Keep the Breeding Herd Cool During Summer

Date of publication : 6/2/2008
Source : Virginia Cooperative Extension
The optimum ambient temperature for swine in the breeding herd is approximately 65° F (range of approximately 55 to 75° F). Thus, the elevated environmental temperature and humidity associated with summers in Virginia can severely affect swine reproduction. Breeding operations often experience a "summer slump" during which reproductive efficiency is compromised. The adverse effects of "heat stress" are manifested in both the male and female. In this Livestock Update, I'll focus on the deleterious effects of elevated temperature on swine reproduction and interventional management practices.

Effects of Elevated Temperature on Reproduction in Boars

Short-term exposure to high environmental temperatures reduces fertility in boars. Boars subjected to heat stress conditions produce ejaculates that have low sperm concentrations, high percentages of abnormal sperm cells and decreased percentages of motile sperm cells. Past research has indicated that the minimum exposure time and critical air temperature above which production of sperm cells is adversely affected is 72 hours and 85° F, respectively.

The negative effects of acute heat stress on semen quality may be somewhat immediate. A "lag" period of approximately 2 weeks, however, is often observed between the initiation of acute heat stress and the first indications of abnormal sperm production. After the cessation of heat stress conditions, six to seven weeks is necessary before fertility returns to normal. Thus, acutely heat stressed boars can have a protracted, negative influence on reproduction in a breeding operation. For example, boars exposed to 95° F temperatures for three consecutive days in late-July may be responsible for suppressed conception rates well into September.

It is likely that boars are also sensitive to chronic exposure to moderately high temperatures (say, 79 to 84° F) that are not generally recognized as "heat stress" conditions. Boars are routinely exposed to these temperatures during the summer in Southeast Virginia.

Researchers at North Carolina State University reported data obtained from seven commercial boar studs in southeastern North Carolina from June through October, when average weekly high temperatures at these facilities never exceeded 84° F. Never the less, during this period there was a significant increase in the number of ejaculates rejected due to poor quality and a decrease in the number of insemination doses per ejaculate.

The effects of elevated environmental temperature on various characteristics of libido have not been extensively studied. However, during the summer boars may become lethargic and display a reluctance or refusal to mount a sow in estrus or an artificial sow. Results from a small study conducted at the Tidewater Agricultural Research and Extension Center (TAREC) in Suffolk supports this concept. During the spring, semen was collected once weekly for 12 weeks from 20 boars trained to mount an artificial sow. During this period, daily high temperatures fluctuated and ranged from 53 to 92° F. Reaction time, defined as the interval from entering the collection room to the start of ejaculation, ranged from 313 to 501 seconds and was positively correlated with environmental temperature. In other words, the hotter it was, the longer boars took to mount the artificial sow and start ejaculating.

Effects of Elevated Temperature on Reproduction in Sows and Gilts

Sows exhibit a reduced appetite due to high temperatures and as a consequence, loose more body condition during lactation. Excessive loss of body condition may result in an increase in the weaning-to-estrus interval.

A small data set generated at TAREC illustrates this point. Feed consumed during an 18-day lactation was recorded for crossbred sows nursing an average of 9.3 pigs during either the summer (n = 7) or winter (n = 9). Sows lactating during the summer consumed less feed, lost more body weight and last rib backfat thickness, and had a longer weaning-to-estrus interval (Table 1).

Table 1. Effect of Season on Performance of Sows During 18-day Lactation






No. Sows





Feed consumed (pounds/days)





Body weight change during lactation





Backfat change during lactation (mm)





Weaning-to-estrus interval (days)





In sows and gilts, increased temperatures can also decrease the number of eggs ovulated during estrus and increase embryonic mortality. In gilts, the attainment of puberty is often delayed during hot weather.

Management Considerations

It is imperative that the breeding herd be kept cool during periods of high environmental temperatures. Following is a list of some key considerations for managers and herdsmen:

     * If swine are pasture-reared, shade should be provided. Better yet is to provide a sprinkler system under a shade. Research conducted in Oklahoma and Florida revealed that boars maintained on outside lots with a shade and sprinklers had 20% higher fertility than boars provided shade only.

     * Insulating the roof or ceiling of confinement units (14 to 25 R value) will minimize solar heat build in hot weather.

     * Managers of confinement units should verify that mechanical ventilation systems are properly serviced and operating in an optimum fashion. During hot weather, ventilation rates should be 500-1000 cubic feet per minute (cfm) for the sow and litter, and 250-300 cfm for boars and gestating sows. These rates may be reduced when supplemental cooling systems (drip cooling, cool cells, etc.) are employed.

     * Thermostatically controlled drip or sprinkler systems allow evaporative cooling. In an experiment conducted by Kansas State University researchers, drip coolers were used to trickle water slowly over the neck and shoulder of lactating sows. A flow rate of 0.8 gallons per hour was used and the system was activated at 85° F. During the study temperatures ranged from 81° F to 95° F (relative humidity was 68%). The cooling system increased sow feed intake by 19.8%, decreased sow weight loss by 78% and increased weaning weight of the litter by 10.5%. Drip or sprinkling is preferred to fogging, because fogging cools the air and the air must then cool the hog. Fog droplets also drift with air movement. Drip and sprinkler systems should be routinely checked for proper operation.

     * Breeding extra sows and gilts to compensate for the lower conception rates expected during the summer is a common practice, but boars must not be overworked and do not overcrowd animals in the breeding-gestation barn.

     * Feed the breeding herd and perform estrus detection and breeding early in the morning or late in the evening when it is cooler.

     * Swine should always have unlimited access to fresh, cool water to drink. During the summer, typical amounts of water consumed per head per day are as follows: sow and litter, 10.4 gallons; gestating sow, 5.2 gallons; and breeding boars, 3.9 gallons. Minimum flow rates for nipple waterers should be 3 to 4 cups per minute.

     * Supplementing lactation diets with fat increases energy density of the diet and is associated with improved performance, particularly during the summer.

     * A single i.m. injection of P.G. 600, a drug marketed by the Intervet America Company (Millsboro, DE), has been shown to accelerate the onset of estrus in sows weaned during the summer and induces estrus in prepubertal gilts.

     * Observe swine regularly to determine if they are being heat-stressed. Producers can get an indication that boars and sows are heat stressed by checking rectal temperatures or respiration rates. Normal values for mature swine are 101.5° F and 13 to 18 breaths per minute, respectively. Values appreciable higher than these are indicative of heat stress.

By Mark Estienne, Swine Research Physiologist, Tidewater AREC
Livestock Update, Virginia Cooperative Extension
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