Heat stress (HS) leads to impairment in body growth, physiology, morphology, blood biochemistry, reproduction, production, milk yield as well as natural immunity in small ruminants across the world (
La Salles et al., 2017). Increased temperature and higher relative humidity in summer can impair the thermal regulatory mechanisms of small ruminants and influence the loss of heat which leads to HS. Exposure of sheep and goats to raised temperature decreases the productive and reproductive performance, milk quantity and quality, body weight gain, daily average weight gain (AWG), growth rate and total body solids. It also reduces the animal’s natural ability of physical immunity and makes them prone to highly-strung diseases
(Marai et al., 2000; Abdel-Hafez, 2002;
La Salles et al., 2017). This causes severe monetary loss for sheep and goat farmers. Heat stress affects ruminants through a combination of environmental factors like higher ambient temperature, lower relative air humidity, higher solar waves, lower wind velocity and rainfall (
Al-Dawood, 2017). Aerial temperature and relative humidity have most direct effects on the production potential of ruminants
(Seixas et al., 2017). However, under extreme environmental conditions, sheep and goats show better heat stress compared to other ruminants
(Bakheit et al., 2017). They are the most adaptable and geographically wide-spread farm animal species, ranging from the high mountains of hypoxia to the extreme lowlands of thermally stressed environment
(Brito et al., 2015; Wei et al., 2015). Sheep and goats adapt to extreme climatic conditions via behavioral, morphological, physiological and mostly genetic principles
(Jose et al., 2016). The main limitations of the arid and semi-arid subtropical climate are its lower biomass production, higher environmental variability and inadequacy in availability of ground water all year round.
Small ruminant population scenario
Small ruminants (sheep and goats) are an important part of livelihood in arid and semi-arid regions of western India. The rural population and landless farmers in the extremely heat stressed (arid and semi-arid) environmental conditions have sustainable and eco-friendly rearing systems. Approximately 50% of the sheep and goat population live in the arid regions of the world, which signifies their adaptability and possible tolerance to increasing temperatures
(Gowane et al., 2017). India’s livestock sector is one of the largest, accounting for 11.6% (536.76 million) of the world’s livestock population. This includes goat and sheep population of 26.40% (148.88 million; second position) and 12.17% (74.26 million; third position) respectively. In India’s 20th livestock census, the state of Rajasthan ranked second as it shares 56.80 million of India’s total livestock population (536.76 million). In case of small ruminants, Rajasthan is first among other states with 20.84 million goat population and is fourth in sheep population (7.90 million) (
DAHD, 2019). In Rajasthan, over than 51% of the livestock is reared in the western region, mostly in the arid and semi-arid parts of the state. Ranching of livestock (23% sheep, 41% goat, 21% cattle, 13% buffalo and 2% camel) has been the main source of income for local migrant/maldhari for centuries.
Heat stress measurement in small ruminants
The temperature humidity index (THI) is a good indicator of heat stress (HS) in livestock. It is a value that shows the discomfort, or is a combined assessment of the ambient temperature and relative humidity which has the objective to examine the contrast in climatic conditions
(Wojtas et al., 2014). Hence, THI enables us to better diction the impressions of a higher environmental temperature and humidity. The method described by
NRC (1971) to calculate the temperature humidity index (THI) is as follows:
THI = (1.8 × Tdb + 32) - [(0.55 - 0.0055 × RH) × (1.8 × Tdb - 26.8)]
Where
Tdb = Dry bulb temperature (°C).
RH = Relative humidity (%)/100, for sheep and goats.
The stress range for ruminants (sheep and goats) on the basis of temperature humidity index (THI): comfortable ≤72; under mild stress 73-78; under severe stress ≥80 (Fig 1). THI values of over 80 are considered to be moderate heat stress (MHS) in sheep and goats (
Silanikove and Koluman, 2015). The degree of thermal stress in all places of Thar Desert (Western India) is that 100% of the places have an average THI of >80 and >35% of the places have a THI of >85 in May and June.
Heat stress responses in small ruminants
Body weight response
Body growth (BG) is defined as the increase in living biomass or cell proliferation. Heat stress (HS) has harmful effects on growth efficiency,
i.e. the body growth rate, daily weight gain and live weight of animals (
Gad, 2013). Among the differing changeable growth attributes, body weight is the most significant factor affected by heat stress in small ruminants (sheep and goats). The body weight performance of sheep and goats is influenced by genetics, surroundings, environment, balanced nutrient diet, hormones and enzyme activities in the body
(Marai et al., 2007). Body weight and average daily gain (ADG) are important selection standard for the meat breed animals (sheep and goat) and it is considered as a decisive trait in determining the meat production
(Pragna et al., 2018). The main factors for retardation of animal body growth can be the declined activity anabolism, decreased voluntary water intake and decreased feed consumption. Essential supplement nutrients are needed for metabolizable energy which gives proper nourishment, body weight gain and improved tissue catabolism (
Gupta and Mondal, 2021).
Among the multifarious environmental stresses, heat stress is a major stress, affecting the performance of body weight in small ruminants, especially sheep’s and goats in the arid and semi-arid hot desert areas. Decreased body growth performance connected with summer heat stress is an ordinary phenomenon in tropical and subtropical regions of the world
(Pragna et al., 2018). In general, sheep and goats begin to experience heat after being exposed to 38oC and above with a THI greater than 75
(Sejian et al., 2018). While the THI values affected the body weight gain due to heat stress, it significantly decreased the feed consumption in Indigenous goat breeds Osmanabadi, Malabari and Salem Black. Heat stress has negative effects on average daily gain (ADG) in all heat stress groups and the weight reduction rates were 3.4, 2.69 and 2.65 kg for Osmanabadi, Malabari and Salem Black goats respectively. The THI value 72 and less are considered comfortable; THI values between 75 and 78 are considered stressful and THI values above 78 are considered extreme distress
(Pragna et al., 2018). In another study carried out on goats exposed to summer heat stress (32.9°C), it was reported that the mean average daily gain (ADG) was reduced due to low feed consumption in the animals
(Popoola et al., 2014). The effects of heat stress on body weight gain, feed consumption and response to water intake in Malpura ewes in semi-arid tropical environment was studied at different temperatures several times a day and it was found that body weight and feed intake were reduced, while water intake increased significantly (Indu
et al.,2014). Thermal stress significantly changed the body weight and feed consumption in ewes under semi-arid hot environmental conditions
(Sejian et al., 2010a). According to
Ismail et al., (1995) the live body weight gain, growth efficiency, body’s total solids and daily solids gain (g) was affected after exposure to higher environmental thermal stress. A similar trend was observed that the body weight decreases under heat stress conditions in the different types of sheep and goats breeds, as mentioned in (Table 1).
Physiological response
High hot ambient temperatures in the arid and semi-arid region are connected to higher ambient temperature and lower humidity. Additional reasonable factors such as discomfort due to increased level of heat stress, which in turn led to a reduction in the physiological activities of these small ruminants. The results of the various experiments show (Table 2) that heat stress or multiple stresses significantly influences the physiological response in sheep and goat breeds. Heat stress has significantly altered some of the physiological responses of small ruminants commonly assessed are rectal temperature, heart rate and respiratory rate, resulting in an incredible financial loss in sheep and goat farming. In addition, the environmental variables change drastically in a day; consequently, physiological reactions are also strongly influenced by the time of day. In addition, physiological responses generally increase from morning (07:00 a.m.- 10:00 a.m.) to noon (11:00 a.m.- 02:00 p.m.). However, from evening to night these responses begin to decrease (11:00 p.m. - 06:00 a.m.) and the values of the physiological variables remain constant throughout this time (
Da Silva et al., 2017). The influence of physiological reactions under heat stress state in small ruminants is clearly described in imagery (Fig 2). The variations in physiological responses have been always recorded not only between species but also between breeds and even between individuals within a breed (Table 2).
Rectal temperature
The body temperature ideally is a very good measurable indicator for the heat load in the body of animals and represents the consequence of all heat gain and heat loss transformation of the body. Rectal temperature is regarded as an important measure of the physiological state of animal
(Koga et al., 2004). Rectal temperature (RT) is a crucial index of body temperature that can be used to determine the adaptability of heat stress. The RT is measured with a diagnostic thermometer by interpolating the thermometer by 6-7 cm into the rectum at an angle to the rectum wall. In addition, increasing RT represents the failure of the thermoregulatory mechanism in the animal body. An increase in rectal temperature of 1°C or less is sufficient to decrease the performance of the animals
(Rashamol et al., 2018). The average daily variant in rectal temperature ranges from 0.3°C to 1.9°C (
Piccione and Refinetti, 2003). The rectal temperature (RT) in sheep and goats fluctuates between 38.0°C to 40.0°C and is often used as an indicator of the body temperature, although there is a variance in body temperature in different parts of the body around the day. The rectal temperatures rates of goats increased from 0800 to 1700 h in heat stress (HS) groups compared to thermal neutral (TN) goats (Hamzaoui
et al., 2013). In contrast, some studies observed that there was no significant difference in rectal temperature in thermal stressed goats
(Alam et al., 2011; Panda et al., 2016). In Malpura ewes the highest rectal temperature (RT) in semi-arid tropical environmental conditions was measured in morning for the normal stress group (101.38°F), while in the multiple stress group, the highest value (102.55°F) was measured in afternoon
(Sejian et al., 2013). Increased rectal temperature with an increase in temperature humidity index in summer season is an excellent indicator of heat stress in animals
(Srikandakumar et al., 2003).
Respiration rate
Breathing is the uptake of oxygen (O
2) and excretion of carbon dioxide (CO
2) under thermo-neutral conditions which leads to evaporation and removal of moisture from the respiratory tract to control thermo equilibrium. The respiration rate (RR) is a realistic and reliable indicator for thermal stress (
Okoruwa, 2014). The RR is determined visually by counting the flank, either its up or down movements per minute using a stopwatch without disturbing the animal
(Shaji et al., 2016). Normally the physiological respiratory rate in sheep and goats is 15 to 30 breaths per minute. The severity of heat stress can be classified based on the respiring rate (breaths/min) (low: 40-60, medium: 60-80, high: 80-120 and severe: >200), as this is the most approachable and easiest way to evaluate the effects of heat stress (
Silanikove, 2000). During the experimental study on Indigenous goats on average (temp. 28.74°C and RH 87.83%) with division into three experimental groups as: 0 hrs, 4 hrs and 8 hrs of heat exposure, the respiratory rate in goats was higher with the increasing heat stress from 0to 8 hrs of heat exposure
(Alam et al., 2011). In another study, goats with heat stress (147 breaths/min) compared to neutral thermal goats (40 breaths/min) showed a positive increase in respiratory rate over the day in accordance with the increase in daily heat-stress rate (HS) from 0800 until 1700 h
(Salama et al., 2020).
However, the significant race differences between hot and cold-adapted goats were recorded for the respiration rates with the highest average values in cold-adapted goats (Gaddi and Chegu) compared to those of hot-adapted goats (Sirohi and Barbari). Another study found that Malpura ewes in a semi-arid tropical setting showed significant changes in respiration rates. The respiration rates for ewes in the heat stress group were lower in morning and significantly highest in the afternoon, compared to ewes in the normal stress group
(Indu et al., 2014). Similarly in another study, the respiratory rates in neutral thermals were significantly lower (52.85 breaths/min) and it was higher after multiple exposures of ewes recorded in the afternoon (68.74 breaths/min)
(Sejian et al., 2013). The impression of heat stress on Naimey sheep in a semi-arid environment with one room was recorded as the normal room temperature (23.6°C) and the other room was used as the heat stress room (33-38.5°C). It was recorded that the increase in respiratory rate is an attempt to increase respiratory evaporation. This is caused by higher skin temperature which occurs due to heat stress in sheep (
Al-Haidary, 2004). It is concluded that both respiratory rate and rectal temperature is a very good indicator of heat stress and it can be used to define the adverse effects of heat stressed environment.
Pulse rate
Pulse rate (PR) is the regularly beating rate of the arteries as blood is pumped through them to the heart. In addition to the general metabolic status, the pulse rate primarily reflects the homeostasis of the circulatory system
(Sejian et al., 2010). The heart rate can change quickly due to external factors such as time of year, time of day, higher ambient temperature, relative humidity and physical exercise
(Marai et al., 2007; Phulia et al., 2010). The pulse rate is generally measured by palpating the hand’s femoral artery in sheep and goats. Hand is placed on the inside of the animal’s thigh: the saphenous artery runs along the inside of the hind leg. Normally, pulse rates for sheep and goats range from 90 to 95 frequencies/min. However, it rises with thermal stress to release more heat to the environment by increasing blood circulation to the body surfaces
(Shilja et al., 2016). In addition,
Shaji et al., (2016) also observed a positively higher pulse rate in heat-stressed Osmanabadi goats, which indicates the role of pulse rate in appraising the amount of heat stress in these animals. In another study,
Alam et al., (2011) observed that the pulse rate in domestic goats heightened with increasing heat stress from the zero-hour heat exposure group (74.30 frequency/min) to the eight-hour heat exposure group (87.30 frequency/min). The sheep in Thar region preferred shade especially of tree scales when THI is above 88. The cooling effect of the leaves due to moisture evaporation and free air flow in the pasture area compared to erected thatched shade along with supplementary concentrate and health management methods maintained a normal heart rate in Jaisalmeri breed of sheep
(Mathur et al., 2019).
Blood biochemical response
In general, the biochemical blood profile indicates the health status, metabolic activities and nutritional conditions of the animal
(Calamari et al., 2016). Heat stress changes the blood plasma content of various blood biochemical parameters such as total protein, total cholesterol, glucose, globulin and albumin (
Gupta and Mondal, 2021). The general fluctuations in blood biochemical parameters of animals in cold and heat stress environments can be due to the inadequacy to adapt in diversified climatic and geographic circumstances which is necessary for their survivability
(Banerjee et al., 2015; Singh et al., 2016). However, variations in the coordination of these entire manners to maintain the production capability under heat stressed conditions have been seen not only among species, but also among breeds and even among individuals within a breed (
Marai and Haeeb, 2010). The sheep and goat breeds differ in various responses of blood biochemical profile in a specific heat stress environmental condition which is included in (Table 3). The fluctuations in the biochemical blood parameters (plasma enzymes, hormones and blood sugar) was significantly noted within breeds and seasons for cold-adapted breeds (Gaddi and Chegu) and heat-adapted races (Sirohi and Barbari)
(Banerjee et al., 2015). Helal et al., (2010) however, conducted an experiment on Balady goat breed which was exposed to thermal stress and recorded a decrease in total plasma protein, globulin and albumin. Some studies showed that the goats in high environmental temperature conditions had lower blood glucose and cholesterol levels, which is an indicator of homeostasis failure (
Sezen and Guney, 2010;
Ribeiro et al., 2016). However, in Malpura ewes during the semi-arid tropical climatic conditions, the blood plasma glucose and whole blood cholesterol were found to be lower. The blood plasma urea was higher in the group of heat-stressed ewes than in the neutral heat-stressed ewes. The total plasma protein, plasma albumin and plasma globulin recorded had non-significant differences in both normal and heat-stress ewe groups
(Indu et al., 2014). While in another study conducted on Chokla, Magra and Marwari sheep breeds, the mean values of the biochemical parameters for total protein and albumin increased whereas cholesterol and triglycerides content decreased during heat stress conditions. The glucose values for Marwari breed increased and for Chokla and Magra decreased. This signifies the higher stress state in Marwari sheep
(Singh et al., 2016). The plasma blood cortisol concentration heightened among the group of heat stress ewes
(Sejian et al., 2010a). The relationship between heat stress and raised secretion of cortisol, the most important glucocorticoid hormone in small ruminants is well indemnified (
Ali and Hayder, 2008). Some of the authors observed that heat stress affects the biochemical blood parameters of sheep and goats
(Alam et al., 2011; Phulia et al., 2010; Sharma and Kataria, 2011;
Kumar et al., 2010).
