Pigs are much more sensitive to heat than other animals because they lack the ability to sweat. Therefore, high temperatures can lead to heat stress which causes poor performance. Often, pork producers think only about grow-finish pigs when they consider the negative effects heat stress has on their pigs. In reality, heat stress also affects the breeding herd.

Dr. Steve Pollmann, Vice President of Smithfield’s Hog Production Division, formly Murphy-Brown LLC. estimated heat stress costs the American swine industry $900 million each year. Of that, about $450 million loss is experienced in the grow-finish stage and the breeding herd suffers the other $450 million loss.

Optimal temperature for housed swine is shown below in the table. As a pig gets older and heavier its optimum temperature decreases. Therefore, the effects of heat stress are more of a concern with older finishing pigs (> 110 lbs) and with sows and boars than with younger pigs. Sows, boars, and finishing pigs begin to feel the negative effects of heat stress at about 70°F. If temperatures remain above 80°F for more than 2 to 4 days, decreases in performance and reproductive efficiency can result if cooling relief is not provided.

Table 1. Optimum temperatures for housed swine of all ages.
Animal age, weight Optimum Temperature (°F) Desirable Temperature Limits (°F)
Lactating sow 60 50 – 70
Litter, newborn 95 90 – 100
Litter, 3 weeks old 85 75 – 85
Nursery, 12 – 30 lbs 80 75 – 85
Nursery, 30 – 50 lbs 75 70 – 80
Grow – finish pigs 60 – 70 50 – 75
Gestating sows 60 – 65 50 – 70
Boars 60 – 65 50 – 70

Effects on the breeding herd

The negative effects of heat stress in the breeding herd are often referred to as seasonal infertility. Seasonal infertility describes the typical decreases in production experienced by sows that are bred during summer months and farrow from November through January. Heat events in the summer that lead to heat stress experienced by the breeding herd is the primary contributer to seasonal infertility. Seasonal infertility is not caused by a seasonal reproductive cycle because domestic swine are not seasonal breeders.

Heat stress experienced by sows during lactation also negatively affects production. Most commonly, increased environmental temperatures lead to a reduction in feed intake by the sow. This reduced feed intake leads to depressed milk production which then reduces piglet body weight gain.

Heat stress experienced by sows in the 14 to 21 days before insemination can have a large negative impact on farrowing rate. Heat stress experienced by sows between 7 days before until 12 days after insemination have a large negative influence on total pigs born per litter (Bloemhof et al., 2013).

Effects of heat stress on sows

  • Decreased farrowing rates
  • Lower total born per litter
  • Reduced number of piglets born alive per litter
  • Reduced number of litters farrowed per week
  • Reduced feed intake
  • Lower milk production
  • Lower piglet body weight
  • Increased wean-to-estrus interval
  • Failure to express estrus
  • Higher embryonic death (if early gestation)
  • Higher stillborns (if late gestation)
  • Failure to maintain pregnancy
  • Increased sow mortality
  • Lower weaned pig throughput

Effects on grow-finish stage of production

Due to selection for increased leanness, total heat production of growing pigs has also increased over the years. The more productive an animal is, the more heat they produce. This elevated heat production makes pigs less tolerant of external heat so they become heat stressed more easily than a less productive pig. Therefore, in response to heat stress, pigs will decrease feed intake as a way to reduce the heat they generate. Reduced feed intake reduces growth rate.

Pigs raised in heat stress conditions will have more fat deposits than pigs reared in cooler environments. This elevated fatness results because the pig’s body will alter nutrient utilization to more fat growth than protein growth when a heat-stressed pig consumes less feed (Baumgard, 2015).

Recently, the effects of heat stress experienced by the sow during gestation has been studied to determine the effects of heat stress on the piglets in the sow’s uterus. Piglets born to sows that experienced heat stress during pregnancy will have increased core body temperature making them more susceptible to heat stress after birth (Baumgard, 2015). Metabolism of these offspring is also modified resulting in less skeletal muscle and more fat tissue being deposited during the growth stage (Johnson et al., 2015).

Effects of heat stress on growing-finishing pigs

  • Reduced feed intake
  • Decreased body weight gain
  • Inconsistent market weights
  • Decreased feed efficiency
  • Higher carcass fat deposits
  • Reduced carcass quality

Take home

The effects of heat stress should not be overlooked. Heat stress causes grow-finish pigs to have reduced feed intake during the summer and negatively affects the sow herd during breeding, gestation, and lactation. Furthermore, heat stress during pregnancy can have lasting negative effects on the offspring. The importance of heat mitigation should not be underestimated.


Baumgard, L. 2015. Assessing the impact of seasonal loss of productivity. National Pork Board Animal Science Webinar. (Accessed 14 October 2015).

Bloemhof, S., P. K. Mathur, E. F. Knol, and E. H. van der Waaij. 2013. Effect of daily temperature on farrowing rate and total born in dam line sows. J. Anim. Sci. 91:2667-2679.

Johnson, J. S., M. V. Sanz Fernandez, J. F. Patience, J. W. Ross, N. K, Gabler, M. C. Lucy, T. J. Safranski, R. P. Rhoads, and L. H. Baumgard. 2015. Effects of in utero heat stress on postnatal body composition in pigs: II. Finishing pigs. J. Anim. Sci. 93:82-92.

Lay, D. 2010. The physiologic response to stress and its effects on swine reproduction. Midwest ASAS Billy N. Day Symposium. (Accessed 20 October 2015).

Myer, R. and R. Bucklin. 2001.Influence of Hot-Humid Environment on Growth Performance and Reproduction of Swine. Pub. AN107 UF/IFAS Extension. (Accessed 29 October 2015).

Pollmann, D. S. 2010. Seasonal effects of sow herds: industry experience and management strategies.Midwest ASAS Billy N. Day Symposium. (Accessed 20 October 2015.)

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