Heat Stress in Sows and Gilts

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With extreme temperatures in the summer, heat stress can occur in sows sooner and more severely. This does have an impact on the well-being and performance of the sows and piglets with significant economic losses as a consequence.

Heat stress can be the leading cause of infertility in sows and gilts during summer months.  When ambient temperatures surpass 27o Celsius, females can begin to exhibit effects of heat stress.  Most noticeably, fertility rates can drop 5 to 10 percent for sows bred during periods of heat stress.  Sow death loss may also increase, particularly in pregnant sows pre-farrowing.  Sow breed-back and weaned to estrus intervals are affected reducing the number of weaned sows available for breeding.    The number of replacement gilts needed to ensure one gilt farrows can triple during the summer months.  Low farrow rates and increased number of replacement gilts can have a negative impact on herd output and the number of non-productive sow days.

The most common sign of seasonal infertility is a reduction in farrowing rates, although reductions in litter size are occasionally observed.

The graph above shows a three (3) year period for farrowing rate in the USA.  Note the drop in farrowing rate in the fall of each year.  This is an example of how heat stress routinely effects sow performance.


  • Associated with hot weather
  • Associated with decreased daylight
  • A test for breeding herd management
  • It is inevitable in many regions of the world.

(Graph and points per Dr. John Deen, PigChamp USA data)


  • Much like humans, pigs feel heat based on temperature and humidity.
  • Heat stress for lactating sows occurs when temperatures are outside of the animal’s comfort zone which ranges from 15 to 21 degrees Celsius. 

Swine are particularly sensitive to heat because they do not sweat.  The majority of cooling done by the sow, is through panting and lying on cool surfaces.  The number one way to cool the sow is by convective cooling – air movement over her body. 

High humidity can also be a major contributor to heat stress.  Thus, the heat index, a combination of heat and humidity should be considered.  A heat index chart can be seen below:


By monitoring respiration rate during rest, producers can determine if females are heat stressed.  Normal breathing rates are 15 to 25 breaths per minute.  Breathing rates above 40 breaths per minute indicate pigs are at risk for heat stress.  Feed intake in lactation will typically be depressed during periods of heat stress.  Extended wean-to-estrus intervals and anestrus following weaning may also be an indication of heat stress.  In particular, first parity sows are more prone to anestrus following weaning due to their smaller body size and lower lactation intake.


Embryo mortality increases when females are heat stressed.  When extreme heat is experienced within the first 30 days of pregnancy sows and gilts can display pseudo-pregnancies and reabsorb fetuses.  Heat stressed sows are uncomfortable.  They may lay on pigs more frequently and exhibit reduced overall mothering ability.  In addition, heat stressed sows in lactation will have reduced feed intake, which affects milk production, resulting in lower weaning weights and sow breed back.

Days PregnantResult
1 – 14Abort – Return to estrus within 21 days of breeding
15 – 30Pseudo-pregnancies, females show pregnant at 30-day check but reabsorb fetus
30 – 60No known/documented effects
60 – 114Increase in Stillborn and decrease in birth weight
Heat stress effects on pregnancy


Choice is committed to assisting you in maximizing the performance of your operation.  Here are some tips to prevent and avoid heat stress:

  • Ensure ventilation systems are operating correctly and are providing adequate air movement.  The minimum ventilation rates in enclosed facilities for breeding and gestating sows is 510 m3/head while the rate for sows with litters is 850 m3/ head during hot weather.
  • Evaluate air movement and distribution and remove or reduce impediments.
  • Improve air temperature with cool cells or drip systems.  If these systems are activated when temperatures are above 27 degrees, they will help keep the facility from heating up too quickly.
  • Ensure a plentiful water supply.  Adequate water supplies are critical for animals in hot weather. Minimum water flow per nipple waterer of 1 liter/minute.
  • Ensure sprinkling systems are correctly placed and operational before heat stress is a threat.  Sprinklers are effective for cooling sows in all places of production if used correctly.
  • To keep farrowing house temperatures comfortable, proper heat lamp management is key during summer months.
    • Utilize 100-watt incandescent bulbs rather than heat lamp bulbs (if they are available).
    • Keep heat lamps away from sows’ head during lactation.
    • Consider controlling heat lamps with a thermostat or environmental control system to make sure the heat lamps are on when needed and as importantly, off when not needed to reduce heat stress on the sow and reduce electricity cost.
    • Turn heat lamps off on hot days (when temperatures exceed 29 degrees Celsius).
    • Provide mats for the litter to maintain their comfort when heat lamps are off.
    • During hot months, turn heat lamps off after pigs are older than one week.
  • Tune up your ventilation system prior to the summer season (generally April).

Ventilation inspections include:

  • Ensure all parts of the ventilation system are operational – fans, baffles, cool cells, drip systems, curtains etc.
    • Ensure fan blades and fan shutters are clean to run efficiently.
  • Avoid moving females during hot temperatures.  Typically, early morning is the best time.
  • The number of weaned sows available to be bred in hot months typically declines due to a decrease in the weaned sow breed back%.  Producers can compensate by increasing the gilt pool.  Many producers will increase the farm inventory by 5 to 10% two months prior to the heat stress season so the farm can maintain its breeding targets.  They may also reduce the sow culling rate to help keep farrowing crates full.
  • Nutritional status before breeding also affects embryo numbers and survival.  Choice recommends feeding weaned sows ad-libitum from weaning to first service.
  • Maintain correct sow body condition.  Correct sow body condition can help sows cope with heat stress.  Over conditioned sows are more prone to heat stress and typically will have a lower lactation intake. 
  • Nutritional programs and feeding practices should be reviewed during periods of heat stress.  The diet should be formulated during periods of high heat to actual lactation intake.
  • Closely monitor sows during early or late season temperature spikes.  These temperature spikes may be more stressful to sows because the sows are not acclimatized to these temperatures.

Depending on geographical location, heat stress can be a serious problem for two to six months of the year.  Proper precautions and management adjustments can reduce the impact that heat stress can have on production levels and enhance the overall comfort of sows.

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