Sustainability in livestock production system is mostly affected by climate change. Climate change affects various factors associated with production, reproduction, heath and adaptability of the animals. Dairy sector is more susceptible to climate change. The temperature humidity index (THI) is the widely used as index to measurement of thermal stress in animals. Environmental stress has adverse effects on health status of dairy animals and decreases the milk production and reproductive performance of dairy cows resulting in huge economic losses. Global climate change is expected to alter temperature, humidity, rainfall, atmospheric carbon dioxide. The dairy sector is a more susceptible to climate change and global warming where it is mostly affected by the temperature and humidity such as temperature humidity index (THI). Combined effect of high ambient temperature and high humidity results adverse effect on reproductive performance of farm animals. The management strategies viz., microclimatic modification, nutritional management, feeding strategies and artificial insemination protocol are to be strictly followed to ameliorate the adverse effects of heat stress in dairy animals.
Livestock sector plays a vital role for livlihood food security in India. The animal husbandry and agriculture are the major resource of income for the farmers and directly affects the economic conditions of farmers. Climate change is one of the major threats for the sustainability of livestock production systems in tropical countries. Sere et al. (2008) reported that heat stress has adverse effects on the productive, reproductive and health performances of dairy animals. Heat stress is a major factor contributing to the decline in fertility in lactating dairy cows (De Rensis and Scaramuzzi, 2003; Dash et al., 2016). Several studies reported 20 to 30% reduction in conception rate (Schuller et al., 2014) and in pregnancy rate (Khan et al., 2013) in hot climatic condition. However, an air temperature above 25-37°C exceeds heat gain than their lost from the body and it induces heat stress in a tropical climate (Vale, 2007; Kumar et al., 2011). According to inter governmental panel on climate change (IPCC, 2007), there is an increase in body surface temperature, rectal temperature (RT), respiration rate (RR) and pulse rate (PR) and decrease in feed intake, production and reproductive efficiency in hot climatic conditions. Increase in temperature of earth per decade by 0.2°C, global average surface temperature would be increased to 1.4-5.8°C by 2100. The major environmental factors affects livestock production system include temperature, relative humidity (RH), solar radiation, precipitation and wind speed (WS) (Hahn et al., 2003). Strategies to ameliorate negative impact of heat stress on production and reproduction in dairy animals include improved housing and management intervention to reduce climatic impacts on livestock. Various cooling system such as use of fogger and sprinkler with or without fan, feeding management, diet manipulation and change in reproductive protocol, will also improve dairy farm profitability. Cooling system is the most effective way to increase both milk production and reproduction in dairy animals during the summer season.
Environmental Factor and Animal Stress
Many environmental factors directly or indirectly affects on production performance of animals. Change in climatic condition directly affects the production and reproduction level of animal about 58.3% and 63.3%, respectively (Singh et al., 2012). High environmental temperature leads to changes in the animal’s body physiology such as rise body temperature (>102.5 ºF), respiration rates (> 70-80/minute) and blood flow (Pereira et al., 2008). The maintenance energy requirement may increase by 20-30% in animals under heat stress, which leads to reduced intake of feed and low energy level for productive functions such as milk production and increased loss of ions like sodium and potassium. This results shift in the acid-base balance and leads to metabolic alkalosis.
Temperature-humidity index (THI) is the universal and most precise indicator of stress assessment as temperature and humidity. Hot climatic conditions lead to decline production and nutrient intake of animal. Heat generated by metabolizing nutrient contributed to body temperature maintenance in a cold environment. However, in a hot climate, heat needs to be dissipated to maintain body temperature and normal physiological functions. Marai et al. (2008) reported that exposure of animals to hot climatic conditions lead to drastic changes in the biological functions which include decrease in feed intake and its utilization, disturbances in enzymatic activity, metabolism of water, protein, energy and mineral balances. THI is account for combine effects of environment temperature and relative humidity and animal response. THI can be calculated by formula (Mc Dowell, 1972),
THI = 0.72 (C db +C wb) + 40.6
Where, C db = dry bulb temperature (0C), C wb = Wet bulb temperature (0C). RH: Relative humidity (RH %) /100.
If THI, 72 there absence of heat stress, 73 to 78= mild heat stress, 79 to 88= moderate heat stress, 89-98 = severe heat stress and <72 = danger for animal. Milk yield decline by 0.2kg per unit increase in thermal humidity index (THI) when it exceeded 72 (Ravagnolo and Misztal, 2000). The production performance of cows is negatively correlated with temperature-humidity index (Shinde et al., 1990; Mandal et al., 2002). When the environmental temperature rises from the upper critical limit, the detrimental effects of heat stress on animals in terms of reduction in production of milk, changes in composition of milk and reduced reproductive performances are observed in cattle and buffaloes. Several studies report the classification of different zones based on THI values whether the animals are comfortable or susceptible to heat stress.
Table1: Classification of stress level based on THI values (Armstrong, 1994).
|THI||Stress Level||Symptom in Livestock|
|< 72||None||Optimum productive and reproductive
|73-78||Mild||Increases respiration rate and rectal temperature, animals seek for shade|
|79- 88||Moderate||Decrease dry matter intake and water intake in buffalo is significantly increase.
Body temperature is increased and reproductive performances are severely affected in cattle and buffalo
|89- 98||Severe||The reproductive performances in animals are significantly decreased. Excessive panting and restlessness are observed|
|>98||Danger||Heat stress is extreme and animals may die|
Effect of Climate Change on Production Performance of Dairy Animals
Climate change adversely affects the milk production and their composition in dairy animals, especially in high genetic merit animals (Upadhyay et al., 2009; Wheelock et al., 2010). Increasing air temperature and THI value above the critical thresholds level lead to decreased dry matter intake (DMI) and milk yield and also disturbance in physiology of animal (West et al.,2003). Several study reported that decrease in DMI by 0.85 kg per cow for every 1°C increase above the thermo-neutral zone and decline in milk production by 36% due to shift in post absorptive metabolism and partitioning of nutrient (West, 2003; Rhoads et al., 2009). THI is negatively correlated to milk yield, an increase of THI value from 68 to 78 decreases DMI by 9.6% and milk production by 21% (Spiers et al., 2004; Bouraoui et al., 2002). Zimbelman et al.(2009) also reported a negative relationship between rectal temperature and milk yield of animal. Johnson et al. (1963) reported that decrease in milk yield by 4 lbs/d per cow for every 0.55 °C increase above the rectal temperature of 38.6 °C. Igono et al. (1985) reported that decrease in milk yield 0.7 kg/day per cow when temperature was increased to 0.6°F above the rectal temperature 102.4°F. Milk constituents are significantly affected by heat stress during summer season. Dairy breeds are more susceptible to heat stress than meat breeds, and higher milk producing animal had increased metabolic heat production and this causes more susceptibility to heat stress as compared to low milk producing animals (Das et al., 2016). Decrease in protein constituent show that reduction in casein, lactalbumin, IgG and IgA. Heat stress cause decline in dry matter intake and feed conversion efficiency which directly affects the body condition and resulting low milk yield (Wilson et al., 1998).
Effect of Heat Stress on Animal Reproduction
Climate change has a great impact on the reproductive activity of cattle and buffaloes (Dash et al., 2015). High temperature combined with high level of relative humidity has detrimental effect on reproduction of cattle in summer season. Heat stress had negative effect on reproductive traits of cattle and buffaloes which can be quantified through formulating temperature humidity index (THI). Conception rates of lactating dairy animals have been declined with increased THI more than 72-73 in cattle (Morton et al., 2007; Schuller et al., 2014) and 75 in buffalo (Dash, 2013). The release of ACTH from anterior pituitary, which stimulate the release of cortisol and glucocorticoids from adrenal cortex occurs during heat stress condition. The release of luteinizing hormone is also inhibited by glucocorticoids. The hyperprolactinaemia, as a result of thermal stress inhibits the secretion of both FSH and LH at hypophyseal level (Singh et al., 2013)
Effects of Heat Stress on Health of Dairy Animals
Heat stress has direct and indirect effects on health performance of animal leading to changes in physiology, metabolism, hormonal and immune system. Increase in environmental temperature has a direct negative effect on voluntary feed intake and efficiency of feed utilization (Baile and Forbes, 1974). Lactating cows start to decline the feed intake at air temperature of 25-26°C and reduces more rapidly above 30°C in temperate climatic conditions and at 40°C it may decline by 40% in cattle, 8-10% in buffalo heifer and 22-35% in goat (Rhoads et al., 2013; Hamzaoui et al.,2012; Hooda et al., 2010). Increased environmental temperature may increase risk of metabolic disorders and health problems and change the basic physiological mechanisms resulting decreasing rumen motility and rumination (Nardone et al., 2010; Soriani et al., 2013). Heat stress changes metabolic patterns results in decreased thyroid activity and reduces metabolic heat production (Helal et al., 2010). Increase in ambient temperature causes increased incidence of in lameness in animals (Cook et al., 2007). This coincides with the change of climate as well the lameness prevalence is higher in hot climates as compared to cooler climates (Sanders et al.,2009). These climatic and seasonal effects are also correlated to mastitis in dairy animals (Dohoo and Meek, 1982; Elvinger et al., 1991).
Strategies to Ameliorate Heat Stress
To reduce the heat stress is the multidisciplinary approach. It should include modification of micro environment; nutritional management and genetic improvement are key components for sustainable livestock production under hot environment conditions.
Modification of Micro Environment
Modification of micro environment to improve heat dissipation mechanism to alleviate heat stress is one of the most important measures to be considered in hot environment. The most common approach to ameliorate heat stress is to modify environment near to cow way through provision of shade, evaporative cooling system by use of fogger, mister or sprinker with fan or without fan (Atrian and Shahryar, 2012). Improve reproductive performance of cows using effective cooling systems that combine evaporative cooling with tunnel ventilation or cross ventilation (Kadokawa et al., 2012).
Reduced dry matter intake with greater availability of key nutrients and to compensate for dietary heat increment while avoiding nutrient excesses. Lower DMI during hot weather reduces nutrients available for absorption, and absorbed nutrients are used less efficiently (West, 1999). Low-fibre, high fermentable carbohydrate diets lower dietary heat increment compared to high fibre diets. Although the metabolic energy of dairy buffaloes increases in a hot environment, heat stress depresses feed intake. Therefore, the course to increase the nutrient density includes feeding of high quality forage, concentrates and use of supplemental fats in the diet of animals. During hot climate, dietary fat content in feed is to be increased to enhanced milk production efficiency and yield. Supplementation of niacin supportive to reducing of heat stress in cattle and supplementation with antioxidants during the heat stress period is an additional to improve fertilty in buffaloes (El-Tarabany and Nasr, 2015). Both Vitamin C and Vitamin E have antioxidant properties. Antioxidant vitamins have proved to protect the biological membranes against the damage of ROS and the role of vitamin E as an inhibitor –“chain blocker”- of lipid peroxidation has been well established (Seyrek et al., 2004).
The identification of heat tolerant animals within high producing breeds and they can be select genetically for crossbreeding programme to improve genetic variation and cooling capability (Kimothi and Ghosh, 2005). Cattle with lighter, thin skin, short hair and greater diameter of hair coat colour are more adapted to hot environments as compared to darker colours and long hair coats (Bernabucci et al., 2010).
Heat stress is a major economic issue in the dairy industry. It affects the production reproduction and health of animal through physiological changes. Environmental stress has adverse effects on health status of dairy animals and decreases the milk production and reproductive performance of dairy cows resulting in huge economic losses. The most common method to reduce heat stress in dairy cows by provision of shades, sprinklers, ventilation and evaporative cooling will be suitable for adapting to climates changes. Environmental modifications and nutritional management are key elements to alleviate the impact of heat stress on animal’s performance during the hot climate. Wallowing and sprinkling are the most effective methods to reduce heat stress in case of buffalo during summer season. Strategies to reduce negative impact of heat stress of animals using cooling system, ration manipulation, change in reproductive protocol, antioxidant, use of buffers, yeast and hormones will improve the economic status of dairy farmers.