Energy Metabolism, Blood Chemistries, and Thermobalance of Large White Male Turkeys Exposed to Temperature Distress

The performance of turkeys is affected by nutrition and ambient temperature. Ambient temperature affects perforrn8IDce primarily by reducing feed consumption which subsequently reduces body weight gain (NRC, 1981). A deeper understanding of the interactions between environmental temperature and feed intake is needed so that turkey rations can be formulated to provide optimal nutrient combinations during different seasons and at various geographic allocations. The reduction in feed intake, occurring as a result of ambient temperatures above the then noneutral zone., is thought to be the factor primarily responsible for reduced growth rate and egg production. With laying hens, Polin (1983) observed a 30% decrease in feed intake at ambient temperature of32 C. Rose and Michie (1987) reported that for each 1 C increase in temperature, feed intake of BUT (British United Turkeys) female turkeys decreased by 1.2% and feed conversion ratio fell by 0.6%. Mitchell and Kosin (1954) and Thomason et al. (1972) reported that moderate increases in ambient temperature depress turkey egg production and egg size. Turkeys reared at high temperatures have reduced body weight gains (De Albuquerque et aI., 1918) and reduced breast yield (Bray, 1985). Nutrient requirements are not directly influenced by environmental temperature according to one review (NRC, 1981). Instead, the decreased feed and nutrient intake could account fully for the adverse responses of poultry undergoing mild heat distress. The reduction in feed intake during heat distress presumably is due to a shift in energy metabolism and consequent physiological chances (Sturkie, 1976). Acutely heat stressed poultry must dissipate the body heat they generate to maintain. a normal body temperature. As the ambient temperature increases, heat dissipation becomes more difficult because nonevaporative cooling, the heat loss due to convection, conduction and radiation, declines as ambient temperature increases. Nonevaporative cooling is the most efficient method for poultry to dissipate heat. Because nonevaporative cooling declines as ambient temperature increases, poultry are forced to depend more on evaporative heat dissipation to remove body heat arising from its maintenance and/or production energy. This is accomplished by panting or by increasing the respiration rate. As ambient temperature increases and exceeds 39 C, dissipating heat to the environment becomes more difficult which may force body temperature to increase. The rate of heat dissipation depends on the difference in temperature between the body surface and the bird's surroundings. As body temperature increases due to heat distress, blood gases become altered. Carbon dioxide is lost as the bird attempts to remove more heat via moisture in its breath. At a critical point, panting (increased respiratory frequency and minute volume but decreased respiratory amplitude and tidal volume) begins. Hyperthermic panting precipitates respiratory alkalosis (Richards, 1970). Chronic heat distressed broilers suffer from intermittent respiratory alkalosis during panting; with acute heat distress, broilers pant continuously and suffer from alkalosis (Teeter et al, 1985). The blood becomes more alkaline (Kohne and Jones, 1975). If the bird is a layer, shell quality declines because the deposition of calcium onto the shell requires a blood pH of 7.4 for proper cation and anion balance. The reduction in partial pressure of carbon dioxide associated with respiratory alkalosis alters electrolyte movement across cell membranes (Fenn and Asano, 1956; Brown and Goot, 1963; Lade and Brown, 1963). During alkalosis, concentrations of cations and anions are shifted in the blood (Harrison and Biellier, 1969; Kohne and Jones, 1975) with excessive loss of potassium through the kidneys (Huston,1978). Finally, extreme heat distress causes death from failure of kidneys and(or) the respiratory system. Although the specific routes of heat loss (nonevaporative and evaporative cooling) are wen defined qualitatively, little research has been conducted with male and female turkeys to quantitatively estimate the irrelative importance. The capacity of turkeys of various ages to lose heat at elevated ambient temperature needs to be quantified (Emmans, 1989). More thorough understanding ofheat dissipation routes and energy metabolism may enhance profitability of turkey production. According to Brody (1945), basal heat production is convenient as a baseline for measuring the heat increments of muscular work, feeding, feed metabolism, lactation, gestation and keeping warm in cold weather. Basal heat production is defined as the heat produced during complete rest in the post-absorptive condition. Basal heat production per unit weight in homeotherms decreases as body weight increases. Consequently, body weight alone is not suitable as a reference base for metabolism. According to the laws of Newton and Stefan - Boltzmann, the rate of cooling of a body is proportional to its surface area. For a cube, Sanus and Rameaux (1837) calculated that surface area, which for a sphere equals 2/3 power of weight, could be used as a reference base for heat production. In 1932, Kleiber reported that the 3/4 power of weight was useful as a reference base across species for adult animals. At about the same time, Brody published results reporting that the 0.73 power of weight was an ideal cross-species reference base. The Conference on Energy Metabolism in 1935 tentatively adopted the 0.73 power of body weight as a reference base for energy metabolism. Smaller, growing turkeys usually have higher heat production than larger turkeys, both per unit weight and per unit of metabolic body size (Wt.75) (Buffington et al, 1974; Afifi, 1975; Nichelmann et at, 1976l Macleod et al., 1980). Macleod et at, (1985) stated that Wt.75 is unlikely to be the most suitable scaling factor for turkeys at different stages of growth even though it seems .adequate over the narrow weight range in domestic fowl. The objectives of our studies were to measure metabolic responses by Large White turkey mates exposed to thermoneutral and acute and heat distress environments. Measurements included: blood chemistries (glucose, triglycerides, albumin, lactate dehydrogenase, aspartate amino transferase, uric acid, creatine, total protein, sodium, potassium, chloride, magnesium, calcium and phosphorus); and bird thermobalance (heat production, body temperature, evaporative cooling, nonevaporative cooling, respiration rate, respiration efficiency and heat content). The resulting thermobalance and blood chemistry data should be usefull to propose rate-limiting aspects of metabolism for maintaining proper bird body temperature and feed consumption under the environmental and age conditions specified. Our aim was to collect baseline data to aid in developing therapeutic regimens to reduce the deleterious consequences of heat distress