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By Youngji Rho
Monogastric Nutritionist,
Grand Valley Fortifiers

Compared to other animals, pigs are naturally more sensitive to heat as they lack sweat glands and have relatively small lungs compared to their body size, making it harder for them to release heat by panting. The thermoneutral zone for sows ranges from 15 to 22°C with 60 to70 % relative humidity. Therefore, when the temperature rises above the sow’s upper critical temperature (24 -25°C), a sow is unable to or has more difficulty maintaining her homeostatic body temperature. This in turn means she will suffer from “heat stress”.

The impact of heat stress on sows has been well documented. However, many underestimate how heat stress negatively influences sow performance and longevity. Modern pigs are more prone to heat stress compared to the ones from decades ago as they are leaner and more productive which contributes to generating more body heat.

Impact of heat stress  in sows

Reduced feed intake (FI) is often seen in sows suffering from heat stress. The reduced FI can be due to the animal trying to reduce the heat produced by consuming feed. Heat stressed sows can also suffer from respiratory alkalosis. Blood alkalosis occurs when sows are panting excessively, leading to increased losses of CO2 in the body, resulting in increased blood pH. Respiratory alkalosis causes imbalances of various biochemical and physiological functions in the sows body, therefore effort should be made to avoid it.

Heat stress can also impair feed digestibility in sows, which can lead to negative energy balance, poor body condition and less milk yield. During heat stress, more blood flow is redistributed towards the periphery of the sow to release excess heat. This results in reduced blood flow to other organs including the gastrointestinal tract (GIT) which can result in inflammation, oxidative stress, and damaged tissues. Consequently, heat stress is in large part an immune response associated with “leaky gut.”

Heat stress can also compromise reproductive efficiency. Sows exposed to high temperatures have smaller and slower follicular growth after weaning. This results in increased number of sows showing anestrus and silent estrus, ovulation failure, and longer weaning to estrus intervals (WEI). It is important to remember that the follicular size at weaning determines the WEI. Additionally, sows mated in the summer months tend to have lower farrowing rates, which is likely due to early pregnancy disruption. Given all the above, we can clearly see that managing heat stress is critical in sows.

Nutritional strategies to alleviate heat stress

There are environmental management and nutritional strategies to avoid or reduce the impact of heat stress on sows. Environmental strategies such as, better ventilation, drip/sprinkler cooling, floor cooling and reducing the pen density can be very effective. However, these environmental changes may not be possible at certain facilities and/or may require more time and investment. Although less direct, nutritional strategies can be quicker and effective when environmental changes can not be made.

Water is the most important nutrient as it plays a role in various metabolic functions. Therefore, water supply is crucial all year round, but especially during the summer months as requirements increase. Water lines should be checked more often to make sure good supply of clean water is being provided, and the drinker height should be checked for easier access.

Changing the feeding time to a cooler time of the day such as the early morning and late evenings may improve the feed intake. However, if this is not possible other nutritional strategies should be considered.

As mentioned above, reduced feed intake in sows during heat stress is one of the major concerns. Feeding a diet formulated to have low thermic effect can reduce the heat generated from feed consumption. This can be accomplished by increasing the use of fat sources, reducing the protein and/or fiber content of the diet. Compared to other energy sources, fat has the highest digestibility and generates the least amount of internal heat.

Reducing protein content in the diet can also reduce the amount of heat being produced during protein digestion. It has been shown that when a low protein diet (13.8 vs 18.7% CP) was fed to lactating sows, heat production decreased from 288.7 to 154.8MJ/(BW0.75) in sows housed under thermal neutral conditions. However, while reducing the protein content, it is important to supply and balance the amino acids required with the currently available crystalline amino acids.

In recent years, a lot of attention has been focused on “fiber”, as gut health has been the hot topic. However, fiber fermentation which occurs in the hindgut generates heat. Therefore, fiber inclusion should be also carefully looked at during times of heat stress.

As mentioned earlier, heat stress can cause alkalosis. Supplying electrolytes such as sodium or potassium can restore the electrolytic balance. This can be supplied in feed or in water.

Betaine supplementation can protect cells from osmotic stress, especially in the GIT as high osmotic regulation occurs in the GIT. Betaine is also known to convert homocysteine to methionine. Sulfur amino acids such as methionine and cysteine are critical in maintaining intestinal function and integrity. During heat stress as gut barrier function is compromised, supplying methionine can increase epithelial regeneration. It has been also found that betaine supplementation to lactating sows experiencing heat stress resulted in increased follicle development and numerically increased farrowing rate. Additionally, supplementing extra levels of feed additives like zinc and antioxidants (e.g., Vitamin E and selenium) have been shown to protect the intestinal barrier integrity during heat stress as well.

Fatty acids especially n-3 and n-6 are critical in oocyte quality as it is an energy substrate for oocyte maturation and are precursors for prostaglandins which is important for uterine health postpartum, follicle develoment, and ovulation. During heat stress, with the reduced FI and negative energy balance, it is important to supplement these fatty acids as they can be depleted.

It is important to understand the detrimental impact of heat stress to sows, and the options to alleviate it. Making management changes to reduce the environmental impact on sows should be priority #1. Combining nutritional strategies along with improved environmental conditions can help reduce the negative impacts of heat stress even further, allowing your sows to remain healthy and productive year-round.

Liu, F., Zhao, W., Le, H. H., Cottrell, J. J., Green, M. P., Leury, B. J., and Bell, A. W. (2021). What have we learned about the effects of heat stress on the pig industry?. Animal, 100349.

Lucy, M. C., & Safranski, T. J. (2017). Heat stress in pregnant sows: thermal responses and subsequent performance of sows and their offspring. Molecular Reproduction and Development, 84(9), 946-956.

Mayorga, E. J., Renaudeau, D., Ramirez, B. C., Ross, J. W., and Baumgard, L. H. (2019). Heat stress adaptations in pigs. Animal Frontiers, 9(1), 54-61.

Williams, A. M., Safranski, T. J., Spiers, D. E., Eichen, P. A., Coate, E. A., and Lucy, M. C. (2013). Effects of a controlled heat stress during late gestation, lactation, and after weaning on thermoregulation, metabolism, and reproduction of primiparous sows. Journal of animal science, 91(6), 2700-2714.

Zhang, S., Johnson, J. S., and Trottier, N. L. (2020). Effect of dietary near ideal amino acid profile on heat production of lactating sows exposed to thermal neutral and heat stress conditions. Journal of animal science and biotechnology, 11(1), 1-16.

This article was written for the Summer 2022 Swine Grist. To read the whole Swine Grist, click the button below.