Climate change may influence livestock systems directly by its effects on animal health, growth, and reproduction, and indirectly through its impacts on productivity of pastures and forage crops.
Domestic livestock in Africa (other than pigs) are concentrated in the arid and semiarid zones. The overwhelming majority of these animals feed predominantly on natural grasslands and savannas. In broad terms, changes in range-fed livestock numbers in any African region will be directly proportional to changes in annual precipitation. Given that several GCMs predict a decrease in mean annual precipitation of 10 to 20 percent in the main semiarid zones of Africa, there is a real possibility climate change will have a negative impact on pastoral livelihoods. Because the CO2 concentration will rise in the future, its positive impact on water use efficiency will help to offset a reduction in rainfall of the same order. Simulations of grassland production in southern Africa indicate an almost exact balancing of these two effects for that region (Ellery, Scholes, and Scholes, 1996).
African cattle are mostly more heat tolerant than European cattle. In extremely hot areas, even the African breeds are beyond their thermal optimum. Under global warming, meat and milk production decline largely because the animals remain in the shade instead of grazing.
In the higher-altitude and higher-latitude regions of Africa, sheep are currently exposed to winter temperatures below their optimum. Mortality often results when cold periods coincide with rains. These episodes are likely to decrease in frequency and extent in the future.
Livestock distribution and productivity could be indirectly influenced by changes in the distribution of vector-borne livestock diseases, such as nagana, and the tick-borne East Coast Fever and Corridor disease (Hulme, 1996). Simulations of changes in the distribution of tsetse fly indicate that with warming it could potentially expand its destructive range.
Simulation studies conducted in Australia (McKeon et al., 1998; Hall et al., 1998) show that CO2 increase is likely to improve pasture growth. There is also a strong sensitivity to rainfall, such that a 10 percent reduction in rainfall would balance out the effect of a doubling of CO2 concentration. A 20 percent reduction in rainfall at doubled CO2 concentration is likely to reduce pasture productivity by about 15 percent, liveweight in cattle by 12 percent, and substantially increase variability of stocking rates, reducing farm income. A substantial reduction in rainfall in many parts of Australia would tend to reduce productivity. However, in the far west rangelands of eastern Australia, summer growing grasses will increase as a result of in creased summer rainfall, and rapid pasture growth will lead to higher stocking rates (Banks, 1996).
An assessment of the response of dairy cattle to heat stress in New South Wales and Queensland indicated significant increases in heat stress over the past 40 years. Physiological effects of heat stress include reduced food intake, weight loss, decreased reproduction rates, reduction in milk yields, increased susceptibility to parasites, and, in extreme cases, collapse and death (Davison et al., 1996; Howden, Hall, and Bruget, 1999). Jones and Hen-nessy (2000) modeled the impact of heat stress on dairy cows in the Hunter Valley in NSW under the climate change scenarios. They estimated the probabilities of milk production losses as a function of time. According to their estimates, under uncontrolled conditions, average milk loss from the cows without shade by the year 2030 will increase by 4 percent. By 2070, the milk loss will increase by about 6 percent of the annual production.
In New Zealand, productivity of dairy farms might be adversely affected by a southward shift of undesirable subtropical grass species, such as Pas-palum dilatatum (Campbell et al., 1996).
Global warming may negatively affect livestock production in summer in currently warm regions of Europe (Furquay, 1989). Warming during the cold period for cooler regions is likely to be beneficial due to reduced feed requirements, increased survival, and lower energy costs. Impacts will probably be minor for intensive livestock systems where climate is controlled (confined dairy, poultry, and pigs). Climate change may, however, affect requirements for insulation and air-conditioning and thus change housing expenses (Cooper, Parsons, and Dernmers, 1998).
In Scotland, studies of the effect on grass-based milk production indicate that for herds grazed on grass-clover swards milk output may increase regardless of site, due to the effect on nitrogen fixation (Topp and Doyle, 1996).
Ranching is a major land use in many parts of Latin America. In Brazil, Argentina, and Mexico, pastures occupy much more area than crops and livestock is almost exclusively raised on rangelands, with no storage of hay or other alternative feeds (Baethgen, 1997). Grass production in rangelands depends on rainfall, and reduced grass availability in dry periods limits cattle stocking rates over most of the region. In areas subject to prolonged droughts, such as northeastern Brazil and many rangeland areas in Mexico, production would be negatively affected by increased variability of precipitation due to climate change. In the Amazonian floodplains, higher peak flood stages would cause losses to cattle kept on platforms during the high-water period.
In Argentina, cattle are mainly fed on alfalfa and some other forage crops. A 1°C rise in temperature would increase alfalfa yields by 4 to 8 percent on average for most varieties, but there will be regional differences. Pasture yields would be reduced in areas north of 36°S and would be increased south of this latitude (Magrin et al., 1999).
Estimates in livestock production efficiency in North America suggest that the negative effects of hotter weather in summer outweighed the positive effects of warmer winters (Adams, 1999). The largest change occurred under a 5°C increase in temperature, when livestock yields fell by 10 percent in Appalachia, the Southeast, the Mississippi Delta, and the southern plains regions of the United States. The smallest change was 1 percent under 1.5°C warming in the same regions. Livestock production could also be adversely affected by an increase in the frequency of blizzards in eastern Canada and the northeastern United States.
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