It is well documented that the stress of hot environments lowers productive and reproductive efficiency in farm animals. As we mentioned earlier, heat stress is a condition where there are imbalance between body heat production and heat loss, thus it make body in uncomfortable situation. The more heat an animal produces internally by its metabolism, the smaller its capacity for tolerating external heat (Bianca, 1976).
Thermoregulatory mechanism is the mean by which animal control its body temperature, so that it will be in normal range. Normal body temperature of a mature chicken is in the range of 41 to 42 °C (Richard, 1971). Where else, the upper and lower lethal body temperatures of chicken are 47.3 °C and 23.4 °C respectively. If the magnitude (intensity and duration) of potential stressors exceeds a threshold, animal are unable to cope and are affected adversely (Hahn et al., 2003). Upper critical temperatures will vary depending on several factors, including degree of acclimatization, rate of production (growth or lactation), pregnancy status, air movement around the animals and relative humidity (J. W. Fuquay 1981).
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Response to heat stress begins with increased respiration rate, continuous with decreased feed intake and also lead to increased rectal temperature (Huynh, 2005), but reduced in feed intake can be considered as a mechanism of thermoregulation (Bianca, 1976). Besides that, the animal will maximized water intake in order to facilitate heat loss to metabolism through increase in water turnover. In chicken, at a body temperature of 44 °C, rate of respiration rises to 140 to 170 per minute (Siegel, 1968).
Heat is produced by essential body metabolism which includes maintenance, growth, exercise, feeding and also production such as lactation. High rate of these activities will result in greater heat production. Besides that, heat also gain from environment through radiation, conduction and convection. During daylight hours, almost of the heat gained from the environment comes directly or indirectly from solar radiation. Heat is gained from convection or conduction only if air temperature is higher than skin temperature or if the animal is resting against a surface hotter than its skin (J.W. Fuquay 1981)
On the other hand, heat also dissipated via metabolism activity, for example milk removal, fecal and urinary elimination. Besides, dissipation of heat also occur through environment such as radiation, conduction, convection and evaporation. Radiation is an important means of heat dissipation when the environment temperature is cooler than the animal's surface especially at night. High humidity and clouds impede cooling by radiation (J.W. Fuquay 1981). In addition, convective and conductive heat losses are more successful at cooler air temperature and have little impact in hot environment. Evaporative heat loss are very important at high temperature. As the ambient temperature rises, evaporative heat loss becomes the major avenue of heat loss because it is not dependent on the thermal gradient, as are conduction and convection (J. W. Fuquay 1981). Since poultry do not sweat, they are depends on this evaporative heat loss mechanism.
2.2 HEAT STRESS PROBLEM
Heat stress often occur in tropical region where the climate can be extremely hot with high relative humidity and small differences between day and night temperature. This situation may give a lot of problem to the animal in term of production and performance. Heat stress can result in chronic hyperthermia, causing severe or prolonged in appetence (Bell et al., 1989). Reduction in growth rate with increases in climate temperature have been widely reported (Daghir, 1995). Milligan and Winn (1964) reported that weight gain, feed conversion ratio (FCR) and feathering were adversely affected for broiler raised under ambient temperature of 29.8°C to 31.3°C. Depression in body weight could be attribute to decline in voluntary feed intake as temperature increased (Davis et al.,1973). In addition, Rose (1987) also indicated that feed conversion ratios had a curvilinear response to ambient temperature which was similar to that for body weight and feed intake.
Management of feeding programme, diet, and also selection of traits that are resistance to heat stress would be desirable in hot climate especially production of poultry in open houses system. For example, since the birds already out of their thermo neutral zone (comfort zone), it is not recommended to feed them during the day. It is because, specific dynamic action (SDA) of the feed may add up the body heat production. It is different with the birds that are in close houses system that are always under thermo neutral zone which they can feed anytime. By providing an optimum environment for birds allow them to reach their maximum genetic potential. Hence, in our study, we are looking if methionine play a key role in physiological stress and the acquisition of thermal tolerance in commercial broiler chicken.
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During exposure to hot ambient temperatures, poultry have a difficult problem keeping themselves cool and maintaining homeothermic body temperature. Since birds do not sweat, they must rely on evaporative cooling which is by panting to keep themselves cool (Koelkebeck., 2003).
Panting is a controlled increase in respiratory frequency accompanied by a decrease in tidal volume, the purpose of which is to increase ventilation of the upper respiratory tract, preserve alveolar ventilation, and thereby elevate evaporative heat loss. Freeman (1966) reported that panting is normally initiated at about 29°C, but it is affected by relative humidity. The panting mechanism tends to be important in smaller mammalian species and in larger species is supplemented by sweat gland.
The respiratory response to heat exposure was biphasic. In the phase 1, which is rapid, shallow breathing, animal will increase the frequency of breathing. It means air will fill in the upper respiratory tract which is dead space. The volume of air enter the dead space are constant while the volume of air enter the alveoli can change. Robertshaw (1985) stated that the increased respiration rates in response to heat stress represent rapid shallow breathing (panting) is to provide for evaporative heat dissipation by the ventilatory system. During panting, only a little of air will enter the alveoli just to maintain respiratory function instead of taking part in heat loss mechanism and normal gasses exchange occur. The increased energy cost of panting is offset by reducing the metabolism of non respiratory muscles. Besides that, in the dead space, the volume of the air entering are constant while the frequency of breathing increase. This will increase the dead space ventilation, (f x VT ) while the ventilation of the alveoli remain constant.
The second phase of panting will be enter only if heat stress in severe condition. During this phase, external respiratory movement changed from rapid, shallow breathing to slow, deeper breathing. In this phase, the frequency of respiration decrease and ventilation of the dead space ( VD ) also decrease. But during this phase, alveoli ventilation (VA) will start to increase due to increasing air volume entering the alveoli space. The increasing in alveoli ventilation will increase the respiration rate, so that heat loss also increase by the evaporation of water from the lung surface. In addition, increasing in respiration rate may lead to hyperventilation that can cause excessive removal of CO2 and resulting in severe fall in hydroxide ( HCO3¯) ion in blood. If this situation occur, the acid-base balance in body will be disturbed and may lead to respiratory alkalosis. The reduction in this hydroxide ion (HCO3¯) can increase blood pH up to 7.4 and above. If the pH keep on increasing until 7.7,animal can even die.
2.4 DIETARY METHIONINE REQUIREMENT IN BROILER CHICKEN
Methionine is an essential nutrient need for healthy and productive poutry. Thus, poultry diet is commonly supplemented with synthetic methionine sources either as dry DL-Met (DLM; 99% pure) or as liquid DLM hydroxy analog free acid (MHA-FA, containing 88% of active substance) to ensure an adequate supply of this essential amino acid (AA). This AA provide the methyl group needed for various metabolic reaction, like carnitine and creatine synthesis. Methionine is also a precursor for polyamines, spermidines and also spermine, which are essential for cell polyferation, and development (Bardocz., 1995). The major limiting AA in corn soybean meal based diet for broiler during the growing period are methionine, total sulfur AA and lysine. However, methionine generally considered as the first limiting AA and thus required proper attention during feed formulation.
For the purpose of feed formulation, amino acid requirements should be deï¬ned as the minimum amount of the amino acid that maximizes the use of essential amino acids for protein synthesis (Ball and Bayley, 1986). The requirement of methionine for growth and maintenance would be expected to vary with factors that inï¬‚uence maximum growth and feed intake. These factors include dietary nutrients, age, sex, physiological status, and environmental conditions (Ishibashi and Kametake, 1985). NRC (1994) recommends 0.5% methionine for 0-3 weeks old broiler chicken, whereas Chamruspollert et al.,(2002) reported higher methionine required for male (0.54%-0.57%) and female (0.52%-0.53%). On the other hand, Klain et al. (1960) reported only 0.18%methionine in the diet methionine the requirement of New Hampshire - Columbian chicks for optimum growth, whereas Dean and Scott (1965) reported that 0.45% methionine was necessary. These variable values of the methionine requirement influenced by the condition of the experiment such as climate, stress factors and also influenced by the interaction between amino acids.
2.5 EFFECT OF AMINO ACID AND CRUDE PROTEIN ON CHICKEN UNDER HEAT STRESS
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It is well documented that rearing chicken under high temperatures resulting poor performance, lower percentage of breast meat and greater fat deposition (Ain Baziz et al., 1996; Geraert et al., 1996). Whereas, in laying hens, heat stress depresses egg production (Marsden et al., 1987) and egg weight (Peguri and Coon, 1991). Temim et al. (1999) reported that broiler fed with high-protein diets under heat stress condition may show better performance. Brake et al., 1994 and Mendes et al., 1997 reported that increasing the Arg:Lys ratio in broiler diets may also reduced mortality and improved feed efï¬ciency of broiler. Dietary supplement of these amino acids are important during hot weather because their availability from dietary sources is decreased due to decrease in feed intake.
In addition, Balnave et al., 1999 found that, in chronically heat-stressed broilers, increasing supplemented methylthio butyric acid (HMB) from 0.16 to 0.32% in a diet with an Arg:Lys ratio of 1.36 produced a signiï¬cant increase in body weight gain and feed intake. This result will show even better when substituting DL-methionine for HMB (Chamruspollert et al.,2002). The increase in dietary crude protein levels also may improved feed conversion and the index of productive efficiency in broilers reared at 22 or 32°C. So, in conclusion, the use of diets with high protein levels is a technically viable practice in broiler chickens reared under thermo neutral or high temperature conditions.
2.6 HEAT SHOCK PROTEIN
Heat shock protein (HSP), also known as stress protein is a class of functional protein which its expression will increase when exposed to high temperature or other stress including infection, inflammation and toxins like arsenic and ethanol. These proteins will be released as a protection to body cells during stress and act as molecular chaperones by binding to other cellular proteins, assisting intracellular transport and folding into the proper secondary structures, thus preventing aggregation of proteins during stress. In a heat-shocked cell, the HSP may bind to heat-sensitive proteins and protect them from degradation, or may prevent damaged proteins from immediately precipitating and permanently affecting cell viability (Etches et al., 1995)
There is considerable evidence that the synthesis of HSP 70 is temperature-dependent (Wang and Edens, 1998; Zulkifli et al., 2003), and thus HSP 70 response is considered a cellular thermometer (Craig and Gross, 1991). During exposure to high temperature, peripheral leukocyte heat shock proteins (HSP: HSP90, HSP70, and HSP23) from broiler chickens and turkey poults were induced by in vitro and in vivo ( Wang and Eden.,1998). Earlier work in our laboratory (Liew et al., 2003) showed that greater HSP 70 response is beneficial in enhancing resistance to viral infection in heat-stressed chickens. Under conditions of this experiment, induction of HSP 70 appears to be associated in the physiological stress responsiveness of chickens varying in the magnitude of underlying fearfulness.
The HSP super family includes a number of different molecular-weight-class families: HSP110, HSP90, HSP70, HSP60, HSP47, and a group of small HSP ranging from 16 to 40 kDa (Jaattela and Wissing, 1992; Arrigo and Landry, 1994). Each family contains both proteins that are present prior to heat treatment but their synthesis is enhanced by it (constitutive HSP), and proteins whose synthesis becomes detectable only following stress. In vitro work suggests an apparent discordance between transcription of message and HSP 70 translation (Bruce et al., 1993). There is a possibility that transcriptional activation of the HSP 70 gene is independent of protein synthesis. These phenomenon merits further investigation in poultry. Despite the reported close relationship between HSP 70 and development of thermotolerance, in vitro work showed that thermotolerance can be generated in the absence of intracellular HSP accumulation. Thus, it is problematic, especially at the whole organism level, to definitely link an increase in HSP 70 expression directly to the acquisition of stress tolerance, partly because animals respond to stress in a multitude of complex, integrated ways.
2.7 OXIDATIVE STRESS
Oxidative stress has been defined as an imbalance between oxidants and antioxidants, which in favor of the oxidants, potentially leading to damage to DNA, protein, lipid and also cause cell death. Acute heat stress can cause serious physiological dysfunction that may result in a decline in animal performance. For examples, feed intake, feed conversion ratio, body weight, egg production and hatchability. Hyperthermia has been proposed to be responsible for stimulating reactive oxygen species (ROS) production because of similarities in the expression patterns of genes (including heat shock, oxidative stress proteins, or both) observed following heat stress compared with that following exposure to oxidative stress (Schiaffonati et al., 1990; Salo et al., 1991). In chickens, oxidative stress was observed on exposure to acute heat stress (Mujahid et al., 2005b; Lin et al., 2006).
Reactive oxygen species (ROS) are produced as a consequences of normal aerobic metabolism. Unstable free radical species attack cellular components causing damage to lipids, proteins, and DNA which can initiate a chain of events resulting in the onset of a variety of diseases (Halliwell., 1996). Free radicals are atoms or molecules which have at least one unpaired electron in the outer orbital. In aerobic organisms free radical containing oxygen and/or nitrogen are continuously generated (Halliwell B., Gutteridge JM. 1999). Mitochondria are the main source of oxygen free radicals under normal conditions. Mujahid (2006) stated that increase in mitochondrial ROS production during the heat stress period was substrate-independent. The primary source of ROS is leakage of electrons from the respiratory chain during the reduction of molecular oxygen to water, to generate superoxide anion (Boveris et al., 1972). Superoxide is a reactive molecule, but it can be converted to hydrogen peroxide by superoxide dismutase and then to oxygen and water by catalase or glutathione peroxidase. Although ROS may play an important role in cellular functions such as cell signaling, it is obvious that high levels of ROS also cause cellular damage (Beckman and Ames, 1998), at least at the mitochondrial level (Raha and Robinson, 2000).
Living organisms are provided a rich system of antioxidant defence whose main purpose is to counteract ROS and to reduce their damage. It has been proposed that antioxidant changes reflect an altered redox balance in several pathological states (Halliwell B., Gutteridge JM. 1999). These antioxidant systems include enzymes such as superoxide dismutase, catalase, and glutathione peroxidise. The sum of endogenous and food-derived antioxidants represent the total antioxidant activity of the system. The combination and cooperation between multiple antioxidants may provide a greater protection against ROS attack, thus preventing oxidative stress from occur.