Estimates of heat exchange in native breeds of sheep kept in a controlled environmentABSTRACT
The aim of this research was to determine the levels of heat stress and sensible and latent heat loss in sheep of the Soinga, Morada Nova and Santa Inês breeds, kept in a controlled environment at thermoneutral temperatures (TN: 20.0, 24.0 and 28.0°C) and under heat stress (TS: 32.0 and 36.0°C). Eighteen non-castrated male sheep of the Soinga, Morada Nova and Santa Inês breeds were used, with an average age of 5±1.0 months and an average weight of 15±2.3kg. The behavior of the physiological variables as a function of temperature was significant (P.0001), and there was an increase in the values of the physiological variables as the temperature rose. Sensible heat loss showed an inverse correlation with the physiological variables, RT, RR and ST of -0.31; -0.65 and -0.46, respectively. The two principal components together explain 84 % of the variation in the data, but with just one PC, there is 70 % of the variation in the data. In thermal conditions of 36°C, the animals lost almost all, plus 90 per cent, of the surplus metabolic heat in latent form (skin and respiratory tract). As AT rose, RR and ST responses showed higher correlations with total latent and sensible exchange, respectively.
Keywords:
heat stress; latent exchange; sensible exchange
RESUMO
O objetivo desta pesquisa foi determinar os níveis de estresse térmico e a perda de calor sensível e latente em ovinos das raças Soinga, Morada Nova e Santa Inês, mantidos em ambiente controlado, em temperaturas termoneutras (TN: 20,0, 24,0 e 28,0°C) e sob estresse térmico (TS: 32,0 e 36,0°C). O delineamento experimental foi inteiramente ao acaso, com fatorial 3x5, três raças (Soinga, Morada Nova e Santa Inês) e cinco temperaturas (20, 24, 28, 32 e 36°C), com seis repetições. O comportamento das variáveis fisiológicas em função da temperatura foi significativo (P<.0001), e houve aumento nos valores das variáveis fisiológicas à medida que a temperatura subia. Por outro lado, as variáveis temperatura do dorso, pernas, olhos e tímpano não apresentaram diferença significativa (P>0,05) dependendo da raça. A frequência respiratória e a temperatura superficial apresentaram a melhor correlação com o calor latente, com coeficiente de correlação de 0,88 e 0,81, respectivamente. A perda de calor sensível apresentou correlação inversa com as variáveis fisiológicas, TR, FR e ST de -0,31; -0,65 e -0,46, respectivamente. Os dois componentes principais juntos explicam 84% da variação nos dados, mas, com apenas um PC, há 70% da variação nos dados. Em condições térmicas de 36°C, os animais perderam quase todo (mais de 90%) o excedente de calor metabólico na forma latente (pele e trato respiratório). À medida que o AT aumentou, as respostas RR e ST mostraram correlações mais altas com a troca total latente e sensível, respectivamente.
Palavras-chave:
estresse térmico; troca latente; troca sensível
INTRODUCTION
Sheep are widely distributed in different regions and climatic conditions and, due to their adaptability, are raised in the most extreme climatic conditions, such as arid and semi-arid regions (Ben Salem, 2010; Alhidary et al., 2012; McManus et al., 2016), have good productive and reproductive performance, providing important sources of protein in the global economy (Al-Dawood, 2017). In Brazil's semi-arid region, native sheep such as the Soinga, Morada Nova and Santa Inês breeds are used for meat and skin production, as they are wild animals that are extremely well adapted to the region's conditions.
The environmental temperature range of tolerance for adult sheep is between 12.0 and 32.0°C (Cwynar et al., 2014) and the exposure of sheep to solar radiation at ambient temperatures above 35.0°C hinders their thermal balance, becoming an important factor in restricting high productivity during the hot seasons and affecting their economic potential (Liu et al., 2012).
One of the limitations on sheep productivity is heat stress in hot climate regions (Silanikove, 2000; Garcia, 2013), however, animals raised in countries located in the temperate zone are also suffering from the effects of this stress, due to rising summer temperatures and the increased occurrence of heat waves in these regions (Mascarenhas et al., 2023).
High temperatures are the realization of heat stress, as sheep mobilize physiological mechanisms to eliminate the excessive heat load by altering their biological functions (Rodrigues et al., 2023), such as an increase in rectal, surface and tympanic temperature, heart rate and respiratory rate (Maia et al., 2016; Sejian et al., 2018; Fonseca et al., 2019; Furtado et al., 2021; Marques et al., 2021).
Heat dissipation in sheep occurs sensitively as a direct function of the environment, which depends on the thermal gradient between the animal's body temperature and that of the environment (Sejian et al., 2018), a gradient which, the higher it is, the greater the animal's ability to dissipate heat. When this thermal gradient is reduced, there is a reduction in sensible heat loss, mobilizing latent thermolysis mechanisms through the evaporation of water via the respiratory tract and from the skin surface through sweating (Marques et al., 2021; Mascarenhas et al., 2023).
The aim of this research was to determine the levels of heat stress and the sensible and latent heat losses of sheep of the Soinga, Morada Nova and Santa Inês breeds, kept in a control environment at thermoneutral temperatures (TN: 20.0, 24.0 and 28.0 °C) and under heat stress (TS: 32.0 and 36.0 °C).
MATERIAL AND METHODS
The experiment was conducted in a climate chamber at the Agricultural Engineering Academic Unit, Rural Constructions and Environment Laboratory of the Federal University of Campina Grande, municipality of Campina Grande-PB.
The chamber measures 6.14 m long, 2.77 m wide, with a built area of 17.00 m2, made of laminated steel sheets with anti-corrosion protection and filled with polystyrene foam.
The research procedures were approved by the Research Ethics Committee (REC) of the Universidade Federal de Campina Grande, Paraíba State, Brazil, CEP Protocol No. 097.2019.
Eighteen non-castrated male sheep of the Soinga, Morada Nova and Santa Inês breeds were used, with an average age of 5±1.0 months and an average weight of 15±2.3kg, dewormed at the beginning of the experiment, were kept in 3 collective stalls inside the chamber, in groups of eight animals. The feeding and water supply to the animals was ad libitum, and the ration was offered at 7:00 am and 5:00 pm, with daily adjustment of consumption based on leftovers, and consumption was quantified by the total supplied minus leftovers in a 24-hour period. Water was supplied ad libitum, and the consumption was quantified according to the total supplied minus the leftovers in the period of 24 hours, the amount of water was weighed on a precision scale.
The diet consisted of Tifton hay (Cynodon dactylon (L) Pers), which constituted 39.9% of the total ration volume, crushed corn (43.4%), soybean meal 33 (11.2%), urea (1.0%), calcitic limestone (1.0%) and vegetable oil (3.5%), according to the composition indicated by (Nutrient…, 2007).
During 15 consecutive days, the sheep breed (Soinga, Morada Nova and Santa Inês) were exposed to temperatures of 20.0, 24.0, 28.0, 32.0 and 36.0 ºC with relative humidity of 65%, with 10 days of adaptation and 5 days of data collection. The animals were exposed for 8 hours to continuous exposure at each temperature. Between exposures and temperatures, the animals were outdoors for 5 days to eliminate the residual effect.
The cooling systems used were SPLIT air conditioners, dehumidifiers and humidifiers. The control room is located next to the chamber where the air temperature and humidity monitoring board is located.
For humidification and dehumidification, commercial humidifiers and dehumidifiers were used coupled to the MT-530 PLUS control system from Full Gauge Controls®, configured via SITRAD software, responsible for acquiring and storing air temperature (AT) and relative humidity data (UR). Data acquisition via SITRAD software was carried out using a thermistor and a humidistat, both located in a permeable envelope and positioned at the height of the animal’s center of mass (±1.50m), and wind speed was recorded using an anemometer.
To determine the relative humidity, air temperature and dew point temperature (DPT), Instrutherm HT-500 datalogger thermohygrometers® were used located at the height of the animals’ center of mass. The black globe temperature was obtained with the aid of black globes made with a plastic sphere and a mercury thermometer and calibrated with the aid of a standard black globe with a copper sphere of the Instrutherm brand, positioned in the center of the installation, approximately 1 meter high from the floor of the installation.
To check the wind speed, an anemometer was installed inside the climatic chamber at the height of the animals' center of mass so that it could record the movement of the air caused by the exhaust fans.
The physiological variables rectal temperature (RT), respiratory rate (RR), and surface temperature (ST) were measured in the morning, from 10 a.m. to 10:30 a.m., and in the afternoon, from 3 p.m. to 3:30 p.m.
Rectal temperature was determined by inserting a veterinary clinical thermometer inserted directly into the animal's rectum (depth of 2.0 cm) with the bulb close to the mucosa, remaining inserted until the reading stabilized. The respiratory rate was obtained by directly counting flank movements during a period of 15.0 seconds, extrapolating to one minute (mov min-1). Heart rate was measured with the aid of a flexible stethoscope, positioned directly over the left thoracic region at the approximate height of the aortic arch, counting the number of movements for 15.0 seconds, and the collected value was multiplied by four to determine the heart rate in bat min-1.
The surface temperature was obtained through an Infrared Thermal Imaging Camera (Fluke Ti 25, USA) with automatic calibration, when the animals remained motionless, without any restriction and with minimum manipulation, avoiding causing possible stress to them. Subsequently, the thermograms were analyzed by the Smartview software, version 4.1, through which mean temperatures were obtained from three regions covering most of the animal's body (including neck, side and thigh), to obtain the average surface temperature, considering an emissivity of 0.98.
The equations relating to the exchange of heat were used as a basis Marques et al. (2021).
The calculated mean values of kinematic viscosity (v), density (ρ), thermal conductivity (k), specific heat of the air (Cp) and partial air vapor pressure (ea), as well as the dimensionless values of Nusselt number (Nu), Reynolds number (Re) and Prandtl (Pr), for the five treatments evaluated (Table 1)
The normality of the data was verified using the Shapiro-Wilk test (P > 0.05) and, subsequently, the data were analyzed by analysis of variance (ANOVA) the means were compared by the PROC GLM of SAS (2001) and the Tukey test was used at 5% probability. A correlation analysis was carried out using PROC CORR from SAS (2001) between the physiological variables (RT, RR and ST) and latent and sensible heat.
The PCA was performed by PRINCOMP procedure (SAS, 2001), relative importance was assessed by eigenvalues (variances), thereby defining the factors to be extracted by the method of varimax rotation for better interpretability.
RESULTS
The physiological variables showed no significant difference (P>0.05) in the temperature vs. breed interaction (Table 2). The behavior of the physiological variables as a function of temperature was significant (P.0001), and there was an increase in the values of the physiological variables as the temperature rose. On the other hand, the back, leg, eye and eardrum temperature variables showed no significant difference (P>0.05) depending on the breed.
As the temperature increased, the heat exchange mechanisms of radiation, convection and total sensible heat exchange were significantly reduced (P < 0.05). Compared to 36 °C, the reductions in total sensible heat exchange (Table 3) were 91.84, 90.35 and 86.74 % at 20, 24 and 28 °C, respectively. On the other hand, respiratory tract latent heat exchange, skin latent heat exchange and total latent heat exchange increased with increasing temperature. At temperatures of 20, 24 and 28°C, latent heat exchange corresponded to 38.56, 48.10 and 62.21%, respectively.
The heat exchanges by radiation, convection and total sensible heat of the breeds at the different temperatures showed no significant difference (P>0.05) (Table 3). However, there was an increase in heat flow in the respiratory tract and skin of the Morada Nova and Soinga breeds (P<0.05), indicating a greater capacity for heat dissipation in these breeds.
At 20, 24 and 28 °C, all three breeds showed very similar values, where, on average, 57.90, 44.05 and 30.19 % of the total dispersion of the animals' surplus metabolic heat was in sensible form. Of these, 45.46, 47.20 and 57.67 % were by radiation, while 54.54, 52.80 and 42.33 % were by convection, respectively. In addition, 42.10, 55.95 and 68.81 % of the excess heat was dissipated latently, with 30.13, 38.00 and 37.35 % being eliminated via the respiratory tract and 68.88, 62.00 and 62.65 % via the skin (Table 3).
Although the animals lost most of their excess metabolic heat through the skin (latent) at 20, 24 and 28 °C, they were in homeostasis conditions. It was observed that in these three conditions, latent heat exchange mechanisms, especially cutaneous heat exchange, played the most important role in controlling homeothermy.
The temperatures of 32 and 36 °C, there was a significant reduction in the ability of the three breeds to dissipate body heat sensibly. Thermal exchange through respiratory and cutaneous evaporation accounted for 81.23 and 94.61 %, respectively. At 32 °C, 68.44 % was attributed to cutaneous mechanisms and 31.12 % to the respiratory tract, while at 36 °C, 85.12 % occurred via the skin and 14.88 % via the respiratory tract, standing out as the most important means of releasing excess heat at high temperatures.
The temperature of 36°C, practically all the surplus metabolic heat produced by the animals was dissipated evaporatively (94.50 per cent), with skin losses making the greatest contribution. However, there was a significant increase (P<0.05) of 539 per cent in the animals' respiratory rate under these conditions, compared to the 24°C temperature (Table 3).
The correlations between the physiological responses rectal temperature (TR), surface temperature (TS) and respiratory rate (FR) with total sensible heat loss (Gs) and total latent heat loss (Et) are shown in Figure 1, with a 95% confidence interval. Among the responses evaluated, respiratory rate and surface temperature showed the best correlation with latent heat, with correlation coefficients of 0.88 and 0.81, respectively. Sensible heat loss showed an inverse correlation with the physiological variables, RT, RR and ST of -0.31; -0.65 and -0.46, respectively.
Correlation between physiological responses and total sensible and total latent heat exchange.
The two main components together explain 84 % of the variation in the data, but with just one PC, there is 70 % of the variation in the data (Table 4). The variables that make up PC1 are neck hair temperature, chest hair temperature, back hair temperature, leg temperature, eye temperature and eardrum temperature, with only eardrum temperature showing inverse behavior in PC1. Radiation and convection behave inversely to PC1 and cutaneous and total sensible heat are representatives of PC1. The PC2 is represented by RR, skin temperature of the neck, chest and back, latent respiratory tract, total latent and total heat exchange.
Principal components (PC), eigenvalue and cumulative variation of the variables studied in the different sheep breeds at different temperatures
DISCUSSION
The heat stress caused by high AT and RH levels makes the dissipation of latent heat via respiratory and skin evaporation one of the essential means of thermoregulation (Silva et al., 2021). RH is another variable of great importance for sheep. Leitão et al. (2013) emphasize its importance, since it has a high effect on the well-being of sheep and can affect their productivity, especially when it contains high values combined with high temperature indices, as is the case in tropical regions. This scenario makes it difficult to dissipate body heat through evaporative processes (Baêta and Souza, 2010).
The difference between the temperature of the epidermis and the surface of the coat can influence the flow of thermal energy in sheep, regardless of whether they have wool or not (Leite et al., 2020). Although the presence of white wool can reflect more radiation, absorb less heat and keep the pelt temperature lower, it can also create a physical barrier that hinders convective heat exchange.
When there is a progressive reduction in the thermal gradient between body temperature and ambient temperature, the processes of conduction, convection and radiation become less efficient, making it necessary to resort to evaporative thermolysis to maintain thermal balance (Fonseca et al., 2019). The Santa Inês breed, on the other hand, has been shown to have a lower heat removal capacity. This can be attributed in part to their black coat, which tends to absorb more heat, resulting in a higher surface temperature, even when the animals are kept in controlled environments (Furtado et al., 2021).
The high contribution of heat exchange through the epidermis, to the detriment of the low contribution of convective exchange to the dissipation of excess metabolic heat produced by the animals in conditions of thermal comfort, may have been influenced by the low air movement in the climate chamber, with an average of 0.5 m/s (Marques et al., 2021).
According to Maia et al. (2016), when sheep are placed in air temperatures below 26°C, they remove the excess metabolic heat to the environment, mainly through free convection and long-wave radiation. When the animals are left in the 26 to 30°C range, latent heat loss starts to become more important, which is why in the two conditions evaluated, 32 and 36 °C, latent heat exchange mechanisms, especially cutaneous heat exchange, played the most important role in controlling homeothermy.
Joy et al. (2020) reported that respiratory evaporation is much less significant than skin evaporation. Given that skin evaporation can account for up to 90 per cent of the total energy dissipated in small ruminants (Costa et al., 2014; Maia et al., 2015, 2016). When the ambient temperature rises, the efficiency of sensible heat loss decreases, due to the lower temperature gradient between the animal's skin and that of the environment. At this point, the animal can maintain its body temperature to some extent through vasodilation, which increases peripheral blood flow and skin temperature. However, if the ambient temperature continues to rise, the animal becomes dependent on heat loss through evaporation, respiration and/or sweating (Souza et al., 2008). This heat loss capacity is related to the thermal gradient between the animal's surface temperature and that of the environment. As the activation of latent heat loss mechanisms (respiration and transpiration) occurs with less efficiency than sensible heat loss (radiation, conduction and convection) (Batista et al, 2014).
The body surface of animals is in direct contact with the surrounding environment, and its temperature is directly influenced by variations in air temperature, causing variations in the capacity for heat exchange by sensible means, and this capacity for heat loss is directly related to the thermal gradient between the surface temperature and that of the environment (Souza et al., 2008). Marques et al. (2021) concluded that, based on the magnitude of Pearson's coefficients of determination, among the physiological responses, surface temperature (r =-0.643) stood out as the physiological response that best represented the behavior of heat exchange in the sensible form of Boer goats subjected to the environmental conditions tested.
Ribeiro et al. (2015) evaluated the adaptive behavior of native Garfagnina goats using multivariate techniques and observed that biochemical and hormonal variables had great discriminatory power. Classification errors depend on the characteristics used in the analysis (Yakubu et al., 2012; Correa et al., 2013). Greater classification errors were observed when considering only anatomical and physiological characteristics, which was also reported by Correa et al. (2013). According to Castanheira et al. (2010), physiological variables are good indicators of animal health, but they must be interpreted appropriately, as they can be influenced by species, age, exercise and stress level.
CONCLUSIONS
In thermal conditions of 36°C, the animals lost almost all, plus 90 per cent, of the surplus metabolic heat in latent form (skin and respiratory tract).
As air temperature increased, respiratory rate and surface temperature responses showed higher correlations with total latent and sensible exchange, respectively
ACKNOWLEDGEMENTS
The authors wish to thank the Brazilian National Council for Scientific and Technological Development (CNPq).
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Publication Dates
-
Publication in this collection
21 Feb 2025 -
Date of issue
Mar-Apr 2025
History
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Received
08 May 2024 -
Accepted
19 Sept 2024
