EFFECTS OF CLIMATE CHANGE ON POULTRY PRODUCTION
1.0 INTRODUCTION
Poultry are birds such as the domestic fowl, Turkey, Duck, Goose, Ostrich e. t. c which are of economic importance to man. Their production is one of the livestock production with significant contribution to human food (Demeke, 2004). It is the primary supplier of eggs and meat as a source of income and employment to people compared to other domestic animals (Avila, 1985). They are raised with relatively low capital investment and readily available household labour. For instance poultry egg contains 74% water, it is a good source of high protein and it is often used by nutritionist as a standard reference for evaluating other protein foods. Studies have shown that the level of performance of poultry does not depends on inherited capacity, but also to a great extent upon the environment (Campbell and Lasley, 1975). The environmental conditions affecting the performance, relative humidity, light, sunshine prevailing at a given time, housing system and ventilation. High temperature and humidity have some negative effects on poultry such as an increase on poultry body temperature: a decrease on feed consumption ( Cowan and Michie, 1978; Howlider and Rose, 1987) and feed efficiency. High temperature also results to a reduction in poultry live weight ( Mowbray and Sykes, 1971), growth speed and high mortality (Arjona et al., 1988) in addition to a decrease on productivity and quality of the eggs (Ozbey and Ozoelk, 2004). Furthermore, harsh rearing environment and imbalanced feeding do not permit expression of the full genetic potential of exotic breeds (Barua and Howlider, 1990). Other environmental factors affecting the performances of layer chicken include feed utilization and body weight ( Harms et al., 1982) water availability (Oluyemi and Roberts, 1979), tier cages ( North, 1984), and infection (McNitt, 1983).
Further, the fowl, like many other animals, can undergo a number of complex and imperfectly understood physiological changes which enable it to minimize the adverse effects of hot climates (Sykes and A1-Fataftah 1980). To understand better the relationship between elevated ambient temperature, nutrition and performance, the physiological mechanisms must also be considered. Acute physiological responses to high environmental temperatures are obviously directed to the maintenance of homeothermy, but small increases in body temperature may be tolerated (Fisher et al. 1985).The effects of high environmental temperatures are well documented; however, in view of the importance of ambient temperature in efficient poultry production, especially in areas where high ambient temperatures are prevalent for most of the year.
2.0 IMPLICATION OF CLIMATIC FACTORS ON PRODUCTION
Temperature is an important bio-climatic factor affecting the physiological function of layer-chickens (McDowell, 1972) though the effect on egg production rate depends on age of laying hens. The effect is more evident at old age when birds are exposed to cold climate. When temperature falls below the thermo neutral zone of below 12.8oC (which is rarely experienced in the tropics) egg production becomes uneconomical (Oluyemi and Robert, 1979). Charles (1980) opined that feed intake of a laying hen decreased by 1.5g a day for every degree rise in temperature above 30oC, decreased egg production by about one egg per bird a year for every degree rise in temperature above 25-30oC and that the depressive effect of environmental temperature by heat stress significantly increases water consumption, reduces egg production , egg weight , shell weight, shell thickness causing a significantly higher production of shellers or very thin-shelled eggs. The optimal laying temperature according to Kekeocha (1985) is between 11o and 26oC .
A humidity level above 75 percent will cause a reduction in egg laying. When ambient temperature is high, chickens have higher energy (feed) needs than when in thermo-neutral environments. Major losses result from a less efficient conversion of feed to meat, which detrimentally impacts poultry health and productivity (Olanrewaju et al., 2010). Poultry flocks are particularly vulnerable to climate change because there is a range of thermal conditions within which animals are able to maintain a relatively stable body temperature in their behavioural and physiological activities. Hence, birds can only tolerate narrow temperature ranges to sustain the peak of their production for human consumption and any unpredictable climatic changes will therefore trigger a series of adjustment and readjustments by livestock and poultry birds in the struggle for survival which may have negative consequence on the viability of poultry production.
Environmental temperatures above 30 °C in the rearing area cause high mortality of broiler chickens (De Basilio and Picard, 2002) or reduction in feed intake, body weight, carcass weight, carcass protein and muscle calorie content (Tankson et al., 2001). Feng et al. (2008) observed significant decrease of initial pH, drip loss, and shear force of breast muscle in heat-stressed broilers.
Heat stress reduces the reproductive performance of laying hens by interrupting egg production. This may be caused not only by a reduction in feed intake but also by disruption of hormones responsible for ovulation and a decrease in responsiveness of granulosa cells to luteinizing hormone (Donoghue et al., 1989; Novero et al., 1991). Significant reduction of body weight and feed consumption occur in heat-stressed hens. Egg production, egg weight, shell weight and shell thickness are considerably compromised by heat exposure (Mashaly et al., 2004). Moreover, heat stress negatively affects the strength, weight, thickness, and ash content of the eggshell (DeAndrade et al., 1976, 1977; Miller andSunde, 1975) resulting in the increased egg breakage.
Stress occurred when birds encounter mass environmental change and lead to the expression of some physiological response such as reduced weight and egg production (Sandercocket al., 2001). Heat stress causes birds to increase their respiration rate and painting which results in reduce of pCO2 and the increase of blood pH (Moraes et al, 2003；Raup and Bottje, 1990). Thyroid gland is responsible for regulation of thermogenesis and is affected by heat stress. Thyroxine secretion should be decreased to avoid thermogenesis (Sahin et al, 2003).
3.0 IMPACT OF CLIMATE CHANGE ON HEALTH OF BIRDS
The effects of climate change on the health of farm animals have not been studied in depth. However, it can be assumed that as in the case of humans, climate change, in particular global warming, is likely to greatly affect the health of farm animals, both directly and indirectly. Direct effects include temperature-related illness and death, and the morbidity of animals during extreme weather events. Indirect impacts follow more intricate pathways and include those deriving from the attempt of animals to adapt to thermal environment or from the influence of climate on microbial populations, distribution of vector-borne diseases, host resistance to infectious agents, feed and water shortages, or food-borne diseases.
The decrease in energy intake due to reduced feed intake, results in a negative energy balance (NEB), and partially explains why cows lose significant amounts of body weight and body score when subjected to heat stress (Lacetera et al., 1996).
If exposure to high air temperature is prolonged, lower feed intake is followed by a decline in the secretion of calorigenic hormones (growth hormone, catecholamines and glucocorticoids in particular), in thermogenic processes of digestion and metabolism, and metabolic rate (Johnson,
1980; Webster, 1991). All these events together (lower feed intake, change in endocrine status and lower metabolic rate) tend to reduce metabolic heat production (Yousef, 1987) and might be responsible for modifications of energy, lipids, protein and mineral metabolism, and liver function.
4.0 MITIGATION MEASURES
Acclimation is a phenotypic response developed by the animal to an individual source of stress within the environment (Fregley, 1996). The acclimation of the animals to meet the thermal challenges results in the reduction of feed intake and alteration of many physiological functions that are linked with impaired health and the alteration of productive and reproductive efficiency (Beede and Collier, 1986; Lacetera et al., 2003a). Acclimation to high environmental temperatures involves responses that lead to reduce heat load. The immediate responses are the reduction of feed intake, increase in respiration rate and water intake and changes in hormonal signals that affect target tissue responsiveness to environmental stimuli (Collier and Zimbelman, 2007).
Increase in temperatures cause severe damage to the physiology, the metabolism and to the healthiness of animals. Modification of existing regimes of precipitation and the increase of aridity will have repercussions on the availability of feedstuff for animals. The increased difficulty in livestock