When you see the term “seasonal infertility,” do you ever wonder which season of the year is causing the problem?

In my opinion, the term should be replaced with a better description of the problem, such as “summer infertility” or “fall infertility.”

Summer infertility clearly identifies that reproductive problems are occurring during the summer months. Summer infertility covers a wide spectrum of problems, including anestrus in gilts and sows, estrous detection problems, decreases in conception rate, decreases in embryonic survival and decreases in fertility of boars.

Fall infertility clearly identifies that reproductive problems are occurring during the autumn months.

The severity of summer and fall infertility varies from farm-to-farm, region-to-region and year-to-year. In the end, what we are most interested in is maximizing fertility and breeding herd reproductive performance regardless of season or location.

Factors Influencing Fertility

Many scientific studies and articles on seasonal infertility in pigs have described the symptoms and the relationship of ambient temperature, photoperiod and nutrition to reproductive performance of gilts, sows and boars.

However, a large volume of scientific literature does not exist on the design and management of swine breeding-gestation facilities to eliminate summer infertility.

Although ambient temperature and photoperiod are thought to be the two major factors influencing lower reproductive performance during the summer and fall, many other factors contribute directly or indirectly to the effect of temperature and photoperiod on reproduction.

Other factors that can influence reproductive performance of sows and gilts during the summer include:

  • Skills of the workforce;

  • Design, maintenance and management of the ventilation and cooling systems of the breeding-gestation building;

  • Functionality of floor plan design to enhance accomplishments of work tasks by employees;

  • Design, ease of functioning and maintenance of equipment;

  • Lactation feed intake; and

  • Management procedures for performing estrus detection and mating activities.

Tasks conducted in marginally acceptable working environments are most likely accomplished in a marginal manner. I am not aware of a swine breeding facility that absolutely eliminates summer infertility.


The intensity and duration of photoperiod in a breeding-gestation facility is not clearly defined.

A study in France used two light regimens that mimicked spring daylight (a gradual increase in daylight from 12 hours up to 16 hours/day; “long” days) or autumn daylight (a gradual decrease in daylight from 12 hours down to 8 hours/day; “short” days) during gestation of first-parity sows (Figure 1 on page 28).

During lactation and postweaning, the long-day group was maintained at 16 hours of light, and the short-day group was maintained at 8 hours of light. Age at weaning was 20.6 ± 1.1 days.

The effect of photoperiod on reproductive performance is also indicated in Table 1 on page 28. The percentage of sows in estrus within 10 days after weaning was higher for sows exposed to short days than for sows exposed to long days.

In sows that farrowed in January, the wean-to-estrus interval was less for sows exposed to short days (5.2 ± 0.3 days) compared to sows exposed to long days (7.0 ± 0.7 days).

However, the wean-to-estrus interval for sows farrowed in July was no different for sows exposed to either short (10.4 ± 2.3 days) or long days (12.6 ± 3.2 days).

Therefore, this data suggests that under high-ambient temperatures, the duration of photoperiod (short or long) does not restore a good wean-to-estrus interval in first-parity sows. The light level for a breeding and gestation facility is indicated in Table 2 on page 29.

Ambient Temperature

High-ambient temperatures seem to have a stronger impact on summer infertility than either long or short light duration. Research in Australia found that when the average daily maximum temperature exceeded 89.6° F during the week of service, there was an increase in the number of sows returning to service (Table 3 on page 30). The number of sows returning to estrus after mating appeared to start increasing when the average daily maximum temperature was in the range of 82.4 to 89.4° F.

Bred females should be kept in a cool environment (85° F or less) during the first 30 days and the last two weeks of gestation. To prevent an increase in stillbirths, keeping sows cool the last 14 days of gestation is critical. Therefore, because a gestation facility houses sows in various stages of gestation, the entire facility needs to be kept cool.

Tunnel Ventilation

Although tunnel ventilation with evaporative pads is widely used to help cool breeding-gestation facilities, scientifically controlled studies have not been published to document the effect of tunnel ventilation on reproductive performance.

In general, the exhaust fans on tunnel-ventilated buildings are placed at one end of the building, and air is brought in from the opposite end of the building. If possible, air inlets should be located on the wall opposite the fans.

Air will not turn 90-degree corners unless forced. If air enters from the sidewall on the opposite end from the ventilation fans, it will generate airflow across the building rather than along the length of it. This perpendicular airflow will eventually turn and move endwise to the exhaust fans, but it will also leave a dead air zone along the sidewall containing the inlet panel. A dead air zone might also occur in the center portion of the building on the opposite end of the ventilation fans.

A weakness of the tunnel ventilation system is a difference in air quality from one end of the building to the other. The true effect of tunnel ventilation on animal comfort, health and performance from one end of the building to the other has not been documented.

Air movement and cooling through the animal zone in crated buildings is a concern. A high-velocity layer of air will be flowing above the crates, but air speed in the space actually occupied by the sow will be far less than the design speed.

Evaporative Pads

Evaporative cooling uses heat in the air to evaporate water, thereby reducing the air temperature. The disadvantage of this system is that it increases the humidity in the building.

Evaporative pad cooling systems work better in geographic areas that have humidity levels less than 80%. In some areas of the United States, the relative humidity is high during the morning hours; however, by the afternoon, the relative humidity is low. This process occurs because sunlight removes the relative humidity when the temperature gets warmer.

The pad is sized to optimize the evaporation and static pressure loss across the pad. The area of the pad is generally between 3 and 5 sq. ft./1,000 cfm drawn through the system. The wetting of the pad should start at 75 to 78° F. The pad should be allowed to dry before switching off the airflow. This procedure will dry the pad and prolong its life.

Scientific literature could not be located that compared the impact of an evaporative pad cooling system with ventilation systems using fans and sprinklers on reproductive performance.

Fans and Sprinklers

Pigs do not sweat. Therefore, evaporation of water from the skin of pigs plays a major role in cooling gestating sows. The key to cooling sows with fans and sprinklers is to thoroughly wet the animals and then let them evaporate dry. This process is called intermittent sprinkling and cooling. The type of flooring can influence the duration of sprinkling. It is important to let the floor surface dry to prevent foot problems.

Heat-Checking Procedure

It is well known that the characteristics of an excellent heat-check boar are sight, sound and smell. The smell of a boar is a key factor in stimulating sows to express the standing response.

A high ventilation rate is utilized during the summer months to enhance cooling of the sows. However, these high ventilation rates are most likely moving the boar odor quickly toward the ventilation fans as the heat-check boar moves down the alley.

The quick removal of boar odor can create two problems. First, if the airflow is moving toward the rear of the boar, the boar odor received by the sow next to the boar might be minimal. And, if the airflow is moving toward the head of the boar, the sows farther down the row may become stimulated before the boar arrives.

If estrous sows are being inseminated at the time of estrous detection, some of the sows farther down the row may be refractory to boar stimuli by the time the boar gets to their stall. Some sows will only stand for 5-10 minutes after responding to boar stimuli (Figure 2 on page 30).

Due to these two situations, it is best to have highly trained and dedicated workers heat-checking and inseminating females early in the morning (Figure 3). The workers might also want to consider using a boar stink stick to supplement the boar odor. The boar stink stick is a 1 in.-diameter PVC pipe with rags attached to one end. The rags are soaked with preputial fluids, saliva and a small amount of boar urine. Depending on the level of use, the stick is recharged once or twice daily.

Group Housing in Gestation

We do not know if heat stress will have a more detrimental effect on reproductive performance when sows are housed in groups compared to individual crates.

The floor plan design and group management will have important effects on reproductive performance. The term “group housing” needs to be clearly defined. If group housing means that the sows are housed in groups all of the time except during farrowing, reproductive performance will most likely be influenced by heat stress.

In addition to heat stress, other stressors (mixing of bred sows, bouts of fighting, inadequate feed intake, etc.) will have detrimental effects on reproductive performance.

In short, the breeding-gestation facility needs to be designed and managed so that the sows are not mixed during the first 30 days of gestation. The combination of heat stress and fighting will increase the body temperature of sows compared to heat stress alone.

If group housing means that the sows are housed in groups after 35 to 42 days of gestation, the detrimental effects of mixing and fighting on reproduction would be prevented. However, it is important to remember that high-ambient temperatures will still have detrimental effects on conception rate, farrowing rate and litter size on sows housed in individual stalls during this critical period.

Operations that house sows in individual stalls from weaning to 42 days after mating have an established area that is used specifically for breeding and the first part of gestation. Sometimes this area is a specific building or area within a building.

Compared to housing breeding and gestating sows in a “snake” system (filling rows of gestation stalls in sequence), the use of a specific breeding and early gestation area provides an opportunity to focus on a means to enhance the cooling of sows. Depending on the current cooling system, enhancement factors might include the addition of an intermittent sprinkling system or adding fans to improve airflow.

Replacement Gilts

Replacement gilts play a key role in reaching breeding targets during the hot months. Heat stress does lower the proportion of gilts reaching puberty, reduces ovulation rate, and reduces the proportion of gilts expressing second estrus.

Although photoperiod might be a concern, many scientific studies used boar exposure to detect estrus when evaluating the effects of photoperiod on puberty attainment. Thus, the true effect of photoperiod has not been established.

Table 4 on page 32 indicates the proportion of gilts reaching puberty is greatest when gilts are exposed to mature boars, regardless of whether duration of daylight is increasing or decreasing.

Instead of maintaining gilts under long photoperiod, it is most economical to maintain developing gilts under cool, white, fluorescent light (270 to 500 lux) for 10 to 12 hours/day. The light is measured at the eye level of a standing pig. The light should be measured at several locations throughout the building.

Because replacement gilts play a key role in keeping the farrowing facility full, pork producers need to seriously consider establishing an effective gilt development unit. A gilt development unit should implement a strict selection program to identify 75-80% of the most fertile gilts, minimize non-productive days, and breed the gilts at second or third estrus when weighing 300 to 330 lb.

This unit should be designed to reduce heat stress on gilts and enhance estrous detection, especially during the hot months.

To enhance and simplify the detection of estrus in replacement gilts, the facility should be designed to individually house the mature heat-check boars in one area. This design should help concentrate boar odor.

Gilts should be moved to heat-check pens located in front of the boar stalls. With the appropriate floor plan, the boar stalls can be designed so the boars can face either direction in the stall.

This design allows the heat-check pen to be established on either end of the boar stall (Figure 3). The boar stalls are 24-30 in. wide × 6-8 ft. long × 46 in. high. The boar stalls have boar-secure latches at both ends. Nipple drinkers are available on either end of the stalls.

This effective boar stimulation system was designed by the Swine Research & Technology Centre at the University of Alberta.

Table 1. Influence of a Step-wise Photoperiod on Reproductive Performance
Item Replication 1 Replication 2
Month bred September March
Temperature during gestation, °F 64.4 to 68.0 68.0 to 100.4
Month farrowed January July
Temperature in farrowing facility, °F 68.0 to 77.0 77.0 to 95.0
Photoperiod,a days Long days Short days Long days Short days
Cycling by day 10, % 53 92b 14 32b
Wean-to-estrus interval, days 7.0 5.2b 12.6 10.4
aBetween 14 and 105 days of gestation, light duration was either increased or decreased to mimic those occurring during spring (long days) or autumn (short days). Figure 1 indicates the two light regimens. The intensity of light at the level of the animal's eye ranged between 150 to 250 lux.
bValues for short days were significantly different (P < .05) from long days.
Source: J. Anim. Sci. 72:1461-1466, 1994.
Table 2. Light Level for Breeding and Gestation Facilities
Level of illumination, foot-candlesa Watts per square foot of floor area
Standard cool white fluorescent Standard incandescent Compact fluorescentb
40 watts 100 watts 150 watts 20 watts 23 watts
15 0.42 1.72 1.50 0.42 0.37
aUnit of illumination equal to one lumen per square foot.
bRetrofit bulbs.
Source: MWPS-43, Swine Breeding and Gestation Facilities Handbook
Table 3. Incidence and Distribution of Returns to Service of Sows Related to the Average Maximum Temperature During the Week of Mating
Temperature range, °F Weeks per year Total services Return to service after mating, %
68.0° to 75.0° 7.78 1,116 12.2
75.2° to 82.2° 15.09 2,584 11.4
82.4° to 89.4° 13.11 2,392 14.4
89.6° 16.02 2,919 19.7
Source: Australian J. Exp. Agric. & Anim. Husb. 18:698-701, 1978.
Table 4. Proportion of Gilts Reaching Puberty When the Duration of Daylight is Increasing or Decreasing
Duration of daylight is increasing Duration of daylight is decreasing
Study Boar exposurea No boar exposure Boar exposurea No boar exposure
1 74.0 13.9 89.4 52.6
2 72.4
3 79.0
aAge of gilts at initiation of boar exposure was 165 to 173 days.
bNumbers in parentheses are average age at puberty in days.
Source: Anim. Reprod. Sci. 24:323-333, 1991; Anim. Reprod. Sci. 23:135-144, 1990; J. Anim. Sci. 57:1235-1242, 1983.