Production measurements

It has been well documented that high ambient temperatures results in decreasing feed consumption, lower BW gain, and high mortality in broiler chickens (Cahaner and Leenstra, Reference CAHANER and LEENSTRA1992; Eberhart and Washburn, Reference EBERHART and WASHBURN1993; Yalçin et al., Reference YALÇIN, SETTAR, ÖZKAN and CAHANER1997; Mushtaq et al., Reference MUSHTAQ, SARWAR, NAWAZ, MIRZA and AHMAD2005; Ahmad and Sarwar, Reference AHMAD and SARWAR2006; Star et al., Reference STAR, KEMP, VAN DEN ANKER and PARMENTIER2008; Quinteiro-Filho et al., Reference QUINTEIRO-FILHO, RIBEIRO, FERRA DE PAULA, PINHEIRO, SAKAI, SÁ, PERREIRA and PALERMO-NETO.2010) and that growth rate of broilers is associated with heat sensitivity (Cahaner and Leenstra, Reference CAHANER and LEENSTRA1992). An increase in the ambient housing temperature from 21.1 to 32.2°C caused a drop in feed intake of about 9.5% per bird per day from the first week to the sixth week of age, and when it rose from 32.2 to 37.8°C, it caused a further drop in feed intake of 9.9% per bird/day, as compared to 21.1°C (North and Bell, Reference NORTH and BELL1990). Washburn et al. (Reference WASHBURN, EL-GEBDY and EBERHART1992) and Yunis and Cahaner (Reference YUNIS and CAHANER1999) concluded that the effect of heat stress was more pronounced in commercial broiler stocks and in broilers with higher growth potential.

Investigators have assessed performance measurements in broilers over several age groups and temperature ranges. High ambient temperatures significantly affected broiler performance, especially between four and six weeks of age (Geraert et al., Reference GERAERT, PADILHA and GUILLAUMIN1996; Garriga et al., Reference GARRIGA, HUNTER, AMAT, PLANAS, MITCHELL and MORETO2006). Yalçin et al. (Reference YALÇIN, ÖZKAN, TÜRKMUT and SIEGEL2001) reported decreases of 23% in body weight (BW) and 15% in food consumption of broilers reared under high temperatures when compared to counterparts reared under optimum temperatures. Quinteiro-Filho et al. (Reference QUINTEIRO-FILHO, RIBEIRO, FERRA DE PAULA, PINHEIRO, SAKAI, SÁ, PERREIRA and PALERMO-NETO.2010) reported that all performance parameters were severely affected in 35- to 42-day-old chickens subjected to daily stressors of 31°C and 36°C. Both temperatures significantly decreased BW as well as food consumption. However, only broilers of the 36°C group presented an increase in feed conversion. There was no mortality in the control (21 ± 1°C) and 31°C groups; however, the high temperature group (36°C) had 43% mortality relative to the control group. Quinteiro-Filho et al. (Reference QUINTEIRO-FILHO, RODRIGUES, RIBEIRO, FERRA DE PAULA, PINHEIRO, SÁ, PERREIRA and PALERMO-NETO2012) reported that all performance variables were altered in chickens submitted to an acute (31 ± 1 ºC) heat stress from day 35 to 42. Acute heat stress at 31ºC decreased BW, feed consumption and feed conversion by 155%, 31.4%, 270% respectively. The control (21 ± 1°C) and the heat stressed group (31°C) experienced 0 and 26% mortality, respectively. However, chickens in the heat stressed group did not experience low water consumption. Azad et al. (Reference AZAD, KIKUSATO, MAEKAWA, SHIRAKAWA and TOYOMIZU2010) demonstrated that broiler body weight gain was reduced due to reductions in both feed consumption and the efficiency of feed utilisation under constant high temperatures.

Zhang et al. (Reference ZHANG, JIA, ZUO, ZHANG, LEI, REN and FENG2012) reported that at a constant high temperature of 34°C and constant high diurnal cyclic temperature (36°C from 1000 h to 1600 h and 23°C from 1600 h to 1000 h) caused decreased BW of 8.1% and 18.2%, respectively, in relation to the control group maintained at a constant temperature at 23°C. Effects of heat stress at older ages may be reduced by exposing broilers to heat stress during the first week post hatch (Arjona et al., Reference ARJONA, DENBOW and WEAVER1988; Yahav and Hurwitz, Reference YAHAV and HURWITZ1996) and by food restriction during heat stress (Francis et al., Reference FRANCIS, MACLOED and ANDERSON1991).

Physiological and biochemical effects

Some studies have shown that heat stress led to cell damage and an increase in cell membrane permeability (Mitchell and Sandercock, Reference MITCHELL and SANDERCOCK1995; Sandercock, Reference SANDERCOCK1997; Sandercock et al., Reference SANDERCOCK, HUNTER, NUTE, MITCHELL and HOCKING2001). With damage to the cell and membrane, intracellular enzymes, such as aspartate aminotransferase and alanine aminotransferase (ALT), were released into the interstitial fluid. As well, serum levels of creatine kinase (CK) and lactate dehydrogenase (LDH) were elevated during heat stress. Other enzymes such as alkaline phosphatase were decreased (Sandercock et al., Reference SANDERCOCK, HUNTER, NUTE, MITCHELL and HOCKING2001). Such elevations were indicative of skeletal muscle damage or myopathy and were a consequence of disruptions in muscle cell membrane (sarcolemma) function and permeability (Ostrowski-Meissner, Reference OSTROWSKI-MEISSNER1981; Mitchell and Sandercock, Reference MITCHELL and SANDERCOCK1995; Sandercock, Reference SANDERCOCK1997). Puvadolpirod and Thaxton (Reference PUVADOLPIROD and THAXTON2000) proposed that, when broilers were exposed to heat stress, blood flow from the gut to skin was greater and liver relative ischemia occurred with increased liver cell permeability, resulting in elevated serum ALT and LDH. Sandercock et al. (Reference SANDERCOCK, HUNTER, NUTE, MITCHELL and HOCKING2001) stated that disturbances in venous blood acid/base status (pvCO₂ and pHv), reflecting thermal panting-induced hypocapnic alkalosis, were proportional to changes in deep-body temperature. They demonstrated that heat stress-induced myopathy and CK loss occur as a result of dysregulation of muscle intracellular Ca²⁺ homeostasis. They inferred that Ca²⁺-mediated changes in muscle membrane permeability were caused by hyperthermia or were the consequences of acid-base disturbance (Sandercock et al., Reference SANDERCOCK, HUNTER, NUTE, MITCHELL and HOCKING2001). However, it has been recognised in humans that a high pH accompanying low pCO₂ in acute respiratory alkalosis decreases blood ionised calcium concentrations (Bowman and Rand, Reference BOWMAN and RAND1980). The decrease in Ca²⁺ increased neuromuscular excitability and the development of tetanic muscle spasms as a consequence of the loss of the stabilising action of Ca²⁺ on muscle cell membranes. These features have been reported in a malignant hyperthermia-like syndrome induced by acute respiratory alkalosis in laboratory rats (Belonje et al., Reference BELONJE, WILSON, BERMAN and HUMAN1995).

The myopathy that result from heat stress is characterised by an increase in the plasma activity of the skeletal muscle derived isoenzyme CK and reflects Ca²⁺-mediated alterations in muscle membrane integrity (Mitchell and Sandercock, Reference MITCHELL and SANDERCOCK1995). Similar increases in plasma CK activity have been reported with hyperthermia-associated stress conditions in pigs and shown to be associated with detrimental effects on meat quality (Sosnicki, Reference SOSNICKI1987). Acute heat stress may cause hyperthermal muscle illness in broilers, increase the activity of muscle CK, change cell membrane integrity in breast muscle glycolytic metabolism and increase osmotic activity in the membrane (Mitchell and Sandercock, Reference MITCHELL and SANDERCOCK1995; Sandercock et al., Reference SANDERCOCK, HUNTER, NUTE, MITCHELL and HOCKING2001; Bao et al., Reference BAO, GONG, HARTUNG, FU, WANG, ZHANG and WANG2004). During heat stress, toxicity from glycolysis and protein metabolism in muscle tissue result in muscle fibre degeneration, necrosis and increased creatine phosphokinase (CPK) release into the blood. The CPK elevation has, therefore, been used as an indicator of heat stress. At the same time, as muscle cells release LDH, its activity will be significantly higher than those in non-heat stress status. This leads to glycolysis causing increases in the quantity of lactic acid and decreasing the activity of serum alkaline phosphatase. It is possible that changes in muscle membrane integrity are involved in the mechanism of post mortem intracellular water loss, as elevations in plasma CK is associated with higher rates of pH decline and increased drip loss.

Many reports have focused on the pathological damage to organ structures and viscera. The study of the heat shock (HS) response at cellular and molecular levels has provided clues for a better understanding of the mechanisms involved in development of thermotolerance in cultured cells. The phenomenon of HS response has been defined as the expression of a small group of highly conserved polypeptides, known as heat shock proteins (HSP), by prokaryotic and eukaryotic cells in response to high ambient temperatures or other environmental stressors (Ashburner and Bonner, Reference ASHBURNER and BONNER1979; Craig, Reference CRAIG1986; Lindquist, Reference LINDQUIST1986; Lindquist and Craig, Reference LINDQUIST and CRAIG1988). HSP is under the influence of adverse factors producing a special set of proteins to maintain cell survival and the stability of the internal environment. HSP70 is the most important group of HSPs in the formation of the thermal resistance of HSP70, and is used as an index for monitoring heat stress.

The HS response appears to be universal and may be detected in organisms as diverse as bacteria and humans (Lindquist, Reference LINDQUIST1986). Evidence of HSP involvement in the development of thermotolerance was provided in early experiments (Li and Laszlo, Reference LI and LASZLO1985; Lindquist, Reference LINDQUIST1986; Hahn and Li, Reference HAHN and LI1990), and the induction of HSP, especially HSP70, has been correlated with the development of thermotolerance in many cell types (Li and Laszlo, Reference LI and LASZLO1985; Li and Mak, Reference LI and MAK1989). HSPs play a vital role in cellular homeostasis during development of thermotolerance (Craig, Reference CRAIG1986; Li and Laszlo, Reference LI and LASZLO1985; Lindquist, Reference LINDQUIST1986; Laszlo, Reference LASZLO1988; Lindquist and Craig, Reference LINDQUIST and CRAIG1988; Hahn and Li, Reference HAHN and LI1990). Cells with acquired thermotolerance to sub-lethal HS were more resistant to subsequent exposures to lethal thermal stressors (Li and Laszlo, Reference LI and LASZLO1985; Lindquist, Reference LINDQUIST1986; Hahn and Li, Reference HAHN and LI1990). Wang and Edens (Reference WANG and EDENS1998) reported that enhancement in HSP expression was evident for periods up to four weeks after termination of the daily heat conditioning episodes, and Sun et al. (Reference SUN, LI, WANG and BAO2005a; Reference SUN, LIU, ZHAO, BAO and WANG2005b) reported changes on constitutive or cognate HSP70 mRNA transcription level and HSP expression in heart muscle.

Effects of heat exposure on muscle oxidation

Heat stress leads to oxidative stress, reflected by an increased production of reactive oxygen molecules and decreased concentrations of serum vitamins and minerals that play a role in the defence system (Halliwell and Gutteridge, Reference HALLIWELL and GUTTERIDGE1989; Sahin and Kucuk, Reference SAHIN and KUCUK2003; Sahin et al., Reference SAHIN, SAHIN and KUCUK2009). Because oxidation is a main factor affecting muscle protein functionality during processing and storage (Ooizumi and Xiong, Reference OOIZUMI and XIONG2004), it is of interest to examine how heat stress-induced protein oxidation alters chicken protein functional properties. Oxidative stress impairs cell membrane and mitochondrial integrity and causes cell damage through lipid peroxidation (Halliwell and Gutteridge, Reference HALLIWELL and GUTTERIDGE1989; Meng et al., Reference MENG, VELALAR and RUAN2008). Lin et al. (Reference LIN, JIAO, BUYSE and DECUYPERE2006) reported that oxidative stress can be induced in five-week-old broilers by acute heat exposure (32°C, 6 h). Their results suggested that elevated body temperature induced the metabolic changes that were involved in the induction of oxidative stress. The liver was more susceptible to oxidative stress than the heart during acute heat exposure in broilers. Thus, oxidative stress should be considered as part of the stress response of broilers to heat exposure.

Malondialdehyde (MDA) and protein carbonyls are compounds derived from lipid and protein oxidation. The amount of these compounds corresponded to the extent of lipid and protein oxidation (Srinivasan et al., Reference SRINIVASAN, XIONG and DECKER1996). Briefly, acute heat stress (34°C for 6 h) stimulates metabolic oxidation capacity in broiler skeletal muscle by increasing the activity or expression of the enzymes associated with β-oxidation, the TCA cycle and fatty acid transport (Mujahid et al., Reference MUJAHID, AKIBA, WARDEN and TOYOMIZU2007a). Broilers exposed to acute heat stress exhibited more than two-fold increases of MDA as a marker for lipid peroxidation in the skeletal muscle (Mujahid et al., Reference MUJAHID, AKIBA and TOYOMIZU2009; Wang et al., Reference WANG, PAN and PENG2009). It has been suggested that elevated MDA levels in the skeletal muscle of ‘acute’ heat-stressed birds were caused by increased substrate oxidation and membrane potential of mitochondria that is associated with a decrease in uncoupling protein (Mujahid et al., Reference MUJAHID, AKIBA and TOYOMIZU2009).

It is well known that oxidative stress can stimulate the production of reactive oxygen species (ROS), induce lipid peroxidation and cause protein and DNA damages (Dröge, Reference DRÖGE2002). It is very likely that elevated body temperature would enhance the generation of ROS via accelerated metabolic reactions in cells and tissues. Mitochondria have long been known to play a critical role in maintaining bio-energetic status under physiological conditions. Mujahid et al. (Reference MUJAHID, UEDA, AKIBA and TOYOMIZU2005; Reference MUJAHID, AKIBA and TOYOMIZU2007b) reported that acute heat stress increased production of mitochondrial ROS in the skeletal muscle of chickens. This increased ROS production may lead to nonspecific modification of lipids and proteins, which results in bio-energetic dysfunctions. On the other hand, acute heat stress resulted in increasing levels of ROS in mitochondria isolated from the skeletal muscle of broilers (Mujahid et al., Reference MUJAHID, SATO, AKIBA and TOYOMIZU2006).

Altan et al. (Reference ALTAN, PABUÇCUOĞLU, ALTAN, KONALIOĞLU and BAYRAKTAR2003) reported that the exposure of broiler chickens at 38 ± 1°C for 3h heightened the extent of fat oxidation in blood and enhance the activity of catalase, superoxide dismutase and glutathione reductase, showing that the antioxidant enzyme activity in the liver was affected by heat stress. Reports with regards to the effects of heat stress on oxidative damage of muscle proteins and protein functionalities were rare until Feng et al. (Reference FENG, ZHANG, ZHENG, XIE and LI2006) studied the effects of daily cycling high temperature on breast muscle mitochondria and discovered that it induced oxidative stress of the breast muscle, pectoralis major. Wang et al. (Reference WANG, PAN and PENG2009) reported that heat exposure increased the oxidation of sarcoplasmic and myofibrillar proteins from pectoralis major of broiler chickens to different extents, whereby the muscle with heat exposure for three hours had the highest lipid and protein oxidative value. According to previous observations, pre-slaughter heat exposure could probably hasten the oxidation process of muscle, possibly by the increased ROS attacking unsaturated lipid in muscle, and then producing elevated MDA. As a result, the advanced oxidative damage of muscle proteins would result in production of protein carbonyls in high quantities.

Meat quality effects of heat stress in birds

Reports with regards to the effects of heat stress on oxidative damage in muscle proteins and their functionalities are rare. Carcass and meat quality traits, such as tenderness and colour, are critical for consumer acceptance. Appearance is critical for consumers’ initial selection of poultry meat as well as for final product satisfaction. Because appearance is so critical for consumers’ selection, poultry producers strive to produce products with the appropriate colour for a particular market and to avoid appearance defects, which will negatively affect product selection or price (Fletcher, Reference FLETCHER2002). Rearing systems and stress factors, such as heat stress and duration of transport, have also been related to changes in meat quality traits in broilers. Feng et al. (Reference FENG, ZHANG, ZHENG, XIE and LI2006) investigated the effects of daily circular high temperature on meat quality effects of heat stress in broilers, revealing that cyclic high temperature induced lower pH, denatured muscle protein and increased drip loss, L* value and shear force.

Investigators reported that meat quality was influenced largely by the ultimate pH (Sales and Mellett, Reference SALES and MELLETT1996; Young et al., Reference YOUNG, WEST, HART and VAN OTTERDIJK2004). As one of the most important physical parameters for the qualitative profile of meat, pH is widely used as a predictor of meat technological and sensory qualities (El Rammouz et al., Reference EL RAMMOUZ, BERRI, LE BIHAN-DUVAL, BABILE and FERNANDEZ2004). Sandercock et al. (Reference SANDERCOCK, HUNTER, NUTE, MITCHELL and HOCKING2001) and Wang et al. (Reference WANG, PAN and PENG2009) reported a significant decrease in pH_30min and pH_24h of pectoralis major from one to five hours of heat exposure, compared with the control group. They theorised that the decrease of pH value during aging was mainly due to lactic acid accumulation in glycolysis and H⁺ from ATP hydrolysation. It was suggested that heat exposure probably increased the release of hormones, accelerated the decomposition of glycogen and increased the rate of muscle glycolysis, leading to the pH decline observed. Wang et al. (Reference WANG, PAN and PENG2009) reported that the accelerated post-mortem pH decrease in heat-stressed broilers could be attributed to the initial high temperature (T_30min) of breast muscle, a condition that could facilitate ATP hydrolysis and glycolysis of muscle hepatin (McKee and Sams, Reference MCKEE and SAMS1997), thereby inducing a low pH.

Water-holding capacity is one of the important sensory qualities of meat. Wang et al. (Reference WANG, PAN and PENG2009) noted that heat exposure increased the drip and cooking losses to various extents, which suggested the water-holding capacity of proteins was susceptible to oxidative damage. Other researchers reported that oxidation could impair water-binding in meat (Huff-Lonergan and Lonergan, Reference HUFF-LONERGAN and LONERGAN2005; Morzel et al., Reference MORZEL, GATELLIER, SAYD, RENERRE and LAVILLE2006). Increased lipid oxidation may change membrane structures and functions, and membrane osmosis, which caused a loss of water. Investigators (Richardson and Jones, Reference RICHARDSON and JONES1987; Daum-Thunberg et al., Reference DAUM-THUNBERG, FOEGEDING and BALL1992; Xiong et al., Reference XIONG, PESCATORE, CANTOR, BLANCHARD and STRAW1993) showed reduced cooking losses, as a result of increasing pH for turkeys and chickens. Because muscle proteins carried more net negative charges as the pH was raised, an enhanced protein-water interaction and therefore a decreased moisture loss can be expected (Xiong et al., Reference XIONG, PESCATORE, CANTOR, BLANCHARD and STRAW1993). Joo et al. (Reference JOO, KAUFFMAN, KIM and PARK1999) reported that the correlation coefficients between sarcoplasmic protein solubility, myofibrillar protein, total protein solubility and drip loss were -0.72, -0.53, and -0.64, respectively.

In theory, changes in muscle tissue caused by heat could affect the meat quality in broiler muscles post-mortem. In swine and cows, PSE is a condition that occurs most often during hot summer months and when there is a continuous increase in muscle metabolism (Froning et al., Reference FRONING, BABJI and MATHER1978; Northcutt et al., Reference NORTHCUTT, FOEGEDING and EDENS1994; Backstrom and Kauffman, Reference BACKSTROM and KAUFFMAN1995; McCurdy et al., Reference MCCURDY, BARBUT and QUINTON1996; D'Souza et al., Reference D'SOUZA, DUNSHEA, WARNER and LEURY1998). Skeletal muscle in the breast and legs (iliotibialis) of broilers often contain damaged fibres that are white with a cooked appearance (Kjolberg et al., Reference KJOLBERG, MANNERS and LAWRIE1963; Thompson et al., Reference THOMPSON, JANKY and WOODWARD1987), and the resulting meat is less juicy and tender (Fox et al., Reference FOX, WOLFFRAM, KEMP and LANGLOIS1980; McKee and Sams, Reference MCKEE and SAMS1997). Northcutt et al. (Reference NORTHCUTT, FOEGEDING and EDENS1994) reported that broilers raised under acute or short duration hot conditions produced PSE-like post-mortem muscle. McKee and Sams (Reference MCKEE and SAMS1997) indicated that seasonal heat stress accelerated ATP depletion and increased the percentage of bird muscles that would be classified as PSE.