Heat Stress and Dietary Electrolyte Balance in Poultry

By F. Mushtaq
November 13, 2022

Heat stress occurs when birds have difficulty in balancing body heat loss and body heat production. Heat stress interferes with broiler comfort and suppresses productive efficiency. When the environmental temperature exceeds birds comfort zone (above 25°C), the birds are likely to experience heat stress. Feed intake changes about 1.72 percent for every one-degree centigrade rise in ambient temperature between 18-32ºC. However the decline is much faster when the temperature rises to 32-38ºC. Squibb et al. (1959) suggested that the decline in growth rate result directly from reduced feed intake. Smith et al. (1983) demonstrated that growth rates of heat stressed birds can be increased by force feeding at levels exceeding ad libitum feed intake but this will result in heat production within the body and will accentuate the problem. As ambient temperature rises above 31ºC the birds tries to maintain their body temperature by dissipating heat through panting (specialized form of respiration, which help to dissipates heat by evaporative cooling), in addition to heat loss through non-evaporative cooling methods. During heat stress birds water intake increases to try to offset water loss but the situation is complicated by the fact that body’s ability to retain water is reduced as the evaporative cooling process escalates. Longer period with maximum temperature of 37-38ºC caused higher mortality than short breaks of 40-41ºC.

Excessive loss of carbon dioxide (CO₂) during panting reduces the partial pressure of CO₂ in the blood plasma. In turn, the bicarbonate buffer system, principle buffer of blood and extra cellular fluid, lowered the concentration of hydrogen ion, causes a rise in plasma pH and plasma bicarbonate levels, called as respiratory alkalosis (Linsley and Burger, 1964; Calder and Schmidt-Neilsen, 1966 & 1967). The birds attempt to correct blood pH by excreting bicarbonate via the urine. Bicarbonates are negatively charged ions that must be coupled with positively charged ions such as K⁺ or Na⁺ to be excreted in the urine. Thus K⁺, Na⁺and HCO^–₃becomes deficit in the body. Belay et al. (1990) reported that respiratory alkalosis induced by heat-distress has been related to negative mineral balance for K⁺ as well as Na⁺.

Efforts to protect birds from high ambient temperatures by cooling of poultry houses below 25ºC during summer season has been attempted but its economical feasibility has been questioned. Further the provision of fans, air coolers etc., does not provide effective cooling of the sheds and inside temperature remains well beyond the desired range. Environmentally controlled poultry houses are yet uncommon, and these might increase the cost of production making poultry uneconomical. This situation therefore, demands that some means to counteract the effect of heat stress in birds without involving high cost must be found out.

In view of the economic effects of reduced growth rates resulting from chronic heat exposure and death losses, there is interest in alleviating the effect of high temperature through chemical manipulation of the blood acid base balance. Without the key electrolytes, lost during heat stress viz., sodium, potassium and bicarbonate, it is impossible to balance the internal environment of the body. As the blood value for CO₂ and HCO₃ concentration also reduced during heat stress, supplementing birds with CO₂ or HCO₃ may be advantageous. Numerous studies have been conducted to check the responses of birds to sodium bicarbonate (NaHCO₃) and potassium bicarbonate (KHCO₃) supplementation of drinking water (Teeter et al., 1985; Branton et al., 1986 ). However sodium bicarbonate has also been used in poultry diets therefore the effect of this compound on acid-base balance during heat stress is ascertained. Dietary supplementation of NaHCO₃ gave significantly better feed conversion and numerically better growth in broilers at 31°C than supplementation of KHCO₃providing similar concentration of bicarbonate (Hayat et al., 1999). This is due to the role of sodium in the regulation of water balance in extracellular fluid volume (Alcantara et al., 1980). Teeter et al., (1985) reported the use of ammonium chloride (NH₄Cl) as a potential blood acidifier and the use of NaHCO₃ as a source of HCO₃ in alleviating the effect of chronic heat exposure in broiler chickens.

The followings are some paragraphs related to previous work on the issue.

Temperature is the most important environmental factor affecting poultry production. This has led researchers to develop a quantitative relationship between production and temperature, in order to arrive at a definitive conclusion on the optimal temperature at which production could be maximized under various farm conditions. Whittow (1966) stated that birds, like mammals, are homeotherms which means that they maintain a relatively constant deep body temperature. Birds are also endotherms, a term indicating that they are able to increase their body temperature by generating a considerable amount of heat within their tissues instead of relying on heat gain directly from their surroundings.

Moskovits and Marder (1975) study the physiological responses of acclimatized White Leghorn x Bedouin fowl exposed to high ambient temperature (40-45ºC). The normal value for body temperature at 25ºC ambient temperature was 40.5ºC, which increased by 2.1ºC after 8-10 hours of exposure at 37-45ºC. Whittow (1965) found that when the environmental temperature is equal to the body temperature of birds, heat can not be lost from the body by non-evaporative means, it can be lost however by the evaporation of moisture from the respiratory tract. King and Farner (1961) reported that the deep body temperature at which panting is initiated varies from 41-43.5ºC in different species of birds. The increase in heat production at the upper critical temperature is possibly related to the increased muscular activity associated with panting, but also to the direct accelerator action of the increase in body temperature on the heat-production chemical reaction in the body (Whittow, 1965). When the environmental temperature are higher than the thermoneutral zone (21-30ºC) birds increase panting by up to 10 times from a normal rate of 25 breaths per min (Nilipour, 2000). Thus panting increases the loss of CO₂ from the lungs, which leads to a reduction in the partial of CO₂ in the blood plasma. In turn, the bicarbonate buffer system, the principal buffer of blood and extracellular fluid (ECF), lowered the concentration of hydrogen ions causing a rise in plasma pH and plasma bicarbonate level, a condition referred to as respiratory alkalosis. Teeter et al (1985) examined the occurrence of respiratory alkalosis and the potential benefit derived from treatment in thermo-stressed, 4-week-old broiler chicks. Blood pH was greater (p>0.05) in heat-stressed (32°C) panting birds (7.395) than either non-panting (7.28) or birds raised at 24°C. Acute heat stress, obtained by elevating ambient temperature from 32°C to 41°C covers a 20 min period and further elevated (p<0.05) blood pH to 7.521. Vo and Boon (1975) reared male and female broilers from 2 to 8 weeks at constant temperatures of 21.1, 29.4, and 37.8C. After 8 weeks of age average weights of birds were 1872, 1692, and 741 gm at the three temperatures respectively. They also noted depression in feed consumption from 852 gm at 21.1°C to 270 gm at 37.8°C.

Wilson et al. (1972) study the deaths of birds with the rise in ambient temperature. They observed that exposure of birds to an abrupt rise in ambient temperature from 10 or 23ºC to 36ºC resulted in some mortality. The rise of body temperatures of chickens and turkeys exposed to high environmental temperatures (32.2 to 37.8ºC) has been reported (Yeates et al., 1941; Parker et al., 1970 & 1982 Smith and Oliver, 1972).

Damron and Harms (1981) found that birds fed on a diet containing sodium bicarbonate as a sodium source, with levels ranging from 0.12 percent achieved a body weight of 889 g at 4 weeks of age as against 861 g of the birds fed a diet containing sodium chloride as sodium source. Supplementation of 0.5 percent sodium bicarbonate (NaHCO₃) in the diet of birds subjected to chronic heat stress enhanced body weight gain by 9 percent. Supplementation of 0.3 percent or 1 percent ammonium chloride (NH₄Cl) to diets decreased blood pH (p<0.01) to 7.194 and increased (p<0.05) body weight gains by 9.5 percent and 25 percent, respectively. Supplementing the 1 percent NH₄Cl diet with 0.5 percent NaHCO₃ increased weight gain by an additional 9 percent. Data indicate that blood alkalosis limits growth rates of broiler chicks reared under chronic thermostress and that the respiratory alkalosis and weight gain depression attributed to thermostress can be partially alleviated dietarily.

Branton et al (1986) supplemented NH₄Cl and NaHCo₃ to the drinking water of 42 to 52 days old broilers in acute heat exposure (40.6ºC). Water intake increased by approximately 20 % in birds given water containing 6.25 gm of NaHCO₃/liter, while both water and feed intake were severely limited by NH₄Cl at 31 gm/liter. The blood pH of birds was substantially lowered by the consumption of NH₄Cl while the consumption of NaHCO₃ did not significantly affect blood pH. A correlation coefficient (r -0.3) existed between blood pH and mortality, while a high negative correlation (r₌-0.72) existed between water consumption and mortality.

Bonsembiante et al. (1988) in a 56 days trial provided a basal diet with no additives (controls) or containing 0.05 percent sodium bicarbonate with or without ammonium chloride to day-old broiler chicks. The temperature was maintained between 26 and 31°C and relative humidity was between 75 and 100 percent. Supplementation of sodium bicarbonate in broiler ration increased weight gain and improved feed efficiency as compared to the control group. However, the performance of chicks gives 0.5 percent sodium bicarbonate plus 1 percent ammonium chloride was not significantly better than that of controls.

Borges (2003) used cobb male broiler chicks on a new litter to evaluate the effect of dietary electrolyte balance (DEB) under tropical summer conditions (23-31ºC). Corn soybean meal-based mash diet having NaCl alone or in combination with NaHCo₃and NH₄Cl was given. There was no significant (p<0.05) effect of treatments on mortality or processing parameters. Water intake increased linearly with increasing DEB (0 to 340 mEq/Kg). Blood pH and HCO₃ increased with the highest DEB (360 mEq/Kg) causing respiratory alkalosis. The DEB of 240mEq/Kg gave the best weight gain and feed conversion ratio