Heat tolerance of goats to increased daily maximum temperature and low salinity of drinking water in tropical humid regions

Study animals, feeding management, and treatments

This experiment was approved by the animal care and use committee of the Faculty of Agriculture, Universitas Sriwijaya (No. KPPHP-2022–4). Twelve female Kacang and Kacang Etawa Crossbreeds (BW, 14.0±0.7 and 18.4±0.9 kg, respectively) between ten to twelve months old were used in a 112-day trial from November 2022 to February 2023. During the trial, the goats were weighted weekly and maintained in individual metabolic pens (1.5×0.75 m) designed to ease the collection of fecal and urine excretion and feed samples.

The goats were offered chopped Guinea hay and a concentrate mixture (57.5% corn meal, 23.3% copra meal, 15% fish meal, 3% molasses, 0.3% urea, and 1% mineral mix, dry matter [DM] basis). The animals were fed (ad libitum) with hay twice a day at 09:00 and 16:00 in equal portions, whereas the concentrate was offered at 2.5% of BW at 09:00.

The experiment was conducted as a complete Latin Square (4×4) with four treatments, four periods, and each lasting 28 days. The goats were divided into three groups based on their initial BW and each goat was allocated into the four treatment groups of three goats each. The groups were switched to the other treatments in the next periods and the treatments order was balanced to minimize carryover effects [22]. The treatments were combinations of two climatic conditions (control, 25°C to 33°C, 83% RH, THI: 76 to 86; and hot environment, 26°C to 39°C, 81% RH, THI: 77 to 94) and two salinity levels (0% and 0.4% NaCl). The THI values were followed formula of Thom [23] as follows: THI = 0.8×T+((RH/100)×(T–14.3))+46.4 (Thom [23]), where T is air temperature and RH is the relative humidity.

The experiment was conducted in two climatic rooms. The climatic environment in the control treatment room followed the daily climatic environment outdoors, where in the hot treatment room, the daily maximum temperature was increased to ≤39.2°C by electric air heaters equipped with a thermostat (XH W3001) from 10:00 to 16:00. The climatic data during the four experimental treatments is presented in Table 1. Drinking water was provided ad libitum throughout the experimental periods. The 0.4% saline water was prepared by adding a defined amount of 99% pure salt (Braun Ltd., Mumbai, India) to the well water. The characteristics of the drinking water offered to the goats are shown in Table 2.

Table 1

Climatic data collected in control and high daily maximum temperature humidity index (hot) during the four experimental periods

Table 2

Concentrations of substances in drinking water offered to treatment groups

Measurements, sample collection, and analysis

Data on indoor ambient temperature and RH in both experimental rooms were recorded continuously every 10 minutes throughout the study using two data loggers (Benetech G1365; Shenzhen Jumaoyuan Science & Technology Co., Ltd, Shenzhen, China). The drinking water sample was collected each week, and then the samples were pooled at the end of each period. The water samples were analyzed to determine pH, electrical conductivity (Hanna HI 98130; Hanna Instruments, Inc., Nusfalau, Romania), total dissolved solids (Milwaukee Mi 180 Bench Meter; Milwaukee Instruments, Inc., Rocky Mount, NC, USA), sodium, and chlorides (inductively coupled plasma atomic emission spectroscopy Varian 715-ES; Agilent, Santa Clara, CA, USA).

For thermo-physiological parameters, rectal (RT), skin (ST), udder temperatures (UT), and respiration rates (RR) were recorded at 0 (11:00), 3, 6, 9, 24, 48, 72, and 240 hours after transferring the goats to the hot conditions. The RT was measured using a digital clinical temperature (Omron MC-343F; Omron Healthcare Singapore Pte Ltd, Singapore), while the UT and ST (in a shaved area at the shoulder) were measured with an infrared thermometer (Beureur FT 90; Beurer Indonesia, Jakarta, Indonesia) at ±2 cm. The RR was determined by counting the flank movements when the goat was lying.

On day 12 of each period, a blood sample of each goat was obtained from the vena jugularis between 11:00 and 12:00. The samples were transferred to an ethylenediaminetetraacetic acid-containing blood tube and then placed in box ice. Other blood samples were also taken in a blood tube containing a gel clot activator for glucose, urea, and creatinine analysis, as well as a blood tube containing heparin for the determination of blood plasma electrolytes. The samples were then transported to the laboratory for hematological analysis. Hemoglobin, erythrocytes, leukocytes, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) levels were measured with Sysmex XP-100. Glucose, cholesterol, and creatinine concentrations were measured with Humastar 100 (Human Diagnostic Asia Pacific Pte Ltd, Singapore), while blood plasma electrolytes (Na⁺, K⁺, Cl⁻) were measured with Amtast CBS-400 (Amtast USA Inc., Lakeland, FL, USA).

Throughout the trial, feed and water consumption were recorded daily. Samples of offered and refused feed and fecal excretion were collected over six consecutive days of the collection week (days 22 to 28) to determine feed intake and digestibility. The hay and concentrate (±100 g each) were sampled twice a week, while refusal feed (±100 g) and feces (±10% of the daily fresh defecation) were sampled daily. The composted samples were dried at 45°C for 72 hours and then ground through a 1-mm mesh for DM content analysis.

The determination of the water balance was also established during the collection week. For the estimation of daily water evaporation, two identical buckets were placed in each room. The buckets were weighed each morning, and the values were used to correct the water intake. Meanwhile, urinary water excretion was determined by weighing the daily urine excretion. A 40-mL of urine sample was taken and stored (−20°C), and at the end of each period, the samples were pooled per animal, and 3 mL samples were then dried at 60°C for two days to correct the water content of the urine. Water retention was calculated from the difference between total water intake and excretion in the urine and feces.

The behavioral responses of goats were observed three times per period at weeks 2 and 3 using a camera system (Hikvision, Hangzhou Hikvision Digital Technology Co., Ltd., Zhejiang, China) for six hours from 10:00 to 16:00. The goat activities such as standing idle, standing, and ruminating, lying idle, lying, and ruminating, and eating were quantified at five-minute intervals to determine the time spent on the behavioral responses. Moreover, the change in position (lying to standing and reverse) and the frequency of urination and defecation were counted during the six hours of recording.

Statistical analysis

The data generated from four treatments, four periods, and twelve goats was analyzed using the PROC MIXED procedure of the statistical analysis system (SAS Inst., Inc., Cary, NC, USA). The treatments (thermal environment and salinity), experimental periods, and their interactions were included in the mixed model as fixed effects and the animal as a random effect. For the behavioral data, the repeated measurements were accounted for via sampling day with compound symmetry as a covariance structure. The significance level was set at p<0.05 and the results are presented as arithmetic means ±standard error.

Thermo-physiological and hematological responses

In tropical humid regions, optimal livestock production is partially limited by the high daily maximum temperature and RH, and the risk of severe heat stress is projected to be elevated in many tropical regions as global warming progresses [19]. With current climatic conditions, the goats in the present study experienced high levels of heat stress for more than 12 hours per day when THI levels were ≥79 Serradilla et al [20]. As homeotherms, the elevated THI levels in the hot thermal environment treatment (≤94) impacted the thermo-physiological systems to maintain their normal body temperature by increasing the body temperature and the evaporative respiratory heat disposal mechanism [16]. The elevated level of thermoregulation response was seen three and six hours after heat exposure (p<0.05). However, after 24 hours, the body parts temperature and breath rate were lowered, indicating a partial adaptation to the heat stress conditions (Figure 1). A similar finding was also reported by previous studies on goats suffered maximum heat stress during the first [24] to third days [25,26] of heat exposure.

Figure 1

Change of respiration rate (A) and temperature of skin, udder, and rectum (B) of goats exposed to a hot thermal environment. The standard errors are shown as vertical bars. The thermoregulation responses reached a maximum level on the first day of heat exposure.

The RT range in this study (38.1°C to 40.0°C) was comparable to that of Brazilian breed goats (38.3°C to 39.5°C), while the ST range (36.0°C to 40.2°C) was higher than that of those breeds (31.4°C to 38.6°C) [21]. A similar range of ST (35.5°C to 40.5°C) was reported in a tropical semi-arid region when the daily maximum temperature reached 43.5°C [27]. However, the RR range in the previous study (27.5 to 38.0 breaths/min) was much lower than the present work (14.9 to 183.1 breaths/min). Similar results (38.3°C to 40.4°C and 30 to 150 breaths/min, for RT and RR, respectively) were reported in Murciano-Granadina goats [25]. The difference was probably due to variances in the treatment conditions, breed, and physiological states.

As shown in Table 3, hematological responses were not significantly different between the treatments. Plasma potassium and chloride levels did not change, while sodium levels were higher at the 0.4% than at 0% NaCl but still within the ranges in previous studies [10,12,28]. Compared to the previous study, elevated sodium concentrations were also reported in goats with higher levels of salinity in drinking water [10,28]. Thiet et al [12] reported that the sodium, potassium, and chloride concentrations did not differ between two levels of salinity, while Zoidis and Hadjigeorgiou [28] and Runa et al [10] found an increase in the electrolyte levels. The urea and creatinine levels represent renal function; an increase in salt concentration and extended exposure may result in a rise in the levels [10,28].

In this study, the increase in RR did not result in a higher hemoglobin level. The elevated RR was linked to an increase in oxygen intake [16]. Abdelatif et al [29] also reported the non-significant influence of THI levels on the erythrocytes, leukocytes, MCV, and MCHC levels in Nubian goats. However, Alam et al [30] found higher hemoglobin, erythrocytes, and leukocytes levels in heat-stressed goats. Considering the effect of the salinity levels, Tulu et al [13] also reported that the levels of hemoglobin, erythrocytes, leukocytes, MCV, and MCHC were not changed.

When feed intake was reduced, a lower glucose level was associated with higher insulin activity [16,21,31–33], whereas Hamzaoui et al [25] found a non-significant change in glucose levels when food intake decreased in lactating goats. The inconclusive results were also observed in cholesterol levels when the animals experienced heat stress. The increased cholesterol level was associated with triglyceride mobilization [21,34], while the decrease was related to increased water retention, lowered thyroid activity, and feed (cholesterol) intake [16,31,33,35]. In the current study, it appears that feed intake had a greater impact on glucose and cholesterol levels than the major shift in body water retention. This also applied to the effect of the salinity levels; the insignificant change in glucose levels in the present study seems to be related to the intake level [28].

Feed intake and water balance

The decreased feed intake and increased water intake in the heat-stressed animals are interpreted as an attempt to minimize heat production and maintain body fluid balance [16,21]. In the present study, the feed and water intake were not changed, but the ratio of water to feed intake was increased by THI and salinity levels. Since the measurement was conducted on days 22 to 28, according to the thermo-physiological responses (Figure 1), the non-significant change in feed intake may represent a goat’s capacity to adjust to the increased THI level. Moreover, the elevated drink water intake (p = 0.0517) at the 0.4% NaCl level might be related to the significant increase in plasma sodium concentration (Table 3) and urinary and fecal water excretion (Table 4) to maintain sodium balance by increasing water intake and excretion. Similar results were also reported in deer [36], lamb [37], and goat [28]. The increased THI, on the other hand, reduced water excretion in order to maintain body water retention (Table 4) for the evaporative cooling process [25]. It appears that a higher level of salinity (≥0.4%) could have adverse effects on heat-stressed goats.

Behavioral responses

The duration and frequency of the various activities reflect goat comfort under hot conditions. Table 5 shows the observed behavioral indices in goats, where only the thermal environment treatment altered the behavioral responses. The increased drinking duration in hot conditions corresponded to the significant increase in the drinking water/feed intake ratio (Table 4) to maintain the body fluid balance.

Furthermore, total lying was longer than standing time in both thermal conditions, and the goats spent more time lying at the elevated THI level. The longer time spent lying during the hot conditions might be an adaptive behavior to reduce additional heat production from body movements, and this corresponded to the decrease in change position frequency. The longer lying time during heat stress was also reported in goats [26,38,39] and sheep [40], but another study in goats [41] found that lying time was lowered during heat stress. In addition, Salama et al [26] reported that changing the position increased under heat stress, reflecting goats’ uncomfortable behavior. The opposing reaction may be related to the period of behavioral observation. The observation in their study was conducted under maximal heat stress (day 3), whereas the observation in this study was taken after the first week of each experimental period.

The longer total ruminating time in the hot conditions could be related to the significant improvement of DM digestibility (Table 4). Chewing time increased as ruminating time increased, and was attributed to a lower feed particle size and a higher ruminal fermentation efficiency [42,43]. A higher digestibility as a result of the lowered feed intake was reported in goats [44] and heifers [45] during hot environments. However, an opposite result was evidenced in cows [46] and goats [38] where ruminating time was shorted during heat stress conditions. In the present study, the goats spent more time for rumination in the lying position than in the standing position. Previous studies reported that rumination time was related to the lying time in dairy cows, indicating that periods of rumination were more frequent when the animal were lying down [47,48].

Experimental periods effect

The significant experimental period effects were found for several parameters. Climatic conditions during the four periods (Table 1) fluctuated and significantly influenced the animal responses (p<0.05). The daily maximum THI elevated for only a six-hour period (10:00 to 16:00), and although significant changes in the animal responses were observed, the influence of the fluctuation of temperature and RH during the non-treatment period (16:00 to 10:00) could not be neglected.