Influence of Hot-Humid Environment on Growth Performance and Reproduction of Swine

Environmental temperatures above the zone of thermoneutrality (comfort zone) for swine are frequent in tropical and subtropical regions, and during the summer months in temperate regions. These warm environmental conditions do not usually result in death losses, but they can cause reduced growth performance in growing-finishing pigs and decreased reproduction in the breeding herd.

Optimum temperatures for housed swine are shown in Table 1. In general, as the pig gets older and larger, its optimum temperature decreases. Thus, the effects of heat stress are more of a concern with older finishing swine (>50 kg or 110 lb) 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 20°C (68° F).

If temperatures remain above 27°C (80° F) for more than a short period of time (2 to 4 days), substantial losses in performance and reproductive efficiency can result unless some type of cooling relief is provided.


Effects of Heat Stress

In general, pigs try to minimize the effects of heat stress by two major methods. These include an increase in heat dissipation and a reduction in heat produced from body metabolism. To increase heat dissipation, pigs will increase their body surface area by sprawling out to increase contact with a cool surface like a floor. Pigs will also increase heat dissipation by increasing respiration rate (panting). The increase in respiration rate will increase the amount of air going in and out of the lungs. The increase in air flow increases evaporation of water from the lungs. The pig is thus cooled via a process called evaporative cooling. In humans, evaporative cooling takes place via sweating on the skin; pigs do not sweat.

Because eating, digestion, and nutrient absorption all generate heat (the heat increment of feeding), pigs can reduce their metabolic heat production by eating less feed. Thus, voluntary reduction in feed intake by the pig is an effort to lower the heat increment of feeding and thereby decreases the amount of heat that will need to be dissipated into the environment. Unfortunately, a reduction in feed intake results in reduced growth, and in lactating sows, a reduction in milk production.

The most noticeable effect of high temperatures on growing-finishing pigs (25 to 110 kg) is a reduction in feed intake (Table 2). With finishing swine (>50 kg), feed intake is reduced at temperatures above 20° C (68°F). The reduction in feed intake will result in reduced daily gain. At temperatures above 30°C (86° F), feed conversion (amount of feed required per unit of weight gain) is adversely affected. With growing pigs (25 to 50 kg), the negative effects of heat stress are not as great as with heavier finishing pigs, but temperatures above 30°C can result in reduced performance.

Research conducted in Guadeloupe, West Indies, summarized in Table 3, clearly demonstrates the negative effect that a hot climate in a tropical country has on growing-finishing pig performance. This negative effect was more apparent with older finishing pigs than with younger growing pigs. This research also noted changes in the pig's physiology in an effort by the pig to cope with the heat stress. Changes included increased body temperature, increased respiration rate, and decreased thyroid hormone production (Christon, 1988). The decrease in thyroid hormone production is an apparent effort by the pig to slow its metabolism and thus its heat production.

Research conducted in North Florida also clearly shows the negative effect that a hot-humid environment has on performance of growing-finishing pigs when raised under farm-like conditions (Table 4; Myer et al., 1998). In this research, pigs reared during the summer grew 11% slower and required 5% more feed per unit of weight gain than a comparable group of pigs raised during the fall when the environmental temperatures were mostly within the pig's thermoneutral zone (comfort zone).

Heat stress has a detrimental effect on efficient swine reproduction. In the sow, temperatures higher than 27°C (80° F) will delay or prevent the occurrence of estrus, reduce conception rate, and increase early embryonic death (Table 5). Heat stress during embryonic implantation (first 13 days postmating) can reduce embryonic survival by 30 to 40% (Curtis, 1981). Also, heat stress during the last few weeks before farrowing can result in a higher number of stillbirths. Surprisingly, sows are rather tolerant of heat stress during mid-gestation (14 to 90 days postmating).

The boar's fertility is also adversely affected by heat stress. High temperatures of 29°C (85° F) or more have been shown to decrease sperm production and decrease sperm quality. Furthermore, the negative effects of heat stress persisted for 4 to 6 weeks after the heat stress was removed (Figure 1) (Curtis, 1981; Stone, 1982). This delayed return to normal results because it takes 6 weeks for sperm production and maturation in the testes and epididymis.

In lactating sows, temperatures above 25°C (75° F) have been observed to result in reduced feed intake (Table 6). The resulting decrease in appetite causes a decrease in milk production and an increase in sow weight loss. Thus, smaller pigs are weaned, and the ability of the sow to return to production after weaning is compromised because of her large weight loss.

Recommended minimum floor space allowances for growing-finishing pigs are given in Table 7. Under conditions of heat stress, it is desirable to increase the minimum floor space allowed for pigs, in particular for larger pigs (given in parentheses in the table). The number of pigs per pen should also be reduced.

The use of shades in pastures and outside lots is an effective method in helping to cool pigs (Table 8). Shades can cut the radiant heat load from the sun by as much as 40%. Shades with straw roofs are best because they have a high insulation value and a reflective surface. Uninsulated aluminum or bright galvanized steel roofs are also good. The best shades have white or reflective upper surfaces.

In confinement (enclosed) buildings, insulation is desirable in the roof or ceilings. The insulation will help to minimize solar heat build-up.

Rapid air movement over animals aids in dispersing heat. Increased ventilation is particularly important in confinement buildings. Air velocity of at least 5 kph (3 mph) over the animals raised in confinement is desirable during warm weather.

Swine must have access to large quantities of water during periods of high environmental temperatures. Much of the water is needed for evaporative heat loss via respiration to help the pig cool off. Table 9 lists water requirements for swine reared under warm conditions.

It is possible to substantially reduce heat by wetting the pig's skin and allowing the water to evaporate. Under natural conditions, pigs will cool themselves by wallowing in mud. As the mud dries, it cools the pig via evaporative cooling. At the same time, the mud protects the pig's skin from the sun. In many swine production systems, however, mud wallows can lead to many health problems. Concrete-lined wallows are quite effective and are much cleaner. Also, water flush waste removal system can be designed so that pigs can wet themselves during warm periods.

Water sprinkler systems are also quite effective for wet-skin cooling. Sprinkling water is preferred to fogging. Fogging uses small water droplets. Sprinkling uses larger water droplets that cool the pig by evaporating off the skin. Fogging cools the air, and the air must then cool the pig. Results of research testing the effectiveness of fogging or sprinkling on cooling and thus improving performance of finishing swine are summarized in Table 10. Pigs cooled with sprinklers should also be provided access to shade to prevent them from being sunburned. Also, air movement across the pig increases the evaporation rate and improves cooling.

Many water sprinkler systems operate by thermostat-controlled timers that wet the pig and then allow the pig to dry. Sprinkler systems are usually designed to run for 1 to 2 min each 30-min period (a few operations use 1 to 2 min each 10-min period) when the temperature is above some set value (typically in the 25 to 30° C (75 to 85°F) range).

Dripping water on the necks and shoulders of sows along with air movement has been shown to be an economically effective way to cool lactating sows in a farrowing house. Best results are usually obtained with drip rates of 2 to 3 L (0.5 gal) of water per hour and with the water dripping for a short time (i.e., 10 to 15 seconds) every 1 to 2 minutes. In one university study, drip-cooled sows consumed 1.7 kg (3.7 lb) more feed per day, and lactation weight losses averaged only 33% of that of the control sows (McGlone et al., 1988; Table 11). There are commercially made systems that can be purchased and installed at relatively low cost. These systems work best in farrowing pens in which the sow is kept in a farrowing crate and the pen flooring is perforated (e.g., plastic-coating expanded metal) so that the pen stays dry.

Since pigs will decrease their feed intake at high environmental temperatures, a more nutrient dense diet will help to minimize production losses due to the high temperatures. Incorporation of fat (e.g., beef tallow, vegetable oil) into the pig's diet will increase energy (calorie) density and help maintain energy intake during times of high temperatures. Fat contains much more energy (calories) per unit of weight than carbohydrates (starch, sugar). Typically, fat is added at a level of 2 to 6% of the diets. If the energy density of the diet is increased by the addition of fat, the other nutrients (e.g., protein) will also have to be increased to prevent nutrient deficiencies (i.e. maintain constant energy to nutrient ratios). Conversely, the use of fibrous feeds for pig diets should be minimized during times of heat stress. Fibrous feeds, such as ground hay, wheat midds, etc. produce much heat when they are digested by the pig, whereas fats produce little heat. A research trial comparing pigs fed diets containing an added fibrous feed or a diet with added fat during the warm and cool seasons in the southeastern United States is summarized in Table 12.

The method by which pigs are fed may also influence pig growth efficiency during times of high environmental temperatures. Growing-finishing pigs, at least in the United States, are typically fed ad libitum (self-fed). The practice of restrictive feeding (feeding 80 to 90% the amount the pig would eat ad libitum) is not widely practiced since growth rate of restricted fed pigs is slower. However, in a hot climate, research results seem to indicate that restricted feeding produces a substantial physiological advantage in the pig, and thus may be of practical interest for pig production within tropical and subtropical areas (Table 13; Christon, 1988).

High humidity in itself does not have a negative effect on swine performance. Combined with high temperatures, however, high humidities can enhance the negative effects of the high temperatures. Since the pig must rely heavily on evaporative heat loss to try to stay cool when it is hot, humidity level is very important. The higher the humidity level in the air, the less effective is the process of evaporative cooling (less moisture can evaporate into humid air than dry air.). Thus, when relative humidity is 50% or higher, the pig will feel the effects of heat stress at a lower temperature than when the air is drier. At temperatures above 30° C (86°F), an 18% increase in relative humidity is equivalent to 1° C (2° F) rise in air temperature. For example, when air temperature is 28°C (82°F) and relative humidity is 80% , the effective temperature is about 30° C (86°F) (NRC, 1981).

The question arises about the effectiveness of wet-skin cooling methods described above during times of high temperature combined with very high humidity. The effectiveness of these systems are decreased as humidity rises. However, research conducted in Florida indicated that water sprinkling is still effective in helping to cool pigs during the hot summer months (Harrison et al., 1983). Florida is noted for high humidity in addition to high temperatures during the summer months.

One of the first major consequences of heat stress is reduced feed intake. Heat stress decreases reproductive performance in both the sow and boar. The negative effects can be minimized by the use of artificial cooling (e.g., water sprinkler system). Cooling systems need not be sophisticated or expensive to be effective, but they must be reliable and easy to maintain. Increasing the nutrient and energy density of the diet can also help to minimize the effects of high temperatures.