High Environmental Temperature Induces Oxidative Stress, Reduced Sow Productivity and Increased Piglet Mortality

Heat stress is a limiting factor for livestock productivity in many countries. This study was designed to evaluate the influence of exposure of exotic pigs to high environmental temperatures on physiology, oxidative stress biomarkers and productivity. Multiparous sows (n = 40) in two equal groups (Groups N and H) were investigated. The pen temperature, relative humidity and Temperature-Humidity Index (THI) were measured for 30 days. The skin temperature, respiratory rate and rectal temperature were measured weekly. Haematology, serum biochemistry and oxidative biomarkers were also evaluated. The production records, including farrowing events (NFE), number of piglets at birth (NPB), number of piglets at weaning (NPW) and piglet mortality (NPM) were obtained per sow for the preceding 24 months. Data were analyzed using descriptive statistics and a student T-test at P <0.05. Pen indoor temperature was significantly higher and relative humidity significantly lower in Group H (30.23+0.99 0 C, 48.90+7.94 %) compared to Group N (25.21+0.94 0 C, 58.97+6.21 %). The THI was higher in Group H (78.40+1.99). There was a significant increase in skin surface temperature and respiratory rate in Group H (37.70+1.40 0 C and 39.70+4.28 bpm) with a slight increase in rectal temperature compared to Group N (33.86+0.95 0 C and 35.79+2.89 bpm). There was a slight increase in PCV, blood urea nitrogen and a decrease in the levels of sodium, chloride, and bicarbonate in sows in Group H. There was a significant increase in hydrogen peroxide generation, nitric oxide contents and malondialdehyde levels, with a decrease in superoxide dismutase activity and glutathione content in Group H when compared to Group N. There was a reduction in farrowing events (N = 3.45 + 0.20; H = 3.00 + 0.19), number of piglets at birth (N = 29.15 + 1.92; H = 27.95+ 2.44), the number of piglets at weaning (N = 27.65 + 1.89; H = 25.90 + 2.25), and an increase in piglet mortality (N = 1.5 + 0.21; H = 2.05 + 0.42) in sows exposed to higher temperatures. The orientation of the pig housing in the north-south direction reduced its effectiveness. Exotic sows exposed to heat stress had altered physiology, experienced oxidative stress, were prone to renal and hepatic injury, and had reduced productivity. Adequate thermal comfort should be ensured in pig husbandry in the tropics for optimal health and productivity.


INTRODUCTION
Heat stress is a major challenge to profitable livestock production in many parts of the world and may increase with future climate change (Sejian et al., 2018).Heat stress occurs when environmental temperature rises beyond the thermal comfort zone of an animal, which is established by an equilibrium between external heat load, internal heat production, and heat dissipation (Rojas-Downing et al., 2017, Rauw et al., 2017, Lacetera, 2019).It remains a major environmental issue negatively affecting animal welfare and production efficiency in almost every livestock sector, especially in the tropical and subtropical regions (Mayorga et al., 2019;Patra and Kar, 2021).Livestock in temperate countries are also affected during warm summer months (Renaudeau et al., 2012) and it remains one of the costliest issues in the U.S. pork industry causing an estimated annual loss of nearly US$1 billion (Pollmann, 2010).
The response mechanisms initiated by animals for adaptation to climatic stressors contribute to reducing their productivity (Pragna et al., 2018).These mechanisms include morphological, behavioural, physiological, neuro-endocrine, blood biochemical, metabolic, cellular, and molecular responses (Sejian et al., 2018).In heat-stressed animals, there is activation of thermoregulatory, physiological, and behavioural responses beyond what is needed for regular maintenance of body temperature (Rauw et al., 2017).Therefore, nutrients are diverted to body temperature maintenance and other survival responses above the synthesis of useful and profitable products such as growth, milk production and foetal development.Higher ambient temperatures associated with climate change also reduce the availability of water, quality, and quantity of feeds and increase outbreaks of livestock diseases (Mutua et al., 2020).
In comparison to other livestock, pigs are prone to heat stress because they do not have functional sweat glands.This is aggravated by their relatively small lungs, which reduce their ability to disseminate heat by panting (D' Allaire et al., 1996).Therefore, when exposed to high environmental temperatures, pigs increase their respiratory rate and reduce voluntary feed intake, leading to significant economic losses.Heat stress often accompanied by a drop in feed consumption, is associated with a reduction in the feed conversion rate, poor sow performance, a drop in milk production, reduced and inconsistent growth, decreased carcass quality, mortality, and morbidity (Rauw et al., 2017;Mutua et al., 2020).
In addition to rising environmental temperatures, the recent fast-growing lean pigs, apart from improved productivity, are known to generate more heat than earlier breeds and their congeners living in the wild (Brown-Brandl et al., 2001) coupled with the intensive system of production that ensures a denser population in confined housing, which makes it difficult for pigs to express natural instincts that aid their body temperature control (Huynh et al., 2006).
When environmental temperature rises above the thermo-neutral zone of pigs, they start making clear physiological changes to dissipate heat.The upper limit of the comfort zone and the thermoneutral zone for sows at different physiological states range between 18-23 0 C and 21-26 0 C, respectively.The upper limit reduces in the order of emptypregnant-lactating sows (Vermeer and Aarnink, 2023).To maintain optimal health and productivity in intensively raised pigs, the housing facility must be effective.
The effectiveness of a swine building/facility is defined as how well a building can provide the conditions needed for optimum pig performance and how well these conditions are maintained throughout the year (Zulovich, 2012).The facility should ensure optimal conditions in the thermal, gaseous, and physical environments (Gonyou et al., 2006).It must meet the requirements of pigs in terms of air temperature, humidity level, air velocity or air movement, indoor air quality, space, manure removal or handling, feed, and water access.
In a swine housing system, the existing modes of heat transfer include: (i) sensible heat transfer (by conduction/contact; by convection or a solid body to an air or gaseous environment; and by radiation); and (ii) latent heat transfer (by evaporation of moisture from the warm, moist surface of a pig and by condensation from the surrounding air with a relatively high dew point temperature to the pig's body with a relatively low dew point or surface temperature) (Zulovich, 2012).
In view of the ongoing global climate change and the tendency of pigs raised in tropical environments to be exposed to high temperatures for an extended period of the year, this study was designed to investigate the effect of exposure of exotic sows to high environmental temperatures on their physiology, oxidative stress biomarkers and productivity.

Study location
The study was conducted on the swine unit of the Teaching and Research Farm (TRF), University of Ibadan, Nigeria, located within Latitude 7° 23' 28.19" N and Longitude 3° 54' 59.99" E. Ibadan is the capital city of Oyo State, located in its south-eastern part, in a wooded area near the border between the rain forest and the savannah climatic zone, in the south-western part of Nigeria.The city is situated about 110 km northeast of Lagos with an average altitude of 200 m and 160 km off the Atlantic coast.The climate of Ibadan is tropical and sub-humid, with distinct wet and dry seasons.It has a bimodal annual rainfall pattern of April-July and August-November with a mean annual rainfall of about 1250 mm and a temperature range of 21 0 C ± 1.5 and 30 °C ± 3.5.Annually, the prevalent winds in the city during the rainy season are the moist maritime south-west monsoon, which blows inland from the Atlantic Ocean, while the dry dust-laden winds blow from the Sahara desert during the dry season, which is from November to February (Ajayi and Olufayo, 2002; Egbinola and Amobichukwu, 2013; Ahmed et al., 2022).

Study design, animal selection and housing conditions
The investigation was prospective and retrospective.The prospective study was designed to investigate the level of exposure of 40 selected sows to heat stress for 30 consecutive days in the dry season (January, 2023) and a retrospective review of the production performance of the same selected sows in the preceding 24 months (January 2021-December 2022).
Only apparently healthy sows without physical deformities and with a proven breeding record were included in the study.They were sows with a minimum of two previous farrowing events within the preceding 24 months without complications.They were within the age range of 38 -50 months and weight range of 64-96 kg.They were in two groups (20 each) sorted from sows that were permanently housed in the existing farm buildings.Based on the farm housing layout, the long-axis of the first pen (Group N) is oriented on the East-West direction, adjacent to the second wing (Group H) oriented on the North-South direction.The dimensions of the building, floor type, roofing design, feeding and watering facilities are described (Table 1).Concrete troughs, open drinkers.Same for both 13 Materials used for the building (Same for both) Cement floors, block walls, metal pillars for roof, wooden rafters and corrugated asbestos roof sheets.*The roof design of both housing does not provide complete cover for the full length of the cubicles.

Measurement of meterological parameters
The pen indoor temperature and relative humidity at peak heat (1-2 pm) were measured using a thermo-hygrometer which was suspended in the pen.Measurements were taken for 30 consecutive days in the dry season (January, 2023).The length of the pen floor exposed to direct sun radiation at sunrise (10 am) and sunset (6 pm) were noted.The heat stress index was determined using the formula: THI ( 0 C) = 0.8T + (RH/100) x (T-14.3)+ 46.4 Reference: THI≤ 74 is safe, 74<THI<79 is critical, 79≤THI<84 is dangerous and THI ≥84 is emergency (NOAA, 1976).

Measurement of physiological parameters
The Skin Temperature (ST), Respiratory Rate (RR) and Rectal Temperature (RT) of the sows were taken once in a week, between 1-2 pm.Skin temperature (°C) were measured using infrared thermometer, the respiratory rate was taken by observation of respiratory movements from a distance while the sows were at rest with measurements recorded as breathes per minutes (bpm) while rectal temperature (°C) was taken with a digital thermometer inserted into the rectum till reading was constant.

Blood sample collection
Blood samples were collected at the end of the experiment (Day 30) from each of the sows for hematology, serum biochemistry and evaluation of markers of oxidative stress.6 mL of blood was collected aseptically using sterile needle and syringe through the anterior vena cava and discharged equally into labelled sample bottles containing ethylene diamine tetra-acetic acid (EDTA) for hematology and plain bottles for serum biochemistry and biomarkers of oxidative stress.All samples were transported on icepack to the laboratory for analysis.

Laboratory analysis Haematology and serum biochemistry
The blood samples were analysed on an autohaematology analyzer (SYSMEX Automated Haematology Analyzer pocH-100iV Diff., United Kingdom).The packed cell volume (PCV), haemoglobin concentration (Hb), red blood cell count (RBC), white blood cell count (WBC), platelet count, neutrophils, lymphocytes, eosinophils, and monocytes were determined.To harvest serum, samples were centrifuged at 2500 x gravitational units (g) for 10 minutes.The harvested serum was stored in cryovials at -20 °C until use.The serum samples were used for the determination of biochemical parameters using analytical-grade reagent kits (Randox Laboratories Limited, United Kingdom) following the manufacturer's instructions.The analytes measured included total protein (TP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), blood urea nitrogen (BUN), and creatinine.(1974).Superoxide dismutase (SOD) activity was estimated by the method of Misra and Fridovich (1972) with slight modification (Oyagbemi et al., 2015).The content of nitric oxide (NO) was evaluated as described by Olaleye et al. (2007).The content of H 2 O 2 generated was measured according to the method of Wolff (1994).

Sow productivity and Piglet mortality
The production records of the 40 selected sows for the preceding 24 months (January 2021-December 2022) were reviewed.Data on parity, number of farrowing events (NFE), number of piglets at birth (NPB), number of piglets at weaning (NPW) and number of piglet mortality (NPM) were obtained for each sow.

Ethical Statement
All of the animals received humane care according to the criteria outlined in the Guide for the Care and the Use of Laboratory Animals prepared by the National Academy of Science and published by the National Institute of Health.The ethics regulations were followed in accordance with national and institutional guidelines for the protection of the animals' welfare during experiments (PHS, 2015).

Statistical Analysis
All values are expressed as mean±SD.Data were analyzed using descriptive statistics and Student T-test to evaluate the difference between the variance and means of the two groups at P<0.05.Analysis was done using excel and SPSS version 26.

RESULTS
The housing design, dimensions and construction materials of the two buildings were similar, except for the orientation of their lengths (long-axis), which was different.The meterological measurements reveal a significant increase in the pen indoor temperature and a significant reduction in relative humidity in Group H (30.23+0.99°C, 48.90+7.94%) compared to Group N (25.21+0.94°C, 58.97+6.21%) (Table 2).The calculated Temperature Humidity Index revealed a significant increase in the risk of heat stress in Group H (78.40+1.99)compared to Group N (72.99+1.43).A greater proportion of the pen floor in Group H (44.8; 48.1 %) was exposed to direct sunrays than in Group N (28.1, 26.8 %) at sunrise and at sunset (Fig. 1).The physiological measurements revealed significant increases in skin surface temperature and respiratory rate in Group H (37.70+1.40°C and 39.70+4.28bpm) compared to Group N (33.86+0.95°C and 35.79+2.89bpm).Although the increase in rectal temperature was not significant, it was higher in Group H (38.88+0.68°C) than in Group N (38.08+0.57°C) (Table 3).The haematology and serum biochemistry values were mostly within physiologic ranges with slight changes.In group H, there was a slight increase in packed cell volume (Table 4), with changes in serum biochemistry including a decrease in the levels of sodium, chloride, and bicarbonate and an increase in blood urea nitrogen (Tables 4 and 5).Changes in specific biomarkers of oxidative stress in group H when compared to group N included an increase in hydrogen peroxide generation, nitric oxide contents and malondialdehyde levels, with a decrease in superoxide dismutase activity and glutathione content (Table 6).*Superscripts (a>b) indicates significant difference in corresponding values along the row at p<0.05.GSH: Glutathione, GPx: Glutathione peroxidase, GST: Glutathione S-Transferase, SOD: Superoxide dismutase, MDA: Malondialdehyde, NPT: Non-Protein Thiol, THIOL: Protein Thiol, NO: Nitric Oxide, H 2 O 2 : Hydrogen peroxide .
There was a reduction in the parameters of sow productivity in the exotic sows exposed to higher environmental temperatures (Group H) compared to those exposed to moderate temperatures (Group N).There was a reduction in the number of farrowing events (N = 3.45+0.20;H = 3.00+0.19),the number of piglets at birth (N = 29.15+1.92;H = 27.95+2.44),and number of piglets at weaning (N = 27.65+1.89;H = 25.90+2.25),and an increase in the number of piglet deaths (N = 1.5+0.21;H = 2.05+0.42;Table 7).

DISCUSSION
Previous investigations have established that factors in the environment, including temperature, relative humidity, solar radiation, and wind speed influence the performance of livestock (Sejian et al., 2018).Therefore, maintaining an adequate indoor microclimate with sufficient thermal comfort is central to improving the welfare and health of intensively raised pigs.The housing design, construction, and facilities should support adequate heat transfer between the pigs and the environment, as excess heat transfer will lead to cold stress while insufficient transfer will lead to heat stress (Lacetera, 2019).In our study, we observed that the housing design, dimensions, and construction materials had low cost and low maintenance implications and were suited for a tropical environment where there is a need for sufficient air circulation for heat dissipation.The observed open-sided housing design is the model usually preferred in tropical environments (Stender et al., 2003;Godyń et al., 2020), but such facilities should be upgraded with additional technology to improve the cooling of the animals in the face of rising ambient temperatures.
The roofing of the pig houses was made of a poor heat conductor which therefore reduced the rate of rise in indoor temperature during the day.We observed that the only difference in the design of the two buildings that was responsible for the difference in indoor temperature was the building orientation.The North-South oriented building allowed more access to direct sunrays at sunrise and sunset, thereby increasing the floor and, hence, the indoor temperature.It is established that housing designs that allow moderate access to direct sunrays have benefits, as the UVB radiation from sunlight aids the synthesis of vitamin D and influences bone health in pigs (Turner and Anderson, 2012;Larson-Meyer et al., 2017).The open-sided housing also improves the pigs' well-being through improved ventilation and the feeling of outdoor experience.Higher indoor temperatures will reduce the efficiency of heat exchange between the sows and the environment, as heat exchange is done by heat gradient.The housing with the higher temperature was observed to have a lower humidity and a higher temperature-humidity index.The THI is an established measure of the risk of exposure of livestock to heat stress.While the pigs in the housing with a moderate temperature (THI: 72.99± 1.43), were within the thermo-neutral zone for sows (Vermeer and Aarnink, 2023), the other group exposed to a higher temperature (78.40±1.49),were critically affected by heat stress and tending towards the dangerous limits based on the THI reference (NOAA, 1976).The sows exposed to higher temperatures were observed to have consistently higher skin surface temperature and respiratory rate, which were significantly higher than the other group.The increase observed in rectal temperature in this group of sows was not significant.This is because rectal temperature is a delayed indicator of heat stress (Huynh and Aarnink, 2005).The sows were observed to present other behavioural coping responses, including panting and wallowing, in drinkers in response to the higher indoor temperatures.These respiratory responses tend to be insufficient in pigs because they do not have functional sweat glands and possess relatively small lungs; therefore, they cannot efficiently dissipate high heat loads through enhanced respiratory and sweating rates like some other livestock (D'Allaire et al.,

1996; Sejian et al., 2018).
We observed that exposure of a larger floor area to direct sunlight was associated with a higher indoor temperature, as expected.Maintaining an optimal floor temperature is important in heat exchange, as pigs are known to spend up to 79% of the day (19 hours) resting with a large surface area of their body in contact with the floor (Huynh and Aarnink, 2005).In group housing for pigs, there should be provision of adequate floor space to aid their resting behaviours and heat exchange.Strategies and facilities for optimal thermal control should be incorporated into farm operations and housing design in pig husbandry.Some of the cooling strategies that may be adopted include increased water supply, skin wetting, nutritional manipulations, adequate ventilation, increased floor space, adequate insulation, and the provision of shades (Adebiyi et al., 2017).Facilities for fogging, misting or showers, as well as evaporative pads or fans, have been effectively used in various pig housing facilities to improve heat loss (Godyń et al., 2020), with each having its merits and demerits.
In our study, water was provided in open drinkers ad-libitum and the sows cooled by wallowing in the drinkers.The use of drinkers as wallows for pigs conserves more water than sprinkling but may aid the buildup of pathogens in the drinkers.We observed that the sows exposed to high ambient temperatures had a higher rectal temperature, showing progression towards excessive heat accumulation.Heat accumulation in the body has been linked to immune dysregulation and elevated oxidant production, which results in cellular oxidative stress and lipid peroxidation of cell membranes, making farm animals more susceptible to infectious diseases and having a higher mortality rate (Varasteh et al., 2015; Patra and Kar, 2021).Additionally, the haematological investigation revealed an increase in PCV and Hb concentrations under heat stress.Previous reports have also observed an association between heat stress and an increase in PCV and Hb concentrations in farm animals (Rana et al., 2014;Okoruwa, 2014;Chaudhary et al., 2015).The findings were attributed to some factors, including the need for more haemoglobin to meet the demand for an increase in oxygen circulation during panting or because of reduced blood volume associated with dehydration.
Serum biochemistry revealed a range of metabolic changes in response to high ambient temperatures.Although most of the values were within reference ranges, comparatively, there was a decrease in the levels of sodium, chloride, and bicarbonate, with an increase in blood urea nitrogen, creatinine, aspartate aminotransferase (AST), alanine aminotransferase (ALT) and alkaline phosphatase (ALP) in the heat-stressed sows.The metabolic changes reveal tendencies for kidney and liver damage in pigs exposed to heat stress.These findings of a rise in liver enzymes have also been reported in other animals, including broiler chickens Factors that initiate an increase in the generation of reactive oxygen species (ROS) or reactive nitrogen species (RNS) beyond their rate of neutralisation by antioxidants lead to oxidative stress (Sirini and Amarakoon, 2017).There are several antioxidants produced in the body to catalyse reactions that neutralize free radicals to prevent free radical-induced cell damage (Gupta et al., 2013).The more important enzymatic antioxidants include superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase, and catalase (CAT), while the non-enzymatic antioxidants include ascorbic acid (Vitamin C), α-tocopherol (Vitamin E), glutathione (GSH), carotenoids, flavonoids and others (Valko et al., 2006).We observed more disruptions in the oxidative balance in sows exposed to high temperatures when compared to those exposed to moderate temperatures.There was a substantial generation of hydrogen peroxide and nitric oxide.There was also an increase in the marker of lipid peroxidation, Malondialdehyde and a decrease in superoxide dismutase and glutathione.This is like the findings in heat-stressed broilers ( Therefore, in pig husbandry in tropical environments, the use of antioxidant supplements should be embraced to improve pig health, in addition to efficient housing. The pride of the pig is their high fecundity and faster growth rate beyond most other livestock.Pigs are choice production animal for achieving animal protein security globally and are increasingly being embraced as such in Nigeria (Omotosho et al., 2016).Despite the expected fecundity and growth performance of the exotic breeds of pigs, their performance while in production over successive generations in the tropics has been observed to reduce with time.This may be due to the suboptimal management conditions, which are rife in the husbandry systems, and the influence of tropical animal diseases.We observed reduced productivity in large white breeds of sows that were exposed to high environmental temperatures.There was a decrease in the number of farrowing events and the number of piglets at birth, while there was an increase in piglet mortality, leading to a reduced number of piglets at weaning.It is known that an increase in free radicals adversely affects reproduction in animals through damage to sperm and oocyte DNA, disruption of testicular functions, dysfunction of the ovary and endometrium, and a decrease in fertilization rate There was also an increase in mortality, leading to a reduced number of weaned piglets in the heat-stressed group.Contributory to this finding is the influence of free radicals in utero and in the preweaning period.Heat stress in late gestation reduces umbilical oxygen supply to the newborn, with an associated higher risk of piglet stillbirth and impairment on neonatal survivability and development (Johnson and Baumgard, 2019;Zhao et al., 2022).Pigs are also known to reduce voluntary feeding to reduce digestion and metabolic heat production, leading to poor performance (Cui et al., 2016).The deleterious effects of heat stress on the welfare, health and productivity of pigs are many (Godyń et al., 2020), with impacts in pregnancy, the nursing period and throughout all stages of production.Our findings provide more insights into the contributing factors to some of the previously reported disease occurrences in pigs in Nigeria

CONCLUSION
In conclusion, the ongoing climate change demands a critical review of pig husbandry systems and housing facilities especially in the tropics, where high ambient temperatures subsists for an extended period of the year.Utilizing effective housing that can provide a stable and suitable microclimate for pig production throughout the year is desirable.The exposure of sows to high environmental temperatures causes heat stress, which interferes with their welfare, health, and productivity.For improved welfare, health, and productivity of exotic pigs in the tropics, optimal housing conditions and antioxidantsupplemented nutrition are recommended.
relationships that could have appeared to influence the work reported in this paper.
method of Rotruck et al. (1973).The Malondialdehyde was determined according to the method of Varshney and Kale (1990).Total sulfhydryl (thiol) groups and the content of reduced glutathione (GSH) in the samples were quantified using the methods of Ellman (1959) and Jollow et al. (1974), respectively.Glutathione S-transferase activity was determined according to Habig et al., Superscripts (a>b)  indicates significant difference in corresponding values along the row at p<0.05.

Fig. 1 :
Fig.1: Proportion of pen floor exposed to direct sunrays at sunrise and sunset in two pens in a tropical environment.
(Hosseini-Vashana et al., 2019), sheep and goats (Nazifi et al., 2003; Gupta et al., 2013; Shilja et al., 2016) and pigs (Lu et al., 2014).An increase in hematocrit level and BUN are established indicators of dehydration, which accompanies heat stress (Rana et al., 2014).Prolonged exposure of sows to heat stress will ultimately reduce their productivity while increasing the cost of production due to expenses on disease control (Patra and Kar, 2021).

(
Abiola et al., 2017; Igbokwe and Maduka, 2018; Ogundijo et al., 2023) and also provide useful information for adjusting pig housing conditions in the tropics for improved productivity.

Table 1
Pig housing design, dimensions and construction materials used in a tropical environment.

Table 2 :
Meterological parameters measured in the pen of two groups of sows exposed to different temperature

Table 3 :
Skin Temperature, Rectal Temperature and Respiratory rate of two groups of exotic sows exposed to different environmental temperature in a tropical environment:

Table 4 :
Hematology values of two groups of exotic sows exposed to different environmental temperature in a tropical environment: * Data are expressed as mean ± Standard Deviation.PCV, packed cell volume; Hb, haemoglobin; RBC, red blood cell count, WBC, white blood cell count.(Klem et al., 2010; MSDVetmanual.com).

Table 5 :
Serum biochemical values of two groups of exotic sows exposed to different environmental temperature in a tropical environment:

Table 6 :
Oxidative stress biomarkers of two groups of exotic sows exposed to different environmental temperature in a tropical environment:

Table 7 :
Productivity of two groups of exotic sows exposed to different environmental temperature in a tropical environment: NFE: Number of farrowing events per sow in 24 months; NPB: Number of piglets at birth per sow in 24 months; NPW: Number of piglets at weaning per sow in 24 months; NPM: Mortality of piglets/sow in 24 months.MT: Moderate Temperature, HT: High temperature.