Translator's Note:
The decrease in the farrowing rate of summer breeding sows caused by heat stress in hot season, the increase in carcass fertility of offspring produced by summer breeding sows, and the growth rate and carcass weight of fattening pigs under high temperature conditions are the three major influencing factors that cause major economic losses in the pig industry. In addition, there are other effects such as increasing the number of non-productive days of sows, increasing mortality, etc. It is estimated that heat stress in the United States costs the pig industry 2.186 billion yuan per year. Incomplete statistics from Australia show that the economic losses caused by the three major impacts of high summer heat on the Australian pig industry are estimated to be 105.3 million yuan per year.
Authors:
F. Liu a,⇑, W. Zhao b, H.H. Le b, J.J. Cottrell b, M.P. Green c, B.J. Leury b, F.R. Dunshea b,d, A.W. Bell e
Research & Innovation Department, Rivaria Australia Limited, Australia
a Research and Innovation Unit, Rivalea Australia Pty Ltd, Corowa, NSW 2646, Australia
College of Veterinary and Agricultural Sciences, University of Melbourne, Australia
b Faculty of Veterinary and Agricultural Sciences, University of Melbourne, Parkville, VIC 3010, Australia
Faculty of Science, University of Melbourne, Australia
c Faculty of Science, University of Melbourne, Parkville, VIC 3010, Australia
School of Biological Sciences, University of Leeds, UK
d Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom
Department of Animal Science, Cornell University, USA
e Department of Animal Science, Cornell University, Ithaca 14853-4801, USA
Literature Review: Effects of Heat Stress on Pig Production (I)
Review: What have we learned about the effects of heat stress on the pig industry? - Part 1
Abstract Abstract
Pig production faces seasonal fluctuations. Low farrowing rates in summer-bred sows, increased carcass fat in their offspring, and slower growth in summer fattening pigs are three important economic impacts of the known season on the pig industry. The aim of this review was to review progress over the past decade in understanding the mechanisms of these three effects associated with summer climatic conditions, particularly heat stress (HS), and to suggest possible strategies for improvement. For the first effect, the low farrowing rate of summer breeding sows is mainly due to pregnancy failure that occurs early in pregnancy. Factors such as spermatocyte DNA damage, poor oocyte quality, local progesterone concentration, and poor embryonic estrogen secretion are discussed here, as these can all contribute to heat stress-mediated effects during conception. Nevertheless, it is not yet clear what the potential mechanism of action is, so viable commercial solutions are currently lacking. For the second effect, the industry has recently made progress in understanding the effects of heat stress during pregnancy on sows and fetuses, which can lead to a decrease in the number of fetal muscle fibers, an increase in the proportion of piglets born with light birth weight, and an increase in carcass fat at slaughter. To date, there are no effective strategies to mitigate the effects of heat stress during pregnancy on the fetus. As for the third effect, the slowdown in summer pig growth is one of the reasons for the loss of carcass weight in summer. Studies have shown that the decrease in growth rate may not only be due to a decrease in feed intake, but also impaired intestinal barrier function and inflammatory responses. In addition, there have been reports that heat stress weakens fat mobilization, which may exacerbate carcass fat fat when carcass weight gains. Novel feed additives have shown potential to reduce the effect of heat stress on intestinal barrier function in growing pigs. Taken together, based on these three effects, the economic losses associated with heat stress can be estimated. These impacts need to be reviewed in order to better align future research directions with the needs of the pig industry. Ultimately, the pig industry benefits by better understanding the underlying mechanisms and continuing to invest in developing commercially viable strategies for heat stress.
Pig production faces seasonal fluctuations. The low farrowing rate of sows mated in summer, increased carcass fatness of progeny born to the sows mated in summer, and slower growth rate of finisher pigs in summer are three economically important impacts identified in the pig industry. The purpose of this review is to examine advances over the past decade in understanding the mechanisms underlying the three impacts associated with summer conditions, particularly heat stress (HS), and to provide possible amelioration strategies. For impact 1, summer mating results in low farrowing rates mainly caused by the high frequency of early pregnancy disruptions. The contributions of semen DNA damage, poor oocyte quality, local progesterone concentrations, and suboptimal embryonic oestrogen secretion are discussed, as these all may contribute to HS-mediated effects around conception. Despite this, it is still unclear what the underlying mechanisms might be and thus, there is currently a lack of commercially viable solutions. For impact 2, there have been recent advances in the understanding of gestational HS on both the sow and foetus, with gestational HS implicated in decreased foetal muscle fibre number, a greater proportion of lighter piglets, and increased carcass fatness at slaughter. So far, no effective strategies have been developed to mitigate the impacts associated with gestational HS on foetuses. For impact 3, the slowed growth rate of pigs during summer is one reason for the reduced carcass weights in summer. Studies have shown that the reduction in growth rates may be due to more than reductions in feed intake alone, and the impaired intestinal barrier function and inflammatory response may also play a role. In addition, it is consistently reported that HS attenuates fat mobilisation which can potentially exacerbate carcass fat- ness when carcass weight is increased. Novel feed additives have exhibited the potential to reduce the impacts of HS on intestinal barrier function in grower pigs. Collectively, based on these three impacts, the economic loss associated with HS can be estimated. A review of these impacts is warranted to better align the future research directions with the needs of the pig industry. Ultimately, a better understanding of the underlying mechanisms and continuous investments in developing commercially viable strategies to combat HS will benefit the pig industry.
Research Implications
Summer environmental conditions, especially heat stress, can affect the production efficiency of pigs. The main known effects are the main known effects of low farrowing rates in sows bred in summer, increased carcass fat in offspring of sows bred in summer, and slower growth rate in summer in fattening pigs. The associated annual economic losses in the pig industry due to these impacts are enormous.
Summer conditions, particularly heat stress, compromise pig production efficiency. The low farrowing rate of sows mated in summer, increased carcass fatness of progeny born to the sows mated in summer, and slower growth rate of finisher pigs in summer are three known major impacts. The annual economic loss associated with these impacts in the pig industry is significant.
Research has led to a better understanding of the physiological effects of heat stress, but the pig industry still needs commercially viable solutions: (1) mitigating early pregnancy losses in summer weaned and bred sows; (2) improve the fetal development of offspring produced by summer breeding and pregnant sows; (3) Improve the growth rate of fattening pigs in the hot season.
Research has progressed our understanding of the physiological impacts of heat stress, but the pig industry still requires commercially viable solutions on (1) mitigating the early pregnancy disruption of sows weaned and mated in summer; (2) improving foetal development of progeny born to the sows mated and gestated in summer, and; (3) increasing the growth rate of pigs finished in hot seasons.
Research Background: Introduction
Summer heat stress (HS) conditions can reduce production efficiency and increase the cost of global pork production. Decreased reproductive performance of breeding herds and slowing growth of growing/finishing pigs are two typical effects associated with heat stress. When the ambient temperature exceeds the upper limit of the thermoneutral zone of the pig, the pig will experience heat stress. The physiological effects of heat stress on pigs are extensive. Understanding and mitigating the physiological effects of heat stress on pigs has become one of the important research topics in the past decade. It is also important to quantify the impact of heat stress on the pig industry, as it helps to study progress in the direction most limited by heat stress. Due to different climatic conditions, production systems and market demands, it is challenging to generalize the impact of heat stress on the pig industry on a global scale. Reviewing the effects of heat stress on pig farming in representative areas is valuable in providing a more targeted and in-depth analysis, and it creates an opportunity to cross-validate and explain the effects of heat stress using knowledge generated by global research. This paper takes Australia, which has a predominantly subtropical climate, as an example, to review the impact of heat stress on the pig industry. The knowledge summarized in this review can be applied to regions where the effects of heat stress are evident or emerging in the global pig industry due to global warming.
Heat stress (HS) conditions in summer can compromise production efficiency and present a high cost to pork production globally. Reductions of reproductive performance in breeding herds and slowed growth rate in grower/finisher pigs are two typical impacts associated with HS. Heat stress can occur in pigs when the environmental temperature goes beyond their thermoneutral zones’ upper limits. The physiological impacts of HS in pigs are comprehensive. Understanding and mitigating the physiological impacts of HS has been one of the key research topics of the past decade. Quantifying the impacts of HS in the pig industry is also important, because it helps research advances in the direction where the pig production is most limited by HS. Generalising the impacts of HS in global pig industry is challenging due to diverse climatic conditions, production systems, and market requirements. Reviewing the impacts of HS on the pig industry in a representative region is valuable for pro- viding a more focused and in-depth analysis, and it creates opportunities to cross validate and interpret the HS impacts using the knowledge generated from global studies. The impacts of HS in the pig industry are reviewed here using Australia, where its pig industry mainly under sub-tropical climate, as an example. The knowledge summarised in this review can be applied to the global pig industry where HS impacts are evident or emerging due to global warming.
Many pig farmers in Australia are located in subtropical regions, where temperatures vary greatly between seasons. High ambient temperatures are one of the most critical features of Australian summer. Figure 1 shows the maximum and minimum temperatures in intensive pig production areas (Croix, New South Wales, Australia, 35.99°S, 146.48°E) between 1970 and 2020. The average maximum temperature from November to March was above the upper limit of the thermoneutral zone for growing pigs (Huynh et al., 2005) and nursing sows (Quiniou and Noblet, 1999). Pig performance is usually expected to deteriorate significantly during these months. As in other heat-stress-affected regions, several research projects are underway in Australia to understand and mitigate the impact of heat stress on pig production, supported by grants from the Pork Centre, Pork Australia Limited, the Department of Agriculture, universities and other industry partners. The study conducted in sows partly used a seasonal comparative design under natural conditions, with physiological effects related to summer conditions (including but not limited to high ambient temperatures) due to the lack of climate-controlled research facilities and the high sample size required. At the same time, a large number of climate control experiments have been carried out on the effects of heat stress on growing pigs. The scope of the current review will focus on three main effects that reduce productivity in the pig industry – (1) increased reproductive failure in summer breeding sows, (2) increased carcass fat in offspring of summer breeding sows, and (3) slower growth rate in summer fattening pigs (Figure 2). This article discusses each impact based on recent global evidence, estimates the economic losses associated with these impacts, and summarizes the solutions assessed. The current review aims to provide an up-to-date research summary on the major impacts of heat stress on the swine industry and to identify knowledge gaps in future research to improve the efficiency of pork production in these challenging and changing climatic conditions.
Many pig producers in Australia are located in sub-tropical regions, where considerable changes of temperature occur between seasons. High environmental temperature is one of the most critical features of summer in Australia. Fig. 1 illustrates the maximum and minimum temperatures in an intensive pig production region (Corowa, NSW, Australia, 35.99°S, 146.48°E) between 1970 and 2020. The average maximum temperature between November and March was above the upper limits of the thermoneutral zone for grower pigs (Huynh et al., 2005) and lactating sows (Quiniou and Noblet, 1999). Significant reductions in pig performance are usually anticipated during those months. Same as other regions that suffer from HS impacts, multiple research projects have been conducted in Australia to understand and mitigate the impacts of HS on pig production with the funding support from the Pork CRC, Australian Pork Limited, Department of Agriculture, universities and other industry partners. The research conducted in sows has partially used a seasonal comparison design under natural conditions, due to the lack of climatically controlled research facilities and high sample sizes required, so the physiological impacts were related to summer conditions (hence include but are not limited to high environmental temperature). At the same time, there has been a considerable amount of climatically con- trolled experiments on the effect of HS on grower pigs. The current review’s scope will focus on three major impacts that are reducing production efficiency in the pig industry – (1) increased reproductive failure of sows mated in summer, (2) increased carcass fatness of progeny of sows mated in summer, and (3) slower growth rate of finisher pigs in summer (Fig. 2). This review discusses each impact based on recent worldwide evidence, estimates the economic loss associated with these impacts, and summarises the solutions that have been evaluated. The current review aims to provide an updated research summary on the major impacts of HS on the pig industry and identify knowledge gaps for future research to improve the efficiency of pork production in these challenging and changing climatic conditions.
Figure 1: Historical temperature records for major Australian pig-producing regions (1970-2020) (mean ± standard deviation). The horizontal axis is the month, the vertical axis is the temperature, the upper dashed line with arrows is the upper limit of the thermoneutral temperature of growing/finishing pigs, and the dashed line with arrows below is the upper limit of the thermoneutral temperature of nursing sows. Data from the Croix Airport Weather Station in New South Wales, Australia (35.99°S, 146.48°E; ID: 074034, Australian Bureau of Meteorology). The upper limit of the thermoneutral zone (orange curve) is defined as the ambient temperature at which pigs begin to reduce total thermogenesis (approximately 23°C in growing pigs (Huynh et al., 2005) and 22°C in nursing soothes (Quiniou and Noblet, 1999)).
Fig. 1. Temperature record of a major pig production region in Australia (1970–2020) (mean ± SD). Data were retrieved from Corowa Airport weather station, NSW, Australia (35.99°S, 146.48°E; ID:074034, Bureau of Meteorology). The upper limit of the thermoneutral zone is defined as the environmental temperature when the pig starts to reduce total heat production (approximately 23 °C for growers (Huynh et al., 2005) and 22 °C for lactating sows (Quiniou and Noblet, 1999)).
Figure 2: Three main effects of summer climatic conditions on pig production. First, summer breeding sows have low farrowing rates, increasing non-productive days, and the need to breed more sows to maintain the market supply of fattening pigs, which increases the feed-to-weight ratio of the herd. Second, for sows that successfully farrowed after summer breeding, the proportion of offspring with lower birth weight (≤ 1.1 kg) was higher due to the effect of gestational heat stress on fetal development. An increase in the proportion of piglets with low birth weight leads to lower growth rates, lower survival rates and higher carcass fat. Third, the slowdown in pig growth under high temperature conditions reduces carcass weight, thereby limiting the income of pig producers.
Fig. 2. Three major impacts associated with summer conditions in pig production. First, the reduced farrowing rate of sows mated in summer increased the non-reproductive days and requires more sows to be mated for maintaining the supply of finisher pigs to the market, which inflates the herd feed conversion ratio. Second, for sows that manage to farrow after summer mating, a greater proportion of progeny is born with light BW (≤1.1 kg) due to the impacts of gestational heat stress on foetal development. The increased proportion of born-light progeny pigs can result in inferior growth rate, reduced survival rate, and higher carcass fatness of the progeny population. Third, the slowed growth rate of pigs during hot conditions reduces carcass weight and consequently limits the revenue of pig producers.
Impact 1: Low farrowing rate of summer breeding sows in summer Evidence for reduced farrowing rate of sows mated in summer Reduced farrowing rate of sows mated in summer Reduced farrowing rates in summer breeding sows remain a major problem for the global swine industry, including Australia. In Australia, common maternal breeds used for end-use commercial pig production are Great White Sow and Long White Sow and the paternal breed is the Duroc Boar (Primary Industry Department, 2016). The latest production data (2010-2018) from Australia's two major pig farms show that the decline in farrowing rates began in December. The average delivery rate from December to March is 5% to 10% lower than the annual average (Hermesch and Bunz, 2020). The recently published farrowing rate of commercially farmed sows bred in summer in Australia ranged from 64% to 83%, lower than the 89% in the colder months (Liu et al., 2019; 2020; Plush et al., 2019)。 High ambient temperatures combined with variations in sunshine length are thought to affect fertility in summer. An analysis of farrowing records from a large Australian farm from 2012 to 2017 found that the occurrence of high ambient temperatures (>29°C) 35 days before breeding led to the greatest decrease in farrowing rates (Bunz et al., 2019). Reduced farrowing rate of domestic sows that were mated in summer remains a major issue in the global pig industry, including Australia. The common breeds used for terminal pig production in Australia are Large White- and Landrace-based genetics for the maternal line and Duroc-based genetics for the sire line (Department of Primary Industries, 2016). Recent production data (2010–2018) from two major Australian piggeries showed that the decline in the farrowing rate began in December. The average farrowing rate between December and March was 5% to 10% lower than the annual average (Hermesch and Bunz, 2020). Recently published Australian commercial farrowing rates of sows mated in summer range from 64% to 83% which is lower than cooler months (89%) (Liu et al., 2019; 2020; Plush et al., 2019). The high environmental temperature coupled with the daylight length change is postulated to affect fertility in summer. An analysis of the farrowing records from a large Australian piggery between 2012 and 2017 found that if a high environmental temperature (>29 °C) occurred 35 days before mating, it caused the largest reduction in farrowing rate (Bunz et al., 2019). Sow farrowing performance depends on a range of factors. The formation of follicles leads to estrus and subsequent ovulation. Conception occurs when a viable oocytes are fertilized with high-quality sperm. The mother's perception of pregnancy triggers endocrine changes and uterine preparation to facilitate embryo implantation (Spencer and Bazer, 2004). Progesterone, secreted by the corpus luteum, plays an important role in maintaining pregnancy. Summer reproductive disorders are mainly manifested by the increased proportion of irregular re-estrus after sow mating, so the number of mating needs to be increased to ensure effective pregnancy. Data from Australia show that total litter size and litter size live born remain constant in different seasons (Lewis and Bunter, 2011). Improving understanding of the physiological mechanisms behind summer reproductive disorders and developing intervention strategies has been a key research focus for the past 40 years (King, 2017). Over the past 5 years, the consensus in summer reproductive disorder research has been that the decrease in farrowing rates in summer breeding sows is mainly due to early pregnancy failure rather than unsuccessful conception. Farrowing success of sows relies on a series of events. Folliculo genesis results in oestrus and subsequent ovulation. Conception can take place when competent oocytes are fertilised by quality sperm. Maternal recognition of pregnancy triggers endocrine changes and the preparation of the uterus to facilitate embryo implantation (Spencer and Bazer, 2004). The progesterone secreted from functional corpora lutea plays an important role in the maintenance of pregnancy. Summer infertility mainly manifests as an increased proportion of sows that have an irregular return after mating, and thus an increased number of matings are required to establish a viable pregnancy. Data from Australia show the total number of piglets born per litter and the number of piglets born alive both remain constant among seasons (Lewis and Bunter, 2011). In the past 40 years, improved understanding of the physiological mechanisms behind summer infertility and developing intervention strategies have been a critical research focus (King, 2017). In the last 5 years, the consensus of summer infertility studies has been that the reduction in the farrowing rate of sows mated in summer is mainly due to early pregnancy disruption rather than a failure to conceive. Changes in estrus to ovulation interval during summer mating Altered oestrus-to-ovulation interval during summer mating Pig producers often schedule artificial insemination (AI) based on detection of behavioral estrus, so knowing the accurate estrus to ovulation interval (OOI) is critical to timing artificial insemination for optimal conception. There are conflicting findings on the estrus to ovulation interval in heat-stressed environments. A climate-controlled study found that heat stress during lactation (31°C, 8:00-16:00; 26°C 16:00-8.00) reduced follicle size from 6.7 mm to 5.8 mm and extended the interval from estrus to ovulation from 1 to 2.5 days (Cabezón et al., 2017). Another study found the opposite result, with summer weaned sows having a heat-to-ovulation interval 10 hours shorter (21.8 hours versus 31.4 hours) than winter weaned sows (van Wettere, 2013). Similarly, sows that breastfed and bred during the summer (December 2019 – March 2020; 25.4±5.01°C mean ± standard deviation; Australia) has a mean interval of 1.1 days from estrus to ovulation (Liu et al., 2021a). Contradictions between the findings may be due to different environmental conditions or genotypes. It is important to note that even though the change in the interval between estrus and ovulation in summer is significant, the timing and success of conception is less likely to be affected by inappropriate mating or insemination when two artificial insemination is used (i.e., the first artificial insemination when estrus behavior is first detected after weaning and the second at 24 hours). Mating at the beginning of estrus remains the best breeding time for weaned sows with a 1.1-day interval from summer heat to ovulation, as insemination 0-24 hours before ovulation results in the best sow conception rate (Kemp and Soede, 1996). Interestingly, sows with an average estrus to ovulation interval of 1.1 days remained low (65%) after two artificial inseminations in the summer (Liu et al., 2021a). Therefore, factors in the first trimester (before day 35), including maternal recognition of pregnancy, embryo survival, uterine environment, sperm and oocyte quality, may be responsible for the lower delivery rate, which we will discuss in the next section. Pig producers usually schedule artificial insemination (AI) based on the detection of behavioural oestrus, so knowing the accurate oestrus-to-ovulation interval (OOI) is essential for scheduling AI to achieve optimum conception. There are conflicting findings on OOI during HS conditions. A climatic controlled study found that HS during lactation (31 °C, 8:00–16:00; 26 °C 16:00–8.00) reduced follicle size from 6.7 to 5.8 mm and prolonged OOI from 1 to 2.5 days (Cabezón et al., 2017). Conversely, OOI was 10 hours shorter in sows whose litters were weaned in summer than winter (21.8 h vs 31.4 h) (van Wettere, 2013). Similarly, the sows lactated and mated under summer conditions (December 2019–March 2020; 25.4 ± 5.01 °C mean ± SD; Australia) had an average OOI of 1.1 days (Liu et al., 2021a). The conflicting findings between studies may be due to different environmental conditions or genotypes. Notably, even if changes in OOI are evident in summer, the timing and success of conception are unlikely to be affected due to a mistimed mating or insemination, when a double AI programme is used ( i.e.: 1st AI at the first detectable behaviour oestrus after weaning and 2nd AI at 24 h after this). Insemination at the onset of oestrus remains the best timing for mating weaned sows with such an OOI (1.1 days) in summer because insemination performed at 0–24 h before ovulation can achieve optimum sow conception rates (Kemp and Soede, 1996). Interestingly, the farrowing rate of the sows with an average OOI of 1.1 days that received two AIs in summer remained low (65%) (Liu et al., 2021a). Hence, factors during early pregnancy (before day 35), including maternal recognition of pregnancy, embryo survival, uterine environment, and sperm as well as oocyte quality, may be responsible for lower farrowing rates and are discussed in the next section.
To be continued... To be continued...
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