How to manage heat stressed AI Boars

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Individual boars react differently to heat stress. Producers often associate heat stress only with summer heat. But boars can also become heat stressed in the winter. Some tips for prevention of AI Boars and heat stress.


Heat stress in boars can occur anytime of the year. Usually the problem is seen in the hot summer months and in warm tropical areas of the world. Symptoms of heat stress in AI boars are a reduction in semen quality and a decrease in conception and farrowing rates. A drop in total born can also occur.  A reduction in semen quality will be displayed in a variety of ways, but the most common is an observance of sperm cells with a high incidence of cytoplasmic droplets. The problem occurs when the temperature of the testicles increases excessively and results in improper spermatogenesis or sperm cell development.  Sperm production occurs at temperatures 3.5o to 7o Fahrenheit (2o to 4o Centigrade) below body temperature. 

Individual boars react differently to heat stress. Some boars are prone to heat stress when the ambient temperature is as low as 83⁰ F (28⁰ C) while others do not exhibit semen quality problems until the temperature reaches 85⁰ F (29.5⁰ C). When the temperature reaches 90⁰ plus F (32⁰ C), virtually all boars are subject to the effects of heat stress.

Producers often associate heat stress only with summer heat. Boars can also become heat stressed in the winter. When a boar gets cold, he pulls his testicles toward his body. If the testicles remain in an ascended position for an extended period of time, the testicular temperature becomes the same as the body temperature. Signs of heat stress can occur if the testicles remain in an ascended position for a significant period.  As stated previously, normal sperm cell development and maturation is optimized at a temperature slightly below normal body temperature. The function of the scrotum is to prevent heat stress damage to the sperm cells by regulating the testicular temperature.  The scrotum does this by rising or lowering depending the boar’s environmental temperature.  In hot weather the testicles descend and in cool weather the testicles will rise into the body.

Boars can also show signs of heat stress if they become sick and spike a high fever.  Some vaccines are known to show similar affects.  The boar’s immune system can spike a fever in reaction to some vaccines that can affect semen quality.

Spermatogenesis, the time to produce a mature sperm cell, typically takes 6 to 8 weeks.  It is typical to see signs of heat stress in semen 6 to 8 weeks after the boar or boars were first exposed to a heat stress event.   Depending on how long the boars were exposed to the heat stress event (days, weeks or months) will affect how quickly the boar recovers.  If heat stress is a regular occurrence, spermatogenesis is constantly affected making semen quality recovery difficult.


Most boars will respond to treatment for heat stress. The following recommendations have been effective for Choice in solving or reducing the effects of heat stress.

  • If boars produce ejaculates with more than 25% proximal droplets, they should be rested for 3 weeks.
  • If boars produce ejaculates with less than 25% proximal droplets and more than 25% distal droplets, they should be rested for 2 weeks.
  • If an ejaculate is evaluated with more than 25% proximal and more than 25% distal droplets, the boar should be rested for 3 weeks. The key measurement to determine if boars should be rested is the proximal droplet evaluation rate of greater than 25%.
  • Boars may not show full recovery within the initial rest period, particularly if they have been severely stressed. However, with additional rest the rate of observed abnormal cells will continue to decrease. For example: If a boar has 40% proximal droplets and 25% distal droplets when heat stress is first recognized,
  • The boar should be rested for 3 weeks. If after 3 weeks of rest, the boar exhibits 10% proximal droplets and 40% distal droplets, he should be rested an additional 2 weeks. This additional rest period typically results in a return to normal semen quality.
  • During the rest period the boars must receive proper nutrition designed for AI boars as well as a cool environment to allow for normal sperm cell development.
Pictures from “Evaluating Semen Quality” Kevin J. Rosenbloom, North Carolina State University, Publication number ANS00-812S
  • Keep day-to-day quality control records of randomly selected samples: Monitoring daily production is important to early recognition of a boar or boars showing signs of heat stress.
    • % Normal sperm
    • % Motility
    • Total Sperm Per Dose (TSD)
  • Routine third-party assessment of randomly selected doses (minimum of 5 doses per assessment) of extended semen every 3-4 weeks provides feedback to the boar stud personnel, management and their production consultants on the current statist of the boar stud.
  • Choice recommends a complete semen analysis from a third party assessor that includes:
    • Dose volume
    • Sperm concentration
    • Total sperm per dose
    • Objective motility assessment
    • A full morphology differential
    • Aerobic culture


Boars should be kept as cool as possible in the hot weather. When ambient temperatures reach 80⁰F (26.5o C), supplemental cooling is recommended. This can be accomplished with fans, cool cells, foggers, or drippers and air conditioning. The number one way to cool the boar is by air movement across his body.  When using drippers, water should be directed to ensure complete coverage of the testicles. Some producers now utilize two dripper lines; one for the neck and the other for the testicles. Water alone is not enough to prevent the effects of heat stress. The combination of air movement and evaporative cooling is a very powerful way to remove body heat and create an environment where the boar feels cooler and performs better.  Air conditioning has become a more common option.  The advantages to using air conditioning; better temperature control and potential for lower temperatures, and the climate is drier because there is no water added to the environment.

  • Introduction of new boars during the heat of the summer. Make any transport movements during the early morning hours (consider for culls as well from a proper animal husbandry aspect; view the boars as yours until they reach market).
  • Maintain functioning cool cells and ventilation systems (fans, controllers etc.).
  • If using a dripper/mister system be sure there is proper dry down time to avoid overheating the boars by being too wet.   Continuous watering of the boars may also cause respiratory issues and feet and leg problems. The run time of dripper/mister systems varies in different facilities but, many systems run one minute on and 9 minutes off in a 10-minute cycle.  The key is the animal is dry or almost dry prior to the beginning of the run cycle.
  • Boars should be in a body condition score of 3 to 4 on a five point BCS program.  Over weight boars will be affected by heat stress more quickly and severely than a boar in proper condition.
  • Boars should have access to a clean ad-libitum water source.  For nipple waters, a minimum of 1 quart/minute (1 liter per minute water) flow is required. 

Ultimately, it is up to the boar stud manager and production team to implement management practices which will optimize the health, productivity and quality of sperm production from the boar stud herd. These strategies and consistent Standard Operating Procedures (SOP’s) should extend to preparation and introduction of the boar into the herd, boar management, environmental management and quality control of the semen product.

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