4.1. Effects of Ambient Temperature on Energy Partition and N Balance of Modern Growing Pigs
Pig are warm-blooded animals. At low ambient temperatures, increased feed intake and heat production can facilitate the maintenance of constant body temperature [
20]. The criteria of “low” temperature are defined according to the thermoneutral zone and the LCT value, which was usually estimated by the following equation: LCT (°C) = 17.9 − 0.0375 × BW [
14]. Based on the temperature settings in the current study, all pigs used in the animal trial were kept above the LCT. When the ambient temperature surpasses the UCT, the heat production of the pig body becomes a “burden”, and pigs can keep the body temperature fluctuate in a small range by increasing the evaporation heat loss and decreasing the heat production [
20]. In the current study, we observed decreased Dmi and ME
i when the ambient temperature increased from 23 °C to 32 °C for pigs at both 25 kg and 65 kg, which were in accordance with the previous reports [
1,
2,
3,
4,
5,
6,
7,
8,
9,
19,
20,
21,
22,
23]. Due to the degradation of the sweat gland, the capacity of heat dissipation through the evaporation of pigs is limited [
20]. Therefore, at high ambient temperatures, the feed intake of pigs will decrease dramatically to control their heat production. The impaired feed and energy intake may also attribute to the damaged gut morphology and function of pigs [
24]. But increased THP adjusted into the same ME
i level was observed for pigs at 65 kg in the current study. Close and Mount (1971) also illustrated that the THP was the lowest during 20 °C to 25 °C, and gradually increased when the temperature reached 30 °C [
21], which could be attributed to the extra energy expenditure used for the additional activity, such as more frequent breathing and heart rating to facilitate the heat dissipation [
9]. The above conflict in heat production results may rely on the specific temperature range, BW, and feeding level of the pigs [
22].
The environmental temperature could affect the partition of ME
i, specifically, into maintenance energy, energy for thermoregulation, and energy for growth (the RE) in growing pigs. In the current study, no significant influence of ambient temperatures on maintenance energy (ME
m, estimated by FHP) and the partial energy utilization efficiency of digestion (DE/GE) and metabolization (ME/DE) of pigs at 25 kg and 65 kg were observed. But decreased RQ during the fed state for pigs at 65 kg, and decreased RE
P for pigs at 25 kg and RE, RE
P, and RE
L for pigs at 65 kg were observed when the environment temperature increased from 18 °C to 32 °C, reflecting reduced maintenance energy partition, and protein and lipid synthesis at high ambient temperature especially for pigs at heavier bodyweight. Close (1978) also demonstrated decreased ME
m from 723 to 469 kJ/kg BW
0.75/d when the environmental temperature increased from 10 °C to 30 °C [
23]. The declined partial energy efficiency estimated for protein deposition, and lipid deposition was also reported [
23], which are similar to our observations, indicating that part of the energy was used to dissipate the heat produced during protein and lipid synthesis at high ambient temperature, leading to the decreased partial energy efficiency. This inference could also be supported by the decreased NE/ME ratio for pigs at 65 kg in the current study. Due to the absence of dietary energy gradient settings, the changes of partial efficiency for protein and lipid synthesis at different temperatures were not tested in the current study.
In the current study, we also focused on the effects of ambient temperatures on N balance. When the ambient temperature increased from 18 °C to 32 °C, the N intake, fecal N output, and N retention of pigs at both 25 kg and 65 kg all decreased, also reflecting the reduced protein synthesis at high temperatures. The environmental temperature had no influence on urine N output for pigs at 25 kg, but altered urine N output for pigs at 65 kg in our study, which was partially supported by Guo (2004) and Verstegen et al. (1973), who reported independence of urine N output with ambient temperatures [
2,
22]. Those discrepancies in results of urine N output could be mainly ascribed to the differences in the growing stages of pigs.
It has been widely proven that pigs at different growth stages have distinct patterns of energy partition as the ambient temperatures change, since the critical temperatures differ for pigs with different BW [
14]. In the current study, we found the coefficients of variation for DMi, N retention, ME
i, RE, RE
P, and RE
L were greater for pigs at 65 kg than those at 25 kg, indicating that pigs at heavier bodyweight were more sensitive to high temperatures on energy intake and partition, which is in consistence with the conclusion from the meta-analysis conducted by Renaudeau et al. [
25]. Therefore, the quadratic models, including the main effects of metabolic BW, ambient temperature, and their interaction effects, were used to fit the raw data in the current study and to predict the VFI, ME
i, as well as RE
P and RE
L.
Similar simulation equations have already been developed in some previous studies to model VFI and ME
i in pigs exposed to high ambient temperatures [
8,
19,
22], which were published about 20 years ago and could be treated as reflections of old-genetic pigs raised at different ambient temperatures. Compared these curvilinear models with those developed in our study, it can be seen that pigs in the current study, as a reprehensive of the modern genotypes, were more sensitive to high temperatures on VFI and ME
i reflected by more rapidly-changing tangent slopes of the curvilinear, especially at heavier BW. This observation was also supported by Renaudeau et al. (2011), who analyzed the effects of high temperature on feed intake of pigs with different BW using meta-analysis based on 86 trials and 202 temperature treatments published from 1970 to 2009, and found that the effect of increased temperature was greater in more contemporary works [
25]. This is reasonable because the genetic selection of modern pigs mainly aimed at improving the quality and efficiency of lean tissue growth, neglecting the capacity dealing with heat stress.
One flaw of the models developed in the current study is that the BW data collected for simulation only came from two growth stages, and the narrow range of BW may produce bias when predicting the responses of pigs with other BWs.
4.2. Effects of Ambient Temperature on Hormone and Biochemical Markers in Serum of Modern Growing Pigs
As the ambient temperature increased from 18 °C to 32 °C, there was linearly decreased cortisol, T
3, T
4, HDL, and TC levels, linearly increased AST level, and quadratically changed glucagon level in serum of 25 kg pigs in the current study. Similar changes in physiological and biomedical parameters in plasma of growing pigs under heat stress were also reported previously [
26,
27]. The decreased cortisol content in plasma may indicate improved protein degradation to provide amino acids for gluconeogenesis [
27]. The decreased cortisol level and increased glucagon and AST levels in plasma may also act as indicators for the stress reaction of pigs under extreme environmental conditions [
27]. In addition, the depressed thyroid hormone (T
3 and T
4) levels have been associated with lowered metabolic heat production in pigs kept at high environmental temperatures [
27], which was also supported by the decreased THP in pigs when the ambient temperature increased from 18 °C to 32 °C in the current study. Meanwhile, a reduction in HDL and TC, indicating the decreased β-oxidation of fat for energy purpose and favored hepatic synthesis of triglycerides, could also support the depressed plasma thyroid hormone levels in pigs reared at high ambient temperature [
26].
4.3. Effects of Ambient Temperature on Plasma Metabolomics Profiles of Modern Growing Pigs
To simplify the analysis, only plasma samples from 18 °C, 23 °C and 32 °C were used for metabolomics analysis in the current study. Among the 13 compounds identified, more compounds showed significant changes in pigs at high ambient temperature than those at relatively low ambient temperature, and most of the metabolites in plasma only exhibited slightly changes in pigs at 18 °C compared to that at 23 °C, indicating that modern growing pigs are more sensitive to heat stress reflected by the plasma metabolites. Another reason may be that the “low” temperature settings in the current study may not produce cold stress, since it is above the LCT defined by NRC (2012) [
14].
At high ambient temperature, most of the metabolites with up-regulated expressions belong to fatty acids ((2’E, 4’Z, 7’Z, 8E)-colnelenic acid, 3-beta-hydroxy-5-cholestenoate, and adrenic acid), or involve in pathways related to lipid metabolism or fatty acid metabolism (dihydrocortisol), indicating elevated lipogenic pathways and suppressed fatty acid oxidation. In agreement with our findings, many previous studies also showed that heat stress could increase lipid retention by enhancing lipogenesis and inhibiting lipolysis in pigs [
26,
28,
29]. Qu and Ajuwon (2018) also detected greater serum linoleic and total polyunsaturated fatty acids levels using metabolomics in pigs stayed at 35 °C [
30], which agrees with the observations in our study. These changes in lipid and fatty acid metabolism are also reflected by the reduced HDL, TC, and thyroid hormone levels in serum in the current study. On the other hand, the metabolites with down-regulated expressions at 32 °C are involved in amino acid metabolism (phenylacetylglycine and spermidine), or lipid metabolism (20-hydroxyeicosatetraenoic acid, beta-sitosterol, and leukotriene C4), which may indicate the enhanced protein degradation and gluconeogenesis, as well as the depressed lipolysis, and was partially supported by some previous studies [
31,
32]. Interestingly, cortisol was also identified in the metabolomics analysis with a down-regulated level at high temperature, consistent with the results in the plasma hormone assay, and can be reflected by the decreased RE
P in the energy partition pattern and the decreased N retention pattern in the current study. Furthermore, the suppressed fatty acid oxidation and elevated lipogenesis and protein degradation at high ambient temperature could also be related to the impaired zootechnical performance, especially the decreased feed intake, which were also observed in pigs during fasting [
18].
4.4. Limitations and Prospects of the Current Study
The originality of the current study was the simultaneous measurement of heat production and metabolic indicators in pigs raised at different ambient temperatures. Unfortunately, it has been done only in pigs at 25 kg, due to the budget limitation. According to our results, pigs at 65 kg were more sensitive to heat stress, thus it will be more pertinent to conduct the metabolomics analysis on heavier pigs under heat stress in the future. Moreover, some physiological indicators of heat stress, such as respiration rate, body temperature, and pig behaviors that could contribute to variations of heat production and energy partition, can be complemented in further study. When the results of the current study were applied to practical situations, the specificity of this study should be paid attention to. For instance, the current study demonstrated the short-term response of pigs to high ambient temperatures, considering the absence of adaptation to the temperature changes before the measurements. Unlike the variable environmental temperatures in practice, the temperatures in respiration chambers are constant over the day. In addition, the single-housing and the metabolic cage housing conditions were all different from settings in some previous studies reporting heat stress responses of pigs, and of course, the practical conditions.