Climatic data
Table 1 shows the THI during the experimental period. The experiment was carried from 1
st May to 31
st August, 2022. During the period the THI ranged from 71.27 to 74.39.
Thermoregulatory responses
Rectal temperature
Fig 1 depicts the trend of rectal temperature (°F) in both the experimental groups. The rectal temperature significantly (p<0.05) increased in HS goats in comparison to CON. Even under challenging weather conditions, goats exhibit a certain level of thermoregulatory control to uphold their body temperature within a narrow range. In a thermo-neutral setting, rectal temperatures usually range from 38.3 to 39.9°C. Rectal temperature measurement is widely employed as a prominent indicator of body temperature, offering insight into the core temperature (
Al-Tamimi, 2007). It is recognized as a crucial physiological parameter for assessing animal welfare in hot environments (
Silanikove, 2000). The outcomes of the present investigation indicated a significant (p<0.05) elevation in rectal temperature on all days, except at 90 days, during periods of HS. Current results were in accordance with the reports of
Kandasami et al., (2023) in Kanni Adu and Kodi Adu goats of Tamil Nadu, where HS caused marked increase in rectal temperature during summer. These heightened rectal temperatures observed in the context of hot summer conditions unequivocally demonstrate that the animals were subjected to HS (
Alamer and Al-Hozab, 2004;
Al-Haidary et al., 2012; Minka and Ayo, 2012). In the face of elevated temperatures, animals employ strategies to maintain thermal equilibrium and dissipate excess heat from their bodies. One notable response is an evident increase in the respiratory rate or panting. However, if the body is unable to effectively maintain this thermal equilibrium, it leads to an elevation in body temperature
(Marai et al., 2007). The outcomes of the present study are in line with published research on Saudi Arabian goats (
Al- Samawi et al., 2014), indicating concurrence with the previous findings. However, these results contradict the reports associated with Black Bengal goats where heat treatments did not increase skin and rectal temperatures
(Alam et al., 2011).
Pulse rate
Fig 2 depicts the trend in pulse rate (bpm) in the different experimental groups. There was no significant (p>0.05) change in pulse rate on day 0, 15 and 30 days, but a significant (p<0.05) increase was recorded from day 45 onwards till 120 days in the HS group in comparison to CON group. The results of our study are consistent with the reports of
Alam et al., (2011) in Black Bengal goats and
Kandasami et al., (2023) in Kanni Adu and Kodi Adu goats. The pulse rate primarily indicates the stability of circulation and the overall metabolic condition. When exposed to high environmental temperatures, the pulse rate tends to increase (
Aboul-Naga, 1987). In goats, it was found that the pulse rate during summer was considerably higher compared to winter
(Ismail et al., 1995). This similar pattern was also observed in grazing goats (
Khan and Ghosh, 1989). An increase in the pulse rate promotes enhanced blood flow from the core to the body’s periphery, leading to increased heat loss through both sensitive means (conduction, convection and radiation) and insensitive means (Water loss through diffusion from the skin)
(Marai et al., 2007).
Respiration
Fig 3 depicts the trend in respiration rate (per minute) in the experimental groups. Results revealed a significant (p<0.05) increase in respiratory rate in the HS group in comparison to the CON. The present results were in agreement with previous findings where it was reported that respiration rate can be elevated through HS in goats
(Habeeb et al., 1992; Minka and Ayo, 2012;
Kandasami et al., 2023). The respiratory rate was employed as a measure of HS and served to assess the detrimental impact of environmental temperature (
Alamer and Al-Hozab, 2004). For goats, the respiration rate is higher during summer compared to winter (
Fahmy, 1994). In essence, the increased respiration rate during summer signifies HS and can be considered one of the mechanisms employed by animals to dissipate excessive heat through evaporation.
Live body weight
Table 2 depicts the average body weight gain in the experimental goats in both the treatment groups. Results revealed a significantly (p<0.05) lower average daily gain in the HS group in comparison to CON. Current results are in agreement with the reports of
Pragna et al., (2018) who reported reduced average daily gain in Osmanabadi and Malabari heat stressed goats. Upon exposure to high temperatures, goats activate their physiological adaptability to maintain homeothermy
(Aleena et al., 2018). These adaptive processes consume considerable energy, leading the animals to redirect their energy from productive pathways towards the adaptive pathway
(Sejian et al., 2018; Aleena et al., 2018). This behavior, characterized by a reduction in production to prioritize life-sustaining activities, is typically observed in adapted goat breeds.
Hormonal profile
Table 3 depicts the serum concentration of hormones
viz., T3 (nmol/L), T4 (nmol/L) and Cortisol (ng/mL) in the experimental groups. T3 and T4 were important hormones in regulating energy balance
(Hefnawy et al., 2011). Environmental temperature was the main external regulator of the thyroid gland activity (
Dickson, 1993). The present study revealed that T3 and T4 serum concentrations were significantly (p<0.05) reduced in HS animals in comparison to CON. These outcomes are in agreement with the findings reported previously, indicating a decrease in blood concentrations of T3 and T4 during thermal stress (
Silanikove, 2000). Furthermore, an inverse relationship between temperature and the activity of thyroid gland hormone secretion (T3 and T4) has been observed in sheep
(Starling et al., 2005) and goats
(Todini et al., 1992). The thyroid and adrenal glands play vital roles in animal adaptation by regulating heat production within the organism. When animals are exposed to high temperatures, the reduction in thyroxine secretion occurs as a response to reduced thermogenesis requirements, representing a significant step in countering heat stress
(Coelho et al., 2008). This decrease in thyroid hormone release is influenced by the impact of heat on the hypothalamic-pituitary-adrenal (HPA) axis, leading to a decrease in thyroid hormone release and subsequently reducing basal metabolism. Similarly, during cold stress, the increased levels of T3 and T4 stimulate oxygen consumption and heat production in cells, resulting in an elevation of basal metabolism
(Bernabucci et al., 2010).
In response to stressful conditions, the activation of the hypothalamic-pituitary-adrenal (HPA) axis leads to an increase in plasma cortisol concentration (
Silanikove, 2000;
Mormède et al., 2011). Our study revealed that the exposure of Assam Hill Goats to heat stress resulted in an elevation of serum cortisol concentration. These results are consistent with the findings of
Zhengkang et al., (1994), who observed a significant (p<0.01) increase in blood cortisol concentrations in relation to higher environmental temperatures. Additionally,
Mormède et al., (2011) reported an increase in plasma cortisol concentration when animals were exposed to direct sunlight during hot summer conditions. Higher plasma cortisol concentrations in animals subjected to heat stress were also reported
(Wise et al., 1988). The elevation in cortisol levels can be attributed to the direct impact of heat stress, particularly from solar radiation, as stress conditions activate the hypothalamic-pituitary-adrenal (HPA) axis (
Mormède et al., 2011). The hypothalamus responds by releasing corticotrophin-releasing factor (CRF), which acts on the anterior pituitary, leading to the release of adrenocorticotropic hormone or corticotrophin (ACTH). This, in turn, stimulates the adrenal cortex to produce corticosteroid (Cortisol) hormones (
Mormède et al., 2011). Cortisol, the primary glucocorticoid, plays a crucial role in various physiological processes, including the regulation of metabolism, body water distribution, electrolyte balance and blood pressure. Its secretion triggers physiological changes that enable animals to cope with the stress caused by a hot environment. In the initial stages of acute heat stress, the animal’s response is primarily emotional rather than thermoregulatory. However, during severe acute heat stress, cortisol likely contributes to the induction of hyperglycemia, facilitating the anticipated increase in glucose utilization
(Bernabucci et al., 2010). 
