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Heat stress has a significant effect on the behaviour, physiology and performance of poultry (Chaiyabutr, 2004) as will be considered in detail in the following sections.

Effect of heat stress on core and body temperatures

The diurnal pattern (i.e. variations during a 24 h period) of CBT in broilers has been related to photoperiod (Lacey et al. 2000) because CBT is always at its maximum in the afternoon and minimum at night, probably due to increased feeding and locomotor activity of the birds during the day time. Core body temperature is considered to be a good indicator of welfare during heat stress as (previously discussed in Section 2.3.2.1) and the magnitude of its increase is highly dependent on the level of heat stress (Toyomizu et al., 2005). However, the increase in CBT when birds are exposed to high temperature can be aggravated by high RH. For instance, CBT in broilers aged 6 weeks was greater by 0.4^oC when kept at 28^oC, 80% RH compared to those kept at 28^oC, 50%

RH (Lacey et al., 2000).

Age of the bird has a great role to play in thermoregulation of broilers (Lin et al., 2005).

Using older birds, rectal temperature of broiler chickens at day 63 (43.8^oC) was greater than those at 35 days (43.1^oC) when these two group of birds were exposed to the same level of acute heat stress (32^oC and 75% RH for 2 h) in a study by Sandercock et al.

(2001). So the bigger the bird, the more difficult it becomes to dissipate heat because of the higher heat production and lower heat loss, which in turn result in a greater increase in CBT.

During heat stress, there is an increased blood flow to the skin, comb, wattles and shanks due to peripheral vasodilation to aid heat dissipation to the environment (Widowski, 2010). Skin temperature is therefore dependent on both environmental temperature and heat loss from the core. A large volume of warm blood is transferred to the body surface through a vascular arrangement called arteriovenous anastomoses present in the legs (Dawson and Whittow, 2000).

A comprehensive study on the redistribution of blood flow in the body during heat stress in laying hens undertaken by Wolfenson et al. (1981) who showed that a 1^oC increase in CBT, which was considered as moderate heat stress, was associated with an increase in blood flow to organs active in heat dissipation such as the back (200%),

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breast skin (679%), comb (427%), wattle (195%), tongue (429%), larynx (323%) and trachea (338%) compared to the control birds in which CBT remained relatively constant. The tongue, larynx and trachea are part of the upper respiratory tract and this increased blood flow to these areas emphasises the importance of heat loss from the respiratory tract. The results of Wolfenson et al. (1981) could be extended to broiler chickens, although broilers are more prone to heat stress than egg-laying strains (Sandercock et al., 2006) because of their fast growth rate which is accompanied by a higher metabolic rate (Gous and Morris, 2005). The increased heat flow to the back may not be readily dissipated to the environment because of the increased feather covering;

however the less feathered parts of the body such as the comb, wattles or breast are more efficient for heat loss. Body surfaces which are less feathered and allow for heat dissipation are referred to as ‘thermal windows’ e.g. comb, legs, under the wing (apteria) and around the cloaca (Gerken et al., 2006), Figure 2.8. In this diagram the hottest parts of the body are those in red colour (comb, eyes, ear, wattle and leg/feet) while those in yellow colour are not as hot as those parts in red colour (wing, beak, nose).

Figure 2.8: Thermograph image of the variation in surface body temperature of a 42 day old Cobb broiler chicken at an ambient temperature of 25.4-31.9^oC (Adapted from Nääs et al., 2010)

Effect of heat stress on Physiological parameters

Blood is a liquid tissue made up of mainly of plasma (pale yellow in colour) and blood cells which flow through the body to perform several functions namely transportation (O2, CO2, food material, waste products, hormones and metabolites), regulation of body temperature, maintenance of acid-base balance and protection against disease (Rastogi,

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2007). The circulatory system is very important for effective control of body temperature especially in altering O₂ supply and heat dissipation (Yahav et al., 1997).

Heat stress could cause both quantitative and morphological changes in blood parameters (Lin et al., 2004; Borges et al., 2007) and this might in turn suppress the aforementioned functions of the blood system.

Some of the quantitative changes caused by high environmental temperature are an increase in plasma and blood volume, which consequently causes a decrease in whole blood viscosity (Zhou et al., 1999). An increase in plasma volume indicates increased flow of water molecules from the interstitial space into the blood needed for the transfer of heat from the core to the periphery Zhou et al. (1999), especially during acute heat stress. On the other hand, chronic heat stress is associated with dehydration due to increased moisture loss from the respiratory tract associated with increased evaporative cooling and increased water loss through the urine. Heat stress (35^oC, 50% RH) caused an increase in the number of heterophils and a decrease in the number of lymphocytes in blood, causing an increase in the H/L ratio in broilers after 4 weeks of exposure compared to control birds kept at 23.9^oC, 50% RH (Mashaly et al., 2004), implying reduced capacity of the immune function and hence following heat stress birds could be more susceptible to a disease challenge.

Blood glucose levels are increased under heat stress due to an increase in secretion of adrenalin, noradrenalin and glucocorticoids (Borges et al., 2007) to increase energy available to survive the stressful condition (Ognik and Sembratowicz, 2012). Apart from heat stress, simulation of stress through exogenous administration of corticosterone has also been shown to increase blood glucose levels (Puvaldopirod and Thaxton, 2000; Post et al., 2003 and Olanrewaju et al., 2006).

In response to heat stress birds may also increase respiratory rate, and a consequence of this is greater exchange in the lungs for CO_2, resulting in a decrease in pCO₂ which in turn leads to an increase in bicarbonate (HCO₃^-) excretion from the kidney and finally a rise in pH (Borges et al., 2007; Hockings et al., 1994). The change in acid-base balance is proportional to the change in core body temperature, and is especially greater in older birds due to their larger body size and concomitant greater susceptibility to heat stress (Sandercock et al., 2001). During heat stress proteins called heat shock proteins are produced which protect the body from damage (Tutar and Tutar, 2010).

37 Effect of heat stress on growth performance

Heat stress results in substantial economic loss to the poultry sector due to reduced productivity and increased mortality (Toyomizu et al., 2005). In a comprehensive study undertaken to estimate the economic losses due to heat stress from cattle, pigs, broilers and layers in the USA, the total loss across all three species was estimated to be $2.4 billion per annum (St-Pierre et al., 2003), equivalent to £1.5 million. This challenge of heat stress has resulted in poultry production in the tropics being inferior compared to that in temperate areas (Ojano-Dirain and Waldroup, 2002).

Regulation of feed intake in response to environmental conditions is referred to as thermostatic theory (Ferket and Gernat, 2006) such that environmental temperature above the UCT will lead to decreased feed intake of birds in order to reduce metabolic heat load. Heat stress can adversely affect voluntary feed intake, body weight gain, carcass characteristics and mortality (Mashaly et al., 2004 and Khan et al., 2012). In fact, a negative correlation (P<0.05) exists between body temperature and weight gain (R²= -0.4), feed intake (R²= -0.31) and feed conversion ratio (R²= 0.24) in birds exposed to heat stress (32^oC) according to Cooper and Washburn (1998). The reduction in feed intake during heat stress could be related to decreased blood flow to the digestive system (Wolfenson et al., 1981), thus heat production associated with digestion, absorption and utilization of nutrients is suppressed (Syafwan et al., 2011).

Hai et al. (2000) reported that during heat stress (32^oC) the amount of chyme (partially digested food passing from the stomach to the duodenum for extraction of nutrients) in the digestive tract of broilers aged 49 days increased (P<0.05), during heat stress conditions but the activities of the intestinal juice (trypsin, chymotrypsin and amylase) were suppressed (P<0.05) compared to control birds kept in 20^oC, 60% RH. The suppression of the intestinal juice could be related to the effect of heat on activity of digestive enzymes. Moreover, the degree of reduction in feed intake during heat stress is dependent on the severity and length of the heat stress (Mashaly et al., 2004). Heat stress also impairs meat quality of broilers by decreasing the pH of meat, increase plasma creatinine kinase levels which indicates a suppression of the integrity of the skeletal muscles (Sandercock et al., 2001).

A considerable amount of literature has been published on the effect of heat stress in broilers (Soleimani et al., 2011; Barbour et al., 2010; Quinteiro-Filho et al., 2010;

Sayed and Downing, 2011). These studies have used different temperatures (30-38^oC) for different period of time (1.5 h to 24 h) to simulate heat stress conditions. Details

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about the different levels of temperature, duration and methodology used and their effects on growth performance, physiology and welfare are summarised in Table 2.2.

In a comparative study of chronic heat stress, broilers aged 5 weeks were exposed to either constant high temperature (35^oC) or diurnal changes in temperature (15:35^oC for 12:12 h) or control moderate temperature (15^oC) for 3 weeks (Yahav et al., 1997).

Although rectal and skin temperatures, blood pH, pCO₂ and haematocrit levels were similar in these two treatments, they were greater than those of control birds kept at 15^oC. The levels of haemoglobin and plasma volume were greater in birds exposed to diurnal heat stress than those exposed to constant heat stress (Yahav et al., 1997), indicative of acclimatisation. Increased plasma volume during heat stress suggests that the birds were not dehydrated. These haematological changes were absent in birds subjected to acute heat stress (35^oC for 6 h). Yahav et al. (1997) deduced that different types of heat stress have different effects on the broilers and this can account for differences in their ability to thermo regulate.

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Table 2.2: Effects of various levels of temperature and relative humidity to simulate ‘heat stress’ on the growth performance, physiology and welfare of broiler chickens

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Table 2.2 continue: Effects of various levels of temperature and relative humidity to simulate ‘heat stress’ on the growth performance, physiology and welfare of broiler chickens

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2.3.2.9 Genetic selection of broilers and consequences for their ability to cope with