There are several components to body heat load which can be divided into two broad categories viz. internal or metabolic heat load (ruminal fermentation and nutrient metabolism) and environmental heat load (Armsby and Kriss 1921; Duckworth and Rattray 1946; Shearer and Beede 1990). Metabolic heat load is typically a result of: (i) basal body functions (heart, lungs and liver), (ii) maintenance, (iii) activity, and (iv) performance (e.g., daily gain, milk, eggs) (McDowell 1974).
Basal body functions contribute between 35% and 70% of daily heat production (McDowell 1974), and will tend to the higher levels during non-basal periods of work (e.g., walking, high respiration rate) or high levels of production. Importantly, core body temperature is dynamic even under thermoneutral conditions (Hahn 1989, 1999), and follows a diurnal pattern which is influenced by interactions between animal and environmental factors.
There are a range of thermal conditions within which animals are able to maintain a relatively stable body temperature by behavioral and physiological means (Johnson 1987; Bucklin et al. 1992). This range is defined for a species based on upper critical and lower critical temperatures. Bligh and Johnson (1973) defined the upper critical temperature (UCT) as 'the ambient temperature above which thermal balance cannot be maintained for a long period and animals become progressively hyperthermic'. This definition was revised in 1987 as 'the ambient temperature above which the rate of evaporative heat loss of a resting thermoregulating animal must be increased (e.g., by thermal tachypnea or by thermal sweating) in order to maintain thermal balance' (IUPS Thermal Commission 1987). The lower critical temperature (LCT) is defined by the IUPS Thermal Commission (1987) as 'the ambient temperature below which the rate of metabolic heat production of a resting thermoregulating tachymetabolic animal must be increased by shivering and/or nonshivering thermogenesis in order to maintain thermal balance'. The thermoneutral zone (TNZ) is defined as the range of ambient temperature at which temperature regulation is achieved only by control of sensible heat loss, i.e. without regulatory changes in metabolic heat production or evaporative heat loss. The TNZ will therefore be different when insulation or basal metabolic rate varies (IUPS Thermal Commission 2001). The UCT, LCT and TNZ of a species are influenced by insulation, nutrition and exercise (Ames 1980; McArthur 1987; Morgan 1997).
Heat stress results from the animal's inability to dissipate sufficient heat or reduce heat influx to maintain homeothermy (Folk 1974). High ambient temperature, relative humidity and radiant energy, particularly with concurrent low air speed, compromise the ability of animals to dissipate heat. As a result, there is an increase in core body temperature, which in turn initiates compensatory and adaptive mechanisms in an attempt to re-establish homeothermy and homeostasis (El-Nouty et al. 1990; Khalifa et al. 1997; Horowitz 1998, 2002; Lin et al. 2006). These readjustments, generally referred to as adaptations, may be favorable or unfavorable to economic interests of humans, but are essential for survival of domestic animals (Stott 1981). However, it is likely that continued genetic selection for improved levels of production (e.g. growth rate, feed intake and milk production) will result in animals that are generally less heat tolerant (Joubert 1954; Young 1985; Johnson 1987; Yahav et al. 2005; Lin et al. 2006).
When animals are exposed to environmental conditions above their UCT core body temperature begins to increase as a result of the animal's inability to adequately dissipate the excess heat load. There is a concomitant decrease in feed intake as core body temperature increases, which ultimately results in reduced performance (production, reproduction), health and well-being if adverse conditions persist (Hahn et al. 1993). Thresholds are genotype/phenotype/species dependent, and are affected by many factors, as noted in Fig. 7.5. For shaded Bos taurus feeder cattle, Hahn (1999) reported respiration rate typically increases above a threshold of about 21°C air temperature, with a threshold for increasing core body temperature and decreasing feed intake at about 25°C. A recent study (Brown-Brandl et al. 2005) showed the influence of condition, genotype, respiratory pneumonia, and temperament on respiration rate of un-shaded Bos taurus heifers). Figure 7.6 illustrates the respiration rate response of different genotypes to hot environmental temperatures.
The lower and upper critical temperatures of both Arabi and Zaraiby goats were 20-25°C and 20-30°C, respectively (El-Sherbiny et al. 1983). Lu (1989) found that the upper critical temperature of goats in maintenance is 25-30°C, and heat stress occurs when they are exposed to ambient temperature above 30°C. He stated that although rectal temperature rose significantly when goats were exposed to 30°C, compared to 20°C, the limit of heat tolerance for goats is between 35°C and 40°C. Dahlanuddin and Thwaites (1993) stated that goats reached the limit of their heat tolerance at 40-45°C ambient temperature. Furthermore, D'miel et al. (1980) stated that goats had a high lower critical temperature of 26°C. Therefore, they must rely mostly on metabolic energy rather than on insulation to keep their body temperature constant during cold weather.
Angus —a—MARC III —•— Gelbvieh —o—Charolais
Fig. 7.6 Respiration rates as a function of ambient temperature for unshaded cattle of four genotypes (Brown-Brandl et al. 2005)
There also appears to be a time-dependency aspect of responses in some species. For example, Hahn et al. (1997) reported that for beef cattle with access to shade, respiration rate lags behind changes in dry bulb temperature, with the highest correlations obtained for a lag of 2 h between respiration rate and dry bulb temperature. For un-shaded beef cattle, respiration rate closely tracks solar radiation; increasing or decreasing with solar radiation. There is also a delay in acute body temperature responses (during the first 3-4 days of exposure) to a heat challenge, with an increasing mean and amplitude, along with a phase shift reflecting entrainment by the ambient conditions (Hahn et al. 1997; Hahn and Mader 1997; Hahn 1999). Even though feed intake reduction usually occurs on the first day of exposure to hot conditions, the endogenous metabolic heat load from existing rumen contents adds to the increased exogenous environmental heat load. Nighttime recovery also has been shown to be an essential element of survival for cattle when severe heat challenges occur (Hahn and Mader 1997). After 3 days, the animal enters the chronic response stage, with mean body temperature declining slightly and feed intake reduced in line with heat dissipation capabilities. Diurnal body temperature amplitude and phase remain altered. These typical thermoregulatory responses (discussed more fully in Hahn 1999), when left unchecked during a severe heat wave with excessive heat loads, can lead to a pathological state resulting in impaired performance or death (Hahn and Mader 1997). The intensity and duration of exposure to a given thermal stress will also determine animal responses (Hahn and Mader 1997; Gaughan and Holt 2004; Beatty et al. 2006). Further studies are required to determine species and breed responses.
Thus, an increase in air temperature, such as that expected in different scenarios of climate change, would directly affect animal performance by affecting animal heat balance. The thermal environment influences animal performance primarily through the net effects of energy exchanges between the animal and its surroundings
(Folk 1974; Hahn 1989; Yahav et al. 2005). There are four modes of energy transfer: radiation (gain or loss of heat from the animal), convection (gain or loss), conduction (gain or loss), evaporation (loss only), all of which are governed by physical laws. Several physical parameters control heat transfer by each mode. Air temperature affects energy exchanges through convective, conductive, and radiative exchanges (not evaporation) (Hahn 1976). In hot conditions, evaporation becomes the most important method of heat loss, as it is not dependent on a temperature gradient (Ingram and Mount 1975). Therefore, the combination of temperature and humidity acquire more relevance, since humidity increases the magnitude of the thermal strain especially at high ambient temperatures.
The temperature humidity index (THI; Thom 1959) is commonly used as an indicator of the intensity of climatic stress on animals, where a THI of 72 and below is considered as no heat stress, 73-77 as mild heat stress, 78-89 as moderate, and above 90 as severe (Fuquay 1981). On the other hand, the Livestock Weather Safety Index (LCI 1970) categories associated with THI are normal (THI < 74), Alert (75-78 THI), danger (79-83 THI) and emergency (THI > 84). Davis et al. (2003) suggest that there is no heat stress for beef cattle when average THI < 70, mild heat stress when 70 < THI < 74, moderate heat stress when 74 < THI < 77, and severe heat stress when THI > 77. Khalifa et al. (2005) indicated that for sheep and goats, there is no heat stress when average THI < 70, mild heat stress when 70 < THI < 74 in sheep and 70 < THI < 78 in goats, moderate heat stress when 74 < THI < 88 in sheep and 78 < THI < 84 in goats and severe heat stress when THI > 84 in goats. It is worth noting that these data were obtained on crossbred sheep and goats which are acclimatized to Egyptian conditions but not well adapted to the subtropical environment like native breeds. Dairy cattle show signs of heat stress when THI is higher than 72 (Johnson 1987; Armstrong 1994); however the actual threshold will be associated with a decline in milk production (Berry et al. 1964; Kadzere et al. 2002), and whether or not heat abatement strategies are implemented (Mayer et al. 1999). Cows with higher levels of milk production are more sensitive to heat load (Johnson 1987; Hahn 1989). Amundson et al. (2006) indicated that a THI threshold of 73 pregnancy rates of beef cattle became negatively affected. Conception rate of dairy cows was affected by just 1 day exposure to THI between 65 and 70 (Ingraham et al. 1974; and Du Preez et al. 1990). The conception rate of water buffalo was significantly lower when THI > 79 (Pagthinathan et al. 2003). Whether these beef cattle, dairy, or buffalo could adapt to a greater THI is not known. A review of heat stress in lactating dairy cows has been published by Kadzere et al. (2002).
Although THI is widely-accepted for evaluating the climatic environment, it is limited because it does not take into account the effects of thermal radiation (solar and long-wave) or wind speed. Modifications to the existing THI to account for wind speed and solar load (Mader et al. 2006) and the development of new indices (Eigenberg et al. 2000, 2005; Khalifa et al. 2005; Gaughan 2008) have been reported. A review of thermal indices used with livestock was undertaken by Hahn et al. (2003). A new heat load index (HLI) which incorporates the effects of solar radiation and wind speed on the heat load status of feedlot cattle has been established (Gaughan et al. 2008a). This index is based on the establishment of thresholds above which cattle gain heat and below which heat is dissipated. The thresholds are adjusted based on genotype, health status, nutritional management, pen management and the provision of shade.
Current indices do not account for cumulative effects of heat load, and/or natural cooling. Cattle may 'accumulate' heat during the day (body temperature rises) and dissipate the heat at night. If there is insufficient night cooling, cattle may enter the following day with an 'accumulated' heat load (Hahn and Mader 1997). The THI-hours model was developed to account for the impact of intensity x duration on thermal status (Hahn and Mader 1997). This concept was further developed for feedlot cattle as the accumulated heat load model (Gaughan et al. 2008a). This model is able to account for genotype differences, management factors and housing factors (e.g. provision of shade).
It is not only the intensity and duration of the thermal challenge, but also the amount of time animals have to recover from the challenge that determines their response (Mendel et al. 1971; Hahn et al. 2001; Gaughan et al. 2008a). In the central Santa Fe region, a major dairy area in Argentina, THI > 72 for 13 h a day is common in January (Valtorta and Leva 1998). These conditions result in poor reproductive performance and milk yield; de la Casa and Ravelo (2003) have estimated the impacts on milk production in Argentina. When considering a global climate change scenario, determined by paleoclimatological studies (Budyko et al. 1994), the hours when THI > 72 would increase to approximately 16 h by 2025 (Valtorta et al. 1996a). The implications of such a change are that the already compromised summer dairy performance measured in terms of reduced milk production (Valtorta et al. 1996b, 1997) and lower conception rates (Valtorta and Maciel 1998), could be further impaired.
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