Heat stress in swine: Impact and strategies to combat

Introduction

Our climate is regularly changing. Since industrial revolution, global temperature has increased to 1.5°C. Numerous health and environmental challenges troublesome the swine industry worldwide.  Heat stress (HS) during summer is one seasonal condition that has an immensely negative effect on pig farmers which may let stifle their profitability if proper management strategies are not adopted in time. Heat stress (HS) is being one of the largest impediments to efficient animal agriculture which causes changes in behavior, physiology, and metabolism of animals. It causes serious health impacts that result in diminished performance, especially in breeding herds and finisher pigs, eventually, economic losses for the farmer.

Pigs are more prone to heat stress compare to other farm animals, because they do not have sweat glands, have relatively small lungs, and possess a thick subcutaneous layer of adipose tissue, making it difficult for them to remove excess internal heat, which jeopardizes their health.  Ambient temperature and humidity both contribute to heat stress and pigs generally develop heat stress at much lower temperatures when the humidity is high.

Impact of heat stress on physiological functions of swine

The two noticeable symptoms exhibited by pigs during stress are loss of appetite and increased respiration rate. By reducing feed intake they try to decrease internal heat production.  During prolong heat stress, pigs start to drink excessive amounts of water which leads to a loss of electrolytes via the urine and accumulate acids produced within the body resulting in acid-base imbalance, which eventually leads to diarrhea or even death in severe condition.

During heat stress, animals shift their blood flow towards the skin to increase heat dissipation and their gastrointestinal blood vessels constrict so that blood pressure is maintained. This reduces the blood flow in the gastrointestinal system for the nutrient absorption and supply of nutrients and oxygen to the intestinal cells that in turn leads to decreased FCR, decreased growth, or milk production. Heat stress is directly or indirectly (via reduced feed intake) causes suboptimal production. So, it is important to minimize heat stress during hot weather to get optimum production.

The intestinal defense systems of pigs are considerably compromised in heat stress (even for a period of just under 2 to 6 hours) with infection probability as pathogenic bacteria can more easily invade the body. Heat stress can therefore lead to secondary infections if the sanitation conditions are poor.

Impact of heat stress in pregnant sows

• Increased higher rectal temperatures in sows post-partum,
• Higher respiration rates
• Prolonged farrowing
• Increase the risk of intrauterine growth retardation in pregnant sows.

Heat stress in lactating sows: causes reduced feed intake with substantial lowered milking performance leading to higher pre-weaning mortality and reduced piglet weaning weight.

Negative impact of heat stress on boar fertility

Remarkable negative effect on semen quality, particularly, increased sperm abnormalities and reduced sperm motility occur in boar under heat stress condition. Altered sperm nucleus size is also one of the consequence of heat stress. Moreover, this altered sperm quality declined conception rates and reduced embryo survival in artificially inseminated gilts.

Negative impact on piglet development

Heat stress in sow leads to changes in the intestinal barrier. This impairment can lead to bacterial translocation and increased permeability for endotoxins, which in turn may result in local or systemic damage or inflammatory responses. Especially effects on milk production can be brought in context with endotoxin circulation as it is reported that circulating endotoxin decreases prolactin level in postpartum plasma. Reduced milk production may lead to impaired piglet immunity and overall growth.

Genetic impact on offspring

Piglets born from heat-stressed sows (during the first half of gestation) have been shown to demonstrate the epigenetic effect of developing higher levels of back fat in the grower phase. They will have less lean meat accretion and increased adipose tissue accretion.

Unfavorable impact on carcass composition

Pigs reared in Heat stress conditions have reduced muscle mass due to reduced protein synthesis in combination with increased muscle proteolysis to provide energy. Besides, increased adipose tissue in carcass due to reduced ability in the utilization of adipose tissue as an energy source combined with increased lipogenesis.

The recommended strategies to combat heat stress in swine

While stress cannot be avoided entirely, it should be the goal to minimize it. Below are some recommended strategies and tips for reduction of heat stress and its negative impact-

1. Increase ventilation and airflow and regularly check cooling system is in good working order (spray cooling).
2. Reduce stocking density.
3. Unlimited access to fresh and cool drinking water

Provide fresh and cool drinking water. During hot environmental temperatures, the growth rate of unwanted bacteria and other microorganisms increases. So, need extra attention to the water quality. Different organic acids have been shown to inhibit unwanted bacteria. To prevent any build-up of biofilm that will actively multiply in hot conditions, sanitizing all the drinking lines during the pre-summer period is better.

4. Adjustments to feeding times and feed composition

a) Feeding time: Avoid feeding during the hottest period of the day (between 10.00am-4.00pm), as animals are more likely to consume feed when it’s cooler.

b) Feed composition:

• • Increase dietary energy density– Considering that feed intake will be reduced (vary from 10 to 25%) during the summer season all of the nutrients must be concentrated. Add a higher level of fat to the diet to make energy-dense feed formulation as fat metabolism generates less endogenous heat than metabolizing protein. Avoid excess protein feeding to reduce the metabolic heat production but pay attention to the amino acid profile and content to fulfill the dietary need for the pig. Scaled up vitamin and mineral levels in feed.
  • Minimise excess fiber in feed- It helps in minimizing intestinal fermentation and therefore heat production. So fermentable fiber should be replaced with non-soluble sources.

5. Supplement electrolytes, antioxidants through the water and feed additives

During periods of heat stress, the dietary electrolyte balance deserves extra focus to correct electrolyte imbalances. With increasing temperatures, the breathing tension of an animal is increased. More carbon dioxide is taken away from the bloodstream and exhaled faster respiration. This changes blood pH levels, leading to metabolic acidosis and reduced feed intake. Blood ‘buffers’ like sodium or potassium may restore electrolytic balance and help to support the intake of feed.

As stated before, heat stress may lead to reduced intestinal integrity. Feed additives like butyric acid and antioxidants like vitamin E and vitamin C, selenium, and betaine can be supplied with the feed to support intestinal integrity and rehabilitation.

6. Combat mycotoxin

Fight toxins that adversely affect animal health. The chances of mycotoxin contamination are increased by hot and humid weather. The liver of an animal is often under stress during heat stress. So, the liver must be kept healthy as possible and further stress from mycotoxins should be avoided.

7. Combat heat stress during transportation

Transportation of pigs should be as early as possible in the morning to avoid the heat of day time. Avoid feeding just before transportation, but give half-rations about 4 hours before loading. The appropriate design of the loading ramp can reduce load stress according to the tailboard height and the use of a hydraulic lift to charge the pigs on the top deck rather than with a steel ramp. It is an advantage to wet the pigs’ skin as they board the vehicle if the transport is inevitable in the hot midday. The lower stocking rates on lorries during hot weather should be considered. Adequate ventilation of the pigs is important to reduce the temperature increase.

Conclusion

Heat stress causes serious impact on variety of reproduction and growth parameters and which ultimately constraint sustainability and profitability of swine farmers. It has direct (increased core temperature) and indirect (reduced nutrient intake and compromised intestinal integrity) effects on multiple physiological systems that ultimately compromise animal health and productivity. Losses due to heat stress include nonproductive days for sows and economic losses in growing-finishing pigs.  To minimize potential problems with animal welfare, swine farmers need to recognize and monitors symptoms of heat stress and actively manage their systems. Implementing all these nutritional and management strategies to combat heat stress can have an positive impact on pig farming in terms of subsequent year’s reproductive and financial performance.

References

1. Mayorga, E.J., Ross, J.W., Keating, A.F., Rhoads, R.P. and Baumgard, L.H. 2020. Biology of heat stress; the nexus between intestinal hyperpermeability and swine reproduction. Theriogenology.
2. Gabler, N.K., Koltes, D., Schaumberger, S., Murugesan, G.R. and Reisinger, N. 2018. Diurnal heat stress reduces pig intestinal integrity and increases endotoxin translocation. Translational Animal Science, 2(1): pp.1-10.
3. Zhao, L., McMillan, R., Zhang, Z., Xie, G., Baumgard, L., El‐Kadi, S., Selsby, J., Ross, J., Gabler, N., Hulver, M. and Rhoads, R. 2015. Effect of heat stress on pig skeletal muscle metabolism. The FASEB Journal, 29:755-7.
4. Pearce, S.C., Gabler, N.K., Ross, J.W., Escobar, J., Patience, J.F., Rhoads, R.P. and Baumgard, L.H. 2013. The effects of heat stress and plane of nutrition on metabolism in growing pigs. Journal of animal science, 91(5): 2108-2118.