This study investigated the effects of a low-protein diet with or without an increase in dietary protein and feed-grade amino acids (AAs) on the growth performance, body composition, metabolism, and serum acute-phase proteins of finishing pigs reared in thermoneutrality or cyclic heat stress conditions. A total of 90 gilts (67.7 ± 6.2 kg) were distributed in a 2 × 3 factorial arrangement (two ambient temperatures and three diets). Ambient temperatures (AT) were thermoneutral (TN, 22 °C for 24 h) and cyclic heat stress (CHS, 12 h to 35 °C and 12 h to 22 °C). The evaluated diets (D) were high crude protein (HP); low CP-free AA-supplemented diets (LPAAs); low CP-free AA-supplemented diets and digestible Lys level (+20%), and Lys:AA ratios above recommendations (LPAA+). The experimental period lasted 48 d (two experimental phases: days 0–27 and days 28–48, respectively). CHS pigs had higher skin temperature (P < 0.05) than TN pigs. Pigs in CHS had higher rectal temperature (P < 0.05) than TN pigs until day 38 but similar (P > 0.10) to TN pigs from 38 to 45 d. For the entire experiment, CHS pigs had lower (P < 0.05) final BW, average daily gain and daily feed intake, net energy intake, body lipid, bone mineral, lipid deposition, energy retention, Lys and CP intake, and nitrogen excretion than TN pigs. The level of CP intake impacted nitrogen excretion, nitrogen retention efficiency, and urea as pigs fed HP had the highest values, and pigs fed LPAA had the lowest values (P < 0.05). On day 27, CHS pigs had lower (P < 0.05) free triiodothyronine than TN pigs. LPAA+ pigs had lower (P < 0.05) insulin than LPAA. On day 48, CHS pigs had lower (P < 0.05) thyroxine, albumin, and lactate than TN pigs. On day 27, pigs fed LPAA+ had higher (P < 0.05) lactate than pigs fed HP or LPAA. Both AT and D were enough to stimulate the immune system as CHS pigs had lower (P < 0.05) transferrin and 23-kDa protein levels than TN pigs, and HP pigs had higher haptoglobin than LPAA on day 27. These results confirm the deleterious effects of high AT on performance, body composition, metabolism, and immune system stimulation in finishing pigs. These data also show that a diet with low levels of CP can be provided to pigs in CHS without affecting performance and body composition while reducing nitrogen excretion. However, the use of a diet with an AA level above the requirements obtained by increasing intact protein and free AA did not attenuate the impact of CHS on performance and body composition of pigs. High ambient temperature and air humidity are the most important climatic factors that jeopardize pig production. Multiple strategies can be applied for pigs under heat stress, including recent research to improve understanding the use of nutrition to attenuate the impact of heat stress. Heat stress impairs digestion, absorption, and amino acid metabolism with changes in amino acid requirements. Updates on the nutritional assessment strategies by differing the diets by protein and amino acid content (protein-bound or feed-grade) seem to be efficient tools for pork producers as amino acids play a functional role in challenged pigs apart from the beneficial effects on performance.

The impact of cyclic heat stress (CHS) and turning the lights on and off on pig feeding behavior (FB) was investigated. The FB of 90 gilts was recorded in real-time under two ambient temperatures (AT): thermoneutrality (TN, 22 °C) or CHS (22/35 °C). The day was divided into four periods: PI _{(06–08 h)}; PII _{(08–18 h)}; PIII _{(18–20 h)}; and PIV _{(20–06 h)}. Automatic and Intelligent Precision Feeders recorded each feed event for each pig. An estimated meal criterion (49 min) was used to calculate the FB variables. Feed behavior in both ATs followed a circadian pattern. The CHS reduced the feed intake by 6.9%. The pigs prioritized feed intake during the coolest hours of the day; however, nocturnal cooling did not allow the pigs to compensate for the reduced meal size due to CHS. The highest meal size and most of the meals were observed during the lighting-on period. The pigs reduced their interval between meals during PII and PIII. The lighting program increased the meal size when the lights were switched on and reduced the meal size when the lights were switched off. Thus, the dynamics of the FB were largely influenced by AT, whereas the meal size was affected by the lighting program.

The pig microbiome composition is affected by factors such as dietary changes, genetics, and diseases. Recent evidence suggests that housing temperature may also contribute to the variability in community structure and composition. Therefore, we investigated the interactive effects of different nutritional strategies and heat stress (HS) on the fecal microbiota composition, community structure, taxon distribution, and taxa correlation structure of pigs. Forty-eight (Landrace × Large White) finishing gilts with an average of 67.7 ± 6.2 kg of body weight (BW) were distributed in a 2 × 3 factorial arrangement: two temperatures [thermoneutral (TN, 22°C for 24 h) and cyclic heat stress (CHS, 12 h to 35°C and 12 h to 22°C)] and three diets varying in the dietary crude protein (CP) contents and amino acid (AA) levels [high CP (HP); low CP-free AA-supplemented diet (LPAA); low CP-free AA-supplemented diet and digestible Lys level (+20%), and Lys:AA ratios above recommendations (LPAA+)] originating six treatments (eight replicates of one pig). Pigs were fed ad libitum throughout the study. The 16S ribosomal RNA (rRNA)-based microbiome analysis was conducted in fecal samples collected on days 0 and 27 (endpoint). Overall, microbiome analysis suggested an increased richness in the fecal microbiome of pigs raised in TN conditions fed a diet supplemented with higher levels of AA (LPAA+). In addition, changes in the fecal microbiome composition indicated that Mogibacterium was significantly diminished in the feces of pigs fed the LPAA diet when compared to pigs fed the LPAA+, both in CHS conditions. Oscillospira was reduced in the feces of pigs fed a diet containing exclusively protein-bound as the source of AA, while the more the feed-grade AA was included in the remaining diets, the more the abundance of this taxon in fecal samples. Despite dietary alterations, Corynebacterium was enriched under CHS compared to TN, whereas the enrichment of Prevotella and Eubacterium hallii group was higher in the TN group. Outcomes of this study suggest that changes in fecal microbiota composition were mainly associated with temperature, pointing toward potential taxa that may contribute to physiological adaptation to heat stress.

The impact of cyclic heat stress (CHS) and lighting events on pig feeding behavior (FB) was investigated. The FB of 90 gilts was recorded in real time under two ambient temperatures (AT): thermoneutrality (TN, 22 °C) or CHS (22°/35 °C). The day was divided into four periods: PI (06-08 h); PII (08-18 h); PIII (18-20 h); and PIV (20-06 h). Automatic and Intelligent Precision Feeders recorded each feed event for each pig. An estimated meal criterion (49 minutes) was used to calculate the FB variables. Feed behavior in both ATs followed a circadian pattern. The CHS reduced the feed intake by 6.9%. Pigs prioritized feed intake during the coolest hours of the day; however, nocturnal cooling did not allow the pigs to compensate for the reduced meal size due to CHS. The highest meal size and most of the meals were observed during the lighting-on period. Pigs reduced their interval between meals during periods II and III. The lighting program increased the meal size when the lights were switched on and reduced the meal size when the lights were switched off. Thus, the dynamics of the FB were largely influenced by AT, while the meal size was affected by the lighting program.

Immune system stimulation (ISS) alters the metabolic demand for amino acids (AA), and amino acid-based nutritional strategies may mitigate performance reduction in pigs subjected to sanitary challenge conditions. This study aimed to evaluate the effect of extra supplementation of a combination of functional AA (FAA, +20% threonine, methionine, and tryptophan) preventively (pre-ISS), curatively (during-ISS) or both on the performance of growing pigs challenged with Salmonella Typhimurium (ST) and poor housing conditions. Fifty-two entire male pigs (21.7 ± 4.6 kg) were allocated to four dietary treatments, with a control AA profile diet fed throughout the experimental period (Control; NRC, 2012) or a FAA supplemented diet (FAA+; supplied +20% Trp: Lys, Thr: Lys, and Met+Cys: Lys above Control) fed only before the ISS (FAA Preventive), during the ISS (FAA Curative), or before and during the ISS-period (FAA Continuous). On day 0, after a 7-d pre-ISS-period (day −7 to −1, pre-ISS), all pigs were inoculated with ST (2 × 109 CFU/mL). Additionally, manure from a commercial pig farm was spread on the solid concrete floor, and the room was not cleaned during the ISS-period for 4 wk (0 to 28 d). Thereafter, all pigs received the same standard diet for 9 wk (days 29 to 91), and the facilities were cleaned daily. Pigs were group-housed and fed using five electronic precision feeders during pre-ISS and ISS-period (−7 to 28 d). There was no effect of providing additional FAA before the ISS-period on growth performance (P > 0.10), indicating that the control diet met at least the requirements under non-challenged conditions. Rectal temperature increased by 1.25 °C on day 1 of the ISS-period (P < 0.05) and remained higher than pre-ISS during days 2 to 7 of the ISS-period (P < 0.05). Control group had higher rectal temperature than the FAA Preventive (P < 0.05). Serum haptoglobin was higher on day 7 and 28 (P < 0.05), while albumin was lower on day 28 (P < 0.05), with lower concentrations in the control group compared with the FAA curative (P < 0.05). Higher final BW (P < 0.05), ADG (P < 0.05), and G:F (P < 0.05) were observed in the FAA Continuous compared to the Control. On day 28, FAA curative pigs showed higher total body protein content than FAA preventive pigs (P < 0.10). Nitrogen utilization efficiency improved by 32.6% (P < 0.05) for the FAA Curative compared with the Control. There were no remaining effects of treatments on BW or body composition on day 91 (P > 0.10). In conclusion, the combination of short-term preventive and curative strategies with FAA dietary supplementation modulated the immune response and improved the growth performance of growing pigs under chronic immune system stimulation.

Sanitary challenges (SCs) may alter the health status, growth performance, and pigs’ welfare. Changes in amino acid (AA) plasma concentrations have been observed in inflammatory-challenged pigs which may be associated with key factors, such as: (1) the synthesis of immune components to support innate and/or adaptive immune responses, (2) the redistribution of nutrients from growth and production functions toward cells and tissues involved in inflammatory and immune responses, and (3) decreased anabolism and/or increased catabolism of skeletal muscle to increase the availability of nutrients, often as a consequence of reduced feed intake. Due to their health-promoting effects, nutritional strategies involving AA may help mitigate the negative impacts of SC. Methionine, tryptophan, and threonine, beyond serving as protein building blocks, are considered functional AA because they support immune system function, enhance intestinal barrier integrity, modulate inflammatory responses, and limit oxidative stress. Additionally, the review highlights the influence of individual variability, such as differences in body weight, on nutritional requirements and responses to AA supplementation for pigs under SC. The integration of nutritional strategies tailored to immune-challenged pigs offers promising avenues to improve productivity and animal welfare in commercial swine production systems with increasing restrictions on antibiotic use.