The livestock sector is socially, culturally and politically very significant. It accounts for 40% of the world's agriculture Gross Domestic Product (GDP). It employs
1.3 billion people, and creates livelihoods for one billion of the world's population living in poverty. Global meat production is expected to more than double from
229 to 465 million tonnes between 1999/2001 and 2050. Milk production is also expected to increase from 580 to 1,043 million tonnes over the same period. In order to achieve these increases, livestock production will intensify. Production of pigs and poultry is expected to account for much of the increase. Grazing occupies 26% of ice-free terrestrial land, and crop production for animal feed accounts for 33% of all arable land. It is estimated that livestock production accounts for 70% of all agricultural land and 30% of the total land surface (Steinfeld et al. 2006). Approximately 3.5 billion hectares are being grazed compared to 1.2-1.5 billion hectares under cropping (Howden et al. 2007).
Climate affects animal agriculture in four ways (Rotter and Van de Geijn 1999) through impacts on livestock: (1) feed-grain availability and price; (2) pastures and forage crop production and quality; (3) health, growth and reproduction; and (4) diseases and pests distributions. Adaptation of practices used by farmers to changing climatic conditions is paramount (ILRI 2006). These changes may result in a redistribution of livestock in a region; changes in the types of animals that are used (e.g., a shift from cattle to buffalo, sheep, goats or camels); genotype changes (e.g., the use of breeds that will handle adverse conditions, such as Brahman cattle); and changes in housing of animals (e.g., protective structures which have allowed the expansion of the dairy industry into areas such as southern USA, Brazil, Israel, Saudi Arabia that would not otherwise be suitable [Darwin et al. 1995]). A lack of thermally-tolerant breeds of cattle is already a major constraint on production in Africa (Voh et al. 2004). Furthermore, it is possible that conflicts over resources may become a problem (Darwin et al. 1995). However, climate change may have a positive impact on livestock production in some areas. For instance, areas that are cooler and wetter may increase forage production and, in turn, livestock production. Warming of areas such as Canada may increase in agricultural production (Arthur and Abizadeh 1988). Increased rainfalls and winter temperatures in India, Pakistan, and Bangladesh may have both positive (e.g., longer growing seasons) and negative (e.g., flooding, increased animal disease risk) effects. Changing conditions in Africa may spread trypanosomosis into previously unaffected areas. Movement of parasites into previously unaffected areas could result in large production and financial losses. Any advantages that may result from climate change could be hampered by an inability (political, social and financial) to change farming practices.
The impact of climate change (higher temperatures) on pastures and rangelands may include deterioration of pasture quality (C3 grasses) towards lower quality tropical and subtropical C4 grasses (Barbehenn et al. 2004) in temperate regions as a result of warmer temperatures and fewer frosts (Briske and Heitschmidt 1991; Greer et al. 2000); however, there could also exist potential increases in yield and possible expansion of C3 grasses if climate change were favorable as a result of an increase in CO2 (Kimball et al. 1993; McKeon et al. 1993; Idso and Idso 1994; Allen-Diaz 1996; Campbell et al. 1995; Reilly 1996), and if precipitation is favorable. An increase in CO2 is likely to have a negative effect on C4 grasses (Collatz et al. 1998; Christin et al. 2008) resulting in declines in pasture productivity and lower carrying capacity.
The impact of climate change on wildlife is deemed to be largely negative (Thuiller et al. 2006). However, in some instances increasing ambient temperature has had little negative impact, at least to date (Johnston and Schmitz 1997; Beaumont et al. 2006), whereas in other cases the impact is largely due to changes in vegetation (Johnston and Schmitz 1997). In some scenarios, a species may be able to extend their current range.
In the animal context, climate change needs to be viewed as more than global warming. As previously mentioned, some areas will become cooler, and this may only have minor impacts on the animals (but could alter feed availability). On the other hand, extreme events such as heat waves can have major impacts on non-adapted animals. Heat waves are recurring events in many current climates, and are projected to increase in number and intensity (Mearns et al. 1984; Gaffen and Ross 1998; IUC 2002). The risk of floods and droughts are also predicted to increase. While there may be little change per se in a region, extreme events may increase in both intensity and duration leading to substantial changes in animal management practices. Climatic variables which need to be assessed include: ambient temperature, relative humidity, the day to night and seasonal variations in ambient temperature, rainfall, wind speed, solar and terrestrial radiation, evaporation rates, and atmospheric CO2 (Folk 1974; Hulme 2005). It is likely that heat and drought will be the major contributing factors to changes in animal production over the next 50 years. Some of the effects will be direct (e.g., heat stress of livestock), and others indirect (e.g., changing pasture composition). In the context of this chapter, we will concentrate on the impact of increasing heat load on livestock, and how animals adapt to increasing heat stress.
Livestock production involves a relatively small group of domesticated animals. Diamond (1999) reported that of the 148 non-carnivorous species weighing more than 45 kg as adults, only 14 have been domesticated. Even fewer bird species (0.001%) have been domesticated (Mignon-Grasteau et al. 2005). However, this does not necessarily make the task any easier.
Mammals are homeothermic endotherms and maintain a core body temperature between 35°C and 40°C depending on the species (Langlois 1994). They are able, through irradiative, conductive, convective, and evaporative exchanges, to generally maintain core body temperature within a fairly narrow range (Langlois 1994; Folk et al. 1998). In many species 5-7°C deviations from core body temperature may cause death, and at least reductions in productive performance. Mammals have a greater capacity for dealing with cold environmental conditions than they do with hot conditions (Folk et al. 1998). The lethal limit of core temperature is about 6°C above normal for healthy animals, and depression of central nervous activity, particularly in the respiratory center, occurs before that (Schmidt-Nielsen 1975). In horses, death may occur if core body temperature decreases by 10°C (27% deviation from normal) or increases by 5°C (13% deviation) (Langlois 1994). In cattle, death has occurred when rectal temperature exceeds 43.5°C (6°C above normal) (J. Gaughan, 2006, personal communication). There is a paucity of information on upper critical body temperature in livestock. Body temperature of some species is more labile, with the capacity to survive large changes in body temperature. For example, the core body temperature of camels can vary between 34.0°C and 42°C (Schmidt-Nielsen et al. 1956; Fowler 1999). Antelope ground squirrels show large fluctuations of core temperature between 37°C to 43°C. These animals will return to burrows or seek shade and rest when core body temperature approaches 43°C. Once body temperature returns to normal (approx 37°C) activity may recommence (Willmer et al. 2000). Animals that hibernate may lower body temperature to only a few degrees above freezing.
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