperature zones to ensure the function of all vital organs, even when the surrounding temperature is not optimal. However, when the environmental temperature is far different from the normal range, the energy cost of thermoregulation spikes. To minimize this energy cost, chickens use a variety of morphological, behavioral and biological responses to adjust their body’s temperature by balancing the ratio of heat loss and heat gain ^[11].

2.1. Physiological and Behavioral Responses

Physiological responses are automatic reactions to a stimulus. In broiler chickens, physiological responses are the initial defensive actions against HS. Broiler chickens attempt to reduce their body temperature through different actions. Featherless parts of the body appear to act as “thermal windows” where heat dissipation is the most efficient, a means of thermoregulation at high temperatures in broilers ^[12]. Plumage related adaptive behaviors in the context of thermoregulation include: keeping the wings away from the body, ruffling feathers, and dustbathing. Furthermore, they limit feed intake and increase water drinking and resting ^[13], and move to a shaded area to avoid absorbing more heat from the environment. Although these initial responses help chickens regulate their body temperature, they may fall short in higher environmental temperatures, and the chicken’s body temperature may still exceed the optimum zone. In that case, other biological mechanisms are required to reduce their body temperature.

2.2. Biological Responses

Though chickens cannot cool their bodies down by the evaporation of moisture from their skin due to a lack of sweat glands, they are equipped with air sacs in the lungs that help them dissipate heat from their body to the environment by improving air circulation on surfaces. When the environmental temperature increases, their principal metabolic option for cooling is to evaporate moisture from the lining of the throat and the air-sacs. The increase in air movement over the air-sacs leads to a significant increase in moisture evaporation, which in turn leads to the loss of body heat and therefore cooling. To further aid radiant heat loss, chickens redistribute blood flow to their skin while panting. Increased peripheral blood flow reduces excessive produced heat, and increased panting facilitates evaporative cooling ^[10]; both are biological responses that help chickens cope with excess body heat. This requires a commensurate reduction of blood flow from elsewhere and is principally derived from the body core. Both mechanisms help chickens to lower their body temperature when the environmental temperature is high. However, the responses are not without consequences, and they could negatively impact the quantity and quality of the final meat products ^[14]^[15].

3. Heat Stress Impairs Production

HS leads to reduced feed consumption, resulting in reduced weight gain. Under HS, the changes in the blood pH impair the immune system’s functions and alter the body’s hormonal activity responsible for metabolic activities ^[7], leading to impaired growth performance. Fast panting leads to respiratory alkalosis and hyperthermia ^[15]^[16] as well as higher oxidative stress for the increased production of reactive oxygen species (ROS). Higher levels of ROS increase excretion from the body ^[17]^[18] which alters vitamins and mineral concentrations which are important for the defense system. Reactive oxygen species are a major reason for oxidation resulting from cellular metabolism and external sources, including feed that contains oxidized fats and lipids in mitochondria ^[19], stimulating metabolic oxidation and increasing the activity of enzymes. Additionally, increased blood flow to the skin leads to a reduction in gastrointestinal, hepatic and renal blood flow, making these organs particularly sensitive to HS. Reduction of blood flow reduces oxygen supply to these organs with high metabolic activities leading to oxidative damage ^[14]. The negative impacts are not limited to the amount of production, but could have adverse impacts on organ functions, leading to lower meat quality. The reduction of blood flow causes mitochondria to malfunction and affects energy-substance aerobic metabolism, resulting in increased glycolysis and intramuscular fat deposition ^[20]; this leads to lower consumer acceptability ^[6] due to paler meat colour ^[21] with lower water holding capacity ^[22] and increased cook and drip losses ^[21].

4. Solutions to Cope with Heat Stress

Genetically modified broiler chickens have a higher growth rate in a shorter period of time compared to natural breeds. They consume more feed, their body produces more heat due to their greater metabolic activity, making them more susceptible to HS. The increasing demand for poultry production around the world calls for an re-evaluation of long-term strategies for modern genetically-modified commercial breeding programmers.

Many strategies have been introduced to the industry, such as improving the housing system to help broiler chickens cope with HS. However, such strategies are expensive to apply or have limited effects on broiler chicken health and performance, while potential supplementations such as betaine (osmolytes) and polyphenols (antioxidants) could help them to improve their health and the body’s heat tolerance by protecting tissues or improving enzyme activities, thereby lowering oxidative stress. One of the major functions of the additives is improving gut health. A healthy digestive system plays an important role in the efficient conversion of feed into absorbable form for optimal nutrient utilization and stability of the microbiota, and what follows is better growth performance. When gut health is compromised, digestion, and nutrient absorption are affected, which in turn, may have a detrimental effect on feed efficiency and lead to economic loss. Therefore, supplementing a diet with potential additive such as osmolytes and antioxidants may help to improve gut heath, reduce metabolic heat production and maintain nutrient intake under HS.

5. Nutritional Additives to Ameliorate Heat Stress

5.1. Macronutrients and Micronutrients

Macronutrients are energy-providing nutrients that are required in larger quantities such as carbohydrates. Carbohydrates break down into smaller sugars like glucose, which will be used as an energy source. Macronutrients are thus essential for growth, developing and repairing tissues that help the body cope with HS by improving thermoregulation pathways without using the stored proteins in tissues. However, to maintain muscle, blood circulation and the immune system, the body requires a supply of different materials including vitamins and other micronutrients.

Micronutrients including minerals and vitamins do not provide energy for the body but they participate in metabolic pathways. They are required in small quantities. Polyphenols, for example, are essential for many pathways including the function of enzymes.

Animals under HS require higher levels of micronutrients, but this may be exacerbated by reduced levels of feed intake. Reduced levels of micronutrients will compromise growth performance ^[23], and therefore the replacement of micronutrients with increased utilisation during HS is one micronutrient amelioration strategy.

5.2. Betaine

Betaine (trimethylglycine) is extracted from various natural sources, including sugar beet, from which its name is derived. Betaine undertakes two major roles in an animal’s body: as an osmolyte to protect cells against osmotic stress, or as the “methyl donor”, being a catabolic source of methyl groups via transmethylation ^[24] to transform excess homocysteine into L-methionine.

As an osmolyte, betaine is a small, highly-soluble organic compound that accumulates in cells without disrupting their function. It affects the process of osmosis by maintaining the balance of fluid levels outside and inside of cells, protecting cells against osmotic inactivation ^[25], alleviates negative impacts of osmotic stress in the intestine ^[26], and maintains water and metabolic balance ^[27]. Imbalanced fluid levels can increase cellular shrinkage depending on the excess fluid on the inside or the outside of the cell. Hyper-osmosis causes water efflux and concomitant reduction in cell volume, leading to more cell deaths ^[28].

Osmolarity effects of betaine can help gut tissues continue regular metabolic activities under stressful conditions. Damage to intestine cells during HS increases osmolarity in the intestine, which impairs cell metabolism and its enzyme activity. Any alterations in the cell’s structure can reduce nutrient absorption, making it easier for external pathogens or toxins to enter the blood. Betaine can enhance intestinal integrity which helps the intestine work normally, resulting in optimized nutrient digestibility and reduced excretion ^[29]. Betaine is also effective in this regard in premature birds. The gut structure of a young chick is not developed sufficiently to absorb or digest nutrients well, leading to osmotic pressure across the tight junctions as the gut structure moves water into the lumen. With betaine accumulated in gut cells, osmolarity increases, and this protects epithelial cell morphology and stabilizes gut mucosa ^[30] as well as reducing movement of water from cells ^[26].

5.3. Polyphenols

Studies have indicated that antioxidant properties of polyphenols helped to improve body weight gain and meat quality of broiler chickens and ameliorate the adverse effects of HS ^[31]^[32] by reducing the production of ROS leading to the reduction of oxidative stress ^[33]^[34]. It also has been reported that polyphenols could maintain and improve meat quality against HS or under normal conditions by increasing the muscle antioxidant capacity and the activity of antioxidants such as glutathione peroxidase ^[35]. It has been reported that polyphenols can reduce DNA damage and protein degradation ^[36]. They could inhibit lipid peroxidation and improve enzyme activity in hepatocytes, thus relieving damages to tissues by HS. Moreover, they act as hormonal and growth regulators ^[37], and enzyme modulators ^[38], leading to improved growth rate. Therefore, the functions of polyphenols make them a promising additive in broiler chickens’ diets especially under HS ^[31] where there is a need to reduce ROS production. It has been found that polyphenols at doses between 0.05 g/kg to 10 g/kg improve weight gain and feed intake of broilers under HS, and that higher doses of polyphenols seem more effective ^[31]^[39]^[40].

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