It’s Not the Heat, It’s the Humidity: Strategies for Navigating Heat Stress

Around this time of year, a common phrase reappears, “It’s not the heat that is so bad, it’s the humidity.” Humans don”t start to notice the heat until the heat index – the combination of temperature and humidity – approaches 90 degrees Fahrenheit (°F). Pigs, though, have a much lower heat tolerance. The thermoneutral zone is a term used to describe ideal temperature ranges for animals. The thermoneutral zone includes an upper critical temperature where the pig begins to experience heat stress and a lower critical temperature where the pig begins to suffer from cold. From weaning through marketing, the upper critical temperature declines from 90°F to 70 – 75°F. Sows have an upper critical temperature of around 70°F.

Humans dissipate heat through breathing and sweating. A pig has two disadvantages: it does not sweat and relative to its size, has low lung output. As a result, pigs cannot manage heat as well as people do, causing them to experience significant heat stress at lower temperatures. Thankfully, heat stress can be detected in a few steps:

  1. Check the barn temperature and humidity. Is the temperature at or above the upper critical temperature for the size of the pigs? Is the relative humidity greater than 45%?
  2. Count the breaths of the pigs that are lying down. Is the respiration rate greater than 25 breaths per minute?
  3. If you’re still unsure, take the pig’s temperature. A pig’s normal rectal temperature is 102°F. If their temperature reads 103°F or higher, coupled with elevated environmental temperatures, the pig is likely experiencing heat stress.

Industry estimates place the cost of heat stress comparable to the financial impact of PRRS (Pollman, 2010 ). Heat stress reduces performance across the entire production cycle.

  • Growing pigs have reduced feed intake and growth rate, leading to increased feed conversion as the pigs use more energy to dissipate heat. Additionally, new research indicates that heat stress compromises gut barrier function, increasing chances of gut health challenges.
  • Sows and gilts’ reproductive efficiency is compromised due to high cycle times, poor conception rates and reduced embryo survival.
  • Boars have reduced semen quality (heat stress reduces sperm motility and morphology) compounding this poor reproduc-tive efficiency). Semen quality issues lag 6 – 7 weeks behind the moderation of temperature within the sow herd, which can prolong the impact of heat stress.

(Artuso-Ponte, 2018 ; Rozeboom, See and Flowers, 2000)

To optimize performance, heat stress should be managed with environmental and nutritional strategies. The first step in managing heat stress is managing the environmental controllers. Make sure the temperature curves are set appropriately for the upper critical temperature for the size of the pig. Ensure that fans are set for adequate airflow for temperature and humidity regulation.

Spring is a great time to clean water lines. Biofilms build up in water lines significantly restricting water flow. Utilize the opportunity between groups to clean water lines to remove the biofilm. You can also remove, clean and inspect water nipples to make sure they will provide the pigs with optimum water availability and minimize water waste.

Nutritional strategies, such as diet composition, protease use, mineral source, antioxidants, and feed quality, can impact a pig’s ability to manage heat stress. Minimizing the heat of digestion is important. Think about a hot day – you probably want a cold sandwich instead of a big steak. Because the steak includes a lot of complex proteins, it takes more metabolic energy to break it down, increasing body heat. To minimize the heat of digestion, we formulate high net energy diets. Typically, we accomplish this by using high levels of synthetic amino acids. We can also reduce heat of digestion by using enzymes called proteases. Proteases break down protein which benefits the animal with more amino acids from ingredients allowing for a lower dietary protein level (through reduced soybean meal). Think of it as reducing the size of the steak the pig is having to eat while still delivering the optimal nutrients.

Oxidative stress occurs because of free radicals that are produced as a result of normal processes like metabolism and immune response. Heat stress increases oxidative stress in the pig. In the case of heat stress, metabolic activity is increased with increased respiration and heart rate as the pig tries to cool itself. Pigs have some defense mechanism against these free radicals. Many of these defenders, called free radical scavengers, are dependent on minerals such as selenium, zinc, copper and manganese. When considering mineral sources for aiding the animal’s ability to manage the free radicals, research shows HMTBa-chelated trace minerals are the most bioavailable sources of Zn, Cu, and Mn. Inorganic minerals in the oxide or sulfate forms are common but have relatively poor availability. Chelated minerals have availabilities 2 to 3 times higher than inorganic minerals enhancing the pig’s natural defenses against free radicals (Wedekind et al. 2014). This is vital for sows that live above their upper critical temperature for much of the year. For sows and some growing pigs, organic selenium has been used for many years because of its increased mineral availability (NOVUS Trial Report Summary, 2011 ).

When a pig’s internal oxidative stress defense mechanisms become overwhelmed (such as during heat stress), we can help the pig manage free radical damage by supplementing anti-oxidants. Traditionally, when we think about supplementing antioxidants, we think of vitamin E or vitamin C. However, studies show that ethoxyquin better manages oxidative stress and spare vitamin E (Lu et al. 2014; Fernandez-Dueñas, 2009).

Feed quality is critical when pigs are facing heat stress – especially mycotoxin management and fat quality. Even with a robust mycotoxin mitigation strategy, some mycotoxins are present and must be managed by the liver. Metabolism of mycotoxins also increases oxidative stress and compromises vitamin A and E status.

For many, fat quality refers to moisture and impurities in fat. However, it is the degree of fat oxidation in supplemental fat sources and the complete diet that impacts performance. Supplemental fat has approximately 2.5 times the energy value of corn. However, if this supplemental fat has been oxidized, as much as 35% of the energy can be lost. This increases feed conversion and can compromise growth when the add-ed fat was being used to balance the reduced intake during hot months. Additionally, oxidized fat has negative effects on the gut further compromising performance. Understanding fat quality in the complete diet can lead to strategies to preserve dietary energy and improve growth and feed conversion.

A study has shown that an antioxidant blend containing ethoxyquin can support the animal during mycotoxin challenges (Harper et al. 2010). Mycotoxins and oxidized fat can increase free radical production in the animal. Antioxidant blends containing ethoxyquin can neutralize these free radicals protecting the animal and preserving the energy value of fats.

Heat stress occurs for a longer period than many producers realize. The impacts of heat stress are wide-ranging, from decreased intake and gain with increased feed conversion, to compromised gut health in growing pigs and reduced reproductive efficiency in sows and boars. To support their animals during heat stress, producers should consider:

  • Ensuring adequate temperature and humidity control in barns.
  • Taking the time in spring to clean and maintain water lines and water nipples.
  • Diet formulation strategies to remove protein from the diet while still providing adequate amino acids, such as including high synthetic amino acids and proteases.
  • Focusing on feed quality, including mycotoxin control and oxidation status of fat. Supplementing a synthetic antioxidant can support oxidative challenges and preserve vitamin E reduction.
  • Supporting the pig’s natural antioxidant defense mechanisms through use of highly available sources of Se, Zn, Cu, and Mn.

Remember that it is the combination of heat and humidity that leads to heat stress, but you can help your pigs manage and thrive.

1 Pollmann, D. S. Seasonal effects on sow herds: industry experience and management strategies. J Anim Sci. 2010;88(Suppl. 3):9 (Abstr).
2 Artuso-Ponte, Valeria. Heat stress in pigs – effect in gut. Pig Progress. 2018
3 NOVUS Trial Report Summary: ZORIEN® SeY (Organic Selenium) in Pig Diets, 2011