Direct effects involve heat exchanges between the animal and the surrounding environment are related to radiation, temperature, humidity, and wind speed (Johnson 1987). Because the thermal environment is more than just heat, the term heat stress is somewhat misleading. The term heat load has been used to highlight the importance of the interactive effects of the fore-mentioned factors. Within breed, animal variation (phenotypes), differences among breeds (genotypes), management factors such as housing and nutrition, physiological status (stage of pregnancy, stage of
lactation, growth rate), age and previous exposure to hot conditions may increase or decrease the impact of hot conditions.
High heat load (and environmental stress in general) has the potential for detrimental effects on susceptible animals. The negative effects on health, growth rate, feed intake, feed efficiency, tissue deposition, milk yield, health status, reproduction, and egg production are well documented (Brody 1956; El-Fouley et al. 1976; Biggers et al. 1987; Fuquay 1981; Johnson 1987; Nienaber et al. 1987a, b; Hahn et al. 1990, 1993; Liao and Veum 1994; Valtorta and Maciel 1998; Mader et al. 1999a, b; Nienaber et al. 1999, 2001; West 1999; Hansen et al. 2001; Wolfenson et al. 2001; Yalchin et al. 2001; Valtorta et al. 2002; Kerr et al. 2003; Faurie et al. 2004; Gaughan et al. 2004; Holt et al. 2004; Mader and Davis 2004; Kerr et al. 2005; Huynh et al. 2005; Wettemann and Bazer 1985). However, the actual numerical impacts are unknown. Furthermore genetic change in livestock animals especially in regards to increase productivity has resulted in animals that more likely to be susceptible to the negative impacts of heat stress.
Differences in their ability to withstand environmental stressors should allow selection of animals (within breeds and between breeds/species) better suited to particular environmental conditions (Scott and Slee 1987; Slee et al. 1991; Langlois 1994; Hammond et al. 1996, 1998; Gaughan et al. 1999; Herpin et al. 2002; Abdel Khalek and Khalifa 2004; Koga et al. 2004; Brown-Brandl et al. 2005; Hamadeh et al. 2006). However, as previously discussed, selection of such animals may result in improved welfare and ability to cope at the expense of lower productivity.
Livestock and poultry are remarkable in their ability to mobilize coping mechanisms when challenged by environmental stressors. However, not all coping capabilities are mobilized at the same time. As a general model for bovines, sheep and goats, respiration rate serves as an easily recognized early warning of increasing thermal stress (Khalifa et al. 1997; Butswat et al. 2000; Gaughan et al. 2000; Eigenberg et al. 2005), and increases markedly above a baseline as the animals try to maintain homeothermy by dissipating excess heat through respiratory evaporation. However, Starling et al. (2002) stated that the use of physiological parameters such as rectal temperature and respiration rate for selection is not enough to evaluate the level of adaptive capability. Clearly this is the case as there are many physiological factors which need to be assessed. However, a full assessment (i.e. changes in body temperature, respiration rate, heat shock proteins, hormones etc. - all of which are indicators of heat load status) of animals is difficult, especially under farming conditions. Increased respiration rate and body temperature do not necessarily indicate that an animal is not coping with the environmental conditions to which it is exposed.
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