Heat stress: Invisible impact on sows and their piglets
Heat stress is one of the most predictable yet disruptive pressure moments in swine production. When temperatures rise, feed intake declines, biological strain increases, and both sow performance and piglet development can be affected. When foetuses develop in heat stressed gestating sows, a condition known as in utero heat stress (IUHS) can occur. Piglets exposed to IUHS may be born smaller and may have reduced immune function and greater sensitivity to stress later in life.
While these effects are not always visible at birth, they can influence performance throughout the pig’s lifetime. Therefore, decisions made months earlier, particularly those related to nutrition, health, and sow condition, often determine how well animals cope when environmental pressure increases.
Modern pig genetics have greatly increased productivity, but this progress has also made animals more prone to heat load due to higher metabolic demands. This is particularly true for today’s lean, high-performing sows, where thermal stress can lead to reduced farrowing rates, longer wean-to-estrus intervals, and poorer-quality piglets – often referred to as ‘seasonal’ or ‘summer infertility’.
To better understand these challenges, Cargill Animal Nutrition & Health (Cargill ANH) conducted an extensive review of the scientific evidence on heat stress and its influence on sow physiology, reproductive outcomes, and fetal development. This knowledge forms the basis for targeted nutritional strategies designed to reduce the impact of heat stress.
Disturbances to gut integrity and endocrine function
What actually happens inside the sow during periods of heat? In response to high temperatures, the animal diverts blood flow away from internal organs and toward peripheral tissues such as the skin and ears to improve heat dissipation. As a consequence, blood flow to internal organs is reduced, particularly to the gastrointestinal tract. Research indicates that this leads to:
- Lower efficiency of nutrient utilisation.
- Heightened oxidative stress and activation of the immune system – processes that require considerable metabolic resources and divert nutrients away from growth and reproduction.
- Reduced mucin production and secretion, increasing intestinal permeability.
- Shifts in gut microbiota, by reducing fibre-fermenting bacteria and short‑chain fatty acid production, while favouring opportunistic pathogens.
Heat stress also interferes with endocrine regulation and reproductive physiology. Altered puberty onset and estrus expression, decreased secretion of key reproductive hormones, and impaired milk production have all been reported (Figure 1). Prolonged farrowing times are another common observation in heat‑stressed sows.
Figure 1 – Maternal and offspring consequences of gestational and lactational heat stress in sows (adapted from Lucy and Safranski, 2017)
Impact of heat stress during lactation
Lactation is the most energy‑demanding stage in a sow’s life, and heat stress greatly amplifies this burden. Under elevated temperatures, sows reduce feed intake to limit metabolic heat production, which in turn reduces milk yield, slows piglet growth, and prolongs the weaning‑to‑estrus interval.
Meta‑analysis by Bjerg et al. (2020) shows that for every 1°C increase above 25°C, sow feed intake decreases by approximately 270 g/day, milk production declines by 0.184 kg/day, and sows experience an additional 1.5 kg of body weight loss over the lactation period. Beyond these productive losses, heat stress disrupts gut integrity by increasing intestinal permeability (‘leaky gut’) and elevating systemic inflammation. Combined, these physiological strains heighten vulnerability to disease and substantially increase the risk of sow mortality during the summer months.
Fetal development under heat stress
The severity of heat stress depends on the stage of pregnancy. Sows bred during hot periods tend to have a higher proportion of low-birth-weight piglets (<1.1 kg) and lower total litter birth weight compared with sows bred under cooler conditions.
Early gestation (mating to ~day 14)
During the first 2 weeks of pregnancy, embryos are developing and implanting in the uterus. Heat stress increases the sow’s core body temperature and alters the uterine environment (blood flow, hormones, and oxidative balance). These changes can reduce embryo survival and increase early embryonic loss, leading to fewer total pigs born.
Mid-gestation (~day 30 to 60)
Between days 30 and 60, major fetal organs and the placenta are developing. Heat stress during this period can reduce placental efficiency and nutrient supply to the fetus.
In female fetuses, this stage also overlaps with important phases of ovarian development. Excessive heat exposure may affect the formation and survival of early germ cells (future oocytes), potentially influencing the reproductive capacity of the offspring gilt.
Late gestation (final weeks before farrowing)
In late pregnancy, heat stress increases oxidative stress and causes the sow to redirect blood flow toward the skin to dissipate heat. This can reduce blood flow to the uterus and placenta. As a result, oxygen and nutrient delivery to the fetuses may be compromised, increasing the risk of stillbirths, lower birth weight, and weaker piglets at birth.
Long‑term impacts of in‑utero heat stress
Research consistently demonstrates that pigs exposed to IUHS face a number of persistent challenges throughout their lives. These range from compromised growth and general health to limitations in reproductive performance and an increased sensitivity to stress. Together, these effects can significantly affect both welfare and productivity in commercial production systems.
Poor growth performance and health
Maternal heat stress has a pronounced impact on life-time growth. Studies show that piglets originating from heat stressed sows tend to be lighter, with body weights reduced by roughly 18% at 10 days of age and by about 17% at weaning. Their feed efficiency is also poorer, which is likely related to higher maintenance energy requirements. Beyond growth retardation, IUHS can alter offspring body composition. Research showed that protein accretion rates may decrease by around 16%, whereas lipid deposition tends to increase by around 33%. These differences in body composition persist regardless of the environmental conditions pigs experience after birth.
Moreover, piglets born at lower birth weights – especially those from sows mated during summer rather than autumn – often display greater fat thickness at slaughter compared with animals born at normal weights. Heat stress during pregnancy can make piglets more reactive to stress (linked with cortisol) and more prone to inflammation throughout life (linked to the higher proinflammatory cytokines).
Impaired reproduction performance
Research showed that IUHS gilts tend to produce fewer total born and born alive piglets. Reproductive performance in female offspring can be affected due to fewer corpora lutea and reduced ovulation rates. Preweaning mortality rises markedly — in some cases by as much as 80% (Figure 2) – ultimately reducing the number of piglets weaned (9.91 compared with 10.85 from in utero thermoneutral (IUTN) controls).
Male reproduction is affected as well. Heat stressed male offspring display significantly lower sperm counts, reduced by roughly 24%, and their testicular growth progresses more slowly – by about 35% – suggesting a delayed onset of puberty.
Figure 2 – Pre-weaning mortality from gilts exposed to in utero heat stress (IUHS) (Source: Safranski et al., 2015)
Elevated stress sensitivity and less activity
Piglets exposed to heat stress before birth show an exaggerated stress response after birth. When exposed to routine stressors such as weaning or transport, these pigs show a stronger stress-hormone response, with higher adrenocorticotropic hormone and cortisol levels. This altered stress regulation is also reflected in their behaviour. IUHS pigs tend to be more reactive, display more stress-related behaviours, and show lower voluntary activity under normal conditions.
Conclusion
Heat stress remains a major challenge in modern swine production. In sows, elevated temperatures can reduce fertility, increase disease risk, prolong farrowing, and reduce colostrum and milk production. Heat stress during gestation can also lead to fetal loss, lower birth weights, altered body composition, and greater sensitivity to stress later in life.
Managing challenges such as heat stress requires a proactive life stage approach to nutrition and management. Through its Micronutrition & Health Solutions portfolio, Cargill ANH brings together targeted micronutrition solutions designed to support resilience across key life stages. In Part 2 of this article series, we will explore nutritional recommendations and micronutritional solutions to mitigate the impact of heat stress.
References are available on request.
