Introduction

Places in the world where outdoor pig production can fare well include some of warmer climates of the world. Outdoor pig production in Texas, for example, was shown to be cost-competitive with upper Midwestern USA costs for both sows and finishing pigs (see detailed cost comparisons on our web page at http://www.pii.ttu.edu).

One limitation to the large potential for outdoor sow units is the negative effects of hot seasons on sow productivity. To get a better idea about outdoor production in warm climates and during warm weather, one must understand how pigs become heat stressed and how best to alleviate the negative effects of heat stress. In my experience, a very warm environment does not limit sow and litter productivity, unless it is a humid environment – then, sow productivity suffers both indoors and outdoors.

Following a brief overview of pig evolutionary biology and a description of modes of heat loss for pigs, I will summarize the scientific literature and my personal experiences on management of sows and litters in hot environments. What we do know is that, in our side-by-side indoor and outdoor production units, the sow and litter productivity is typically better among the outdoor sows, especially during warm weather. We of course have exceptions to these generalities largely because poor management can cause great dips in productivity in outdoor units. Outdoor pig units are much less forgiving of poor stockmanship than are indoor production units.

This paper will first describe the biological effects of heat stress on sows and boars. After background material on the evolutionary biology of the pig and a discussion of modes of heat flux, information will be provided on management of an outdoor sow herd to minimize the negative effects of heat stress.

Evolutionary Biology of the Domestic Pig

The domestic pig evolved in the forests, riverbeds and fields of either Europe or Asia (the argument on its origins continues). The domestic pig originated from the species known as the European Wild Boar (Sus scrofa). The European Wild Boar is a scavenger that benefited from food scraps around human early dwellings. As a scavenger, the pig evolved a powerful rooting disc on the end of its snout. The pig’s snout, worth about $0.15 to the meat processor, is the appendage that the pig uses to experience its world. The snout acts as a filter of earth and air while food and oxygen are sifted out of the forest floor and into, respectively, the stomach and lungs. Besides a physical tool, the pig’s sense of smell and touch (using the snout as “fingers”) are very well developed.

Besides a well-developed snout, the pig has a sparse hair coat. Wild pigs actually have a thicker hair coat than domestic pigs – one assumes early pig breeders selected against a thick hair coat in common breeds. The sparse hair coat facilitates heat loss from the skin surface of the pig since the pig’s skin is open, unimpeded by hair, to evaporate water. The trouble is, the pig does not have functional sweat glands on its skin.

The pig has a form of sweat gland on its skin, but they do not work. The pig can lose a small amount of water, and thus heat, through passive diffusion through the skin – but not much. So instead of sweating, the pig uses behavioral thermoregulation to cool itself. Behavioral thermoregulation is actually a more powerful mode of heat loss than is sweating – and the pig does not become dehydrated. Behavioral thermoregulation is an effective means of cooling as long as the animal has access to cool substrate.

Sows in fact prefer a cool surface to lay on (Bull et al., 1997). They actually preferred conductive cooling to other forms of cooling (water or cool air). Thus the domestic pig seeks cool surfaces and would be comforted by cool surfaces both by reducing heat gain (by exposing their body surface to cool rather than warm surfaces) and by the cooling effect of the water and mud. Understanding that sows seek out cool surfaces rather than wet surfaces during warm weather should help to manage the sows better.

Biological Effects of Heat Stress

Reproduction

Heat stress has negative effects on both sows and boars. For sows, heat stress can cause failure to express estrous behavior, loss of pregnancy in the first 30 days and an increase in stillbirth numbers.

Early research showed that heat stress has a negative effect on embryo survival during the first 30 days of pregnancy, but heat stress during mid pregnancy did not influence the numbers of pigs born per sow (Omtvedt et al., 1971). However, when pregnant gilts were heat stressed during the last 2 weeks of pregnancy, the Oklahoma investigators demonstrated a large increase in the numbers of stillborn pigs per sow (from 0.4 stillborn per gilt to 5.2 during heat stress). Days 1 to 5 of pregnancy are especially critical days to protect sows from heat stress since heat stress during these days can reduce the numbers of viable embryos (Thomkins et al., 1967).

Sows must be protected from very hot temperatures at breeding, days 0-30 of gestation, during parturition, and during lactation.

Heat stress kills immature sperm cells, but it has little effect on mature sperm cells. Thus a bout of very hot temperatures today will not affect breeding success for several days (until those young sperm cells were due to mature). But for 3-10 weeks after the heat stress experience, the boar may be infertile (Stone, 1982). Boars should be continuously protected from the negative effects of heat stress when the air temperature exceeds 84 F (29 C). The problem with boars, particularly under natural service, is that their fertility declines above 84 F, but they continue be interested in sex until the air temperature exceeds 105 F (40 C).

Lactation

Heat stress during lactation causes a reduced feed intake. During heat stress, eating less while nursing a litter of pigs can lead to reduced milk production and extreme sow lactation weight loss (McGlone et al., 1988).

Modes of Heat Flux

To know the air temperature is to know only one measure of the thermal environment. To understand how pigs perceive their thermal environment, one needs to know the Effective Environmental Temperature (EET). The EET includes consideration of heat flux (heat gain relative to loss) by radiation, convection, conduction and evaporation or condensation.

Thermal radiation is the heat gained or lost by radiating of non-touching bodies in the environment that are warmer or colder than the pig. The best source of radiant heat gain is the warming effect of the sun. The coolest radiant body is the cool night sky on a cloudless night. Pigs may gain or lose heat by radiation. Radiation is more intense when at higher altitudes.

Table 1. Modes of heat flux for pigs (and other animals)

Modes of Heat Flux
Loss	Gain
Radiation	Radiation
Convection	Convection
Conduction	Conduction
Evaporation	Condensation

Convection is heat flux in streams of air as they pass over a body. The pig can lose heat by convection, as when a cool breeze blows over its body. Pigs can also gain heat by convection, as when a furnace warms the air in a building.

Conduction is heat flux when two bodies touch. Lying on a cold floor will cause heat loss. Resting on a warm surface will cause a heat gain.

Evaporation is heat loss from a surface when fluid evaporates from its surface. Evaporation is a powerful means of heat loss (only). Heat can be gained by condensation, but evaporation requires heat as fluid changes form from a liquid to gaseous state. The energy contained in gaseous water vapor is said to be latent or insensible. The ability to evaporate and cool is dependent on the humidity in the air -- the drier the air, the more evaporation is effective.

The four modes of heat flux are more than additive. For example, when water is on the skin surface it cools the skin while it evaporates. Adding a cool breeze to a wet surface greatly increases heat loss. Evaporation is less effective in humid conditions. For these and other reasons, people have suggested use of combined measures such as temperature-humidity index (THI) or wind chill factor. Each of these indexes that combine two modes of heat flux are less descriptive than the EET. The EET, in principle, is the best indicator of the pig’s thermal experience.

Is there a connection between preweaning mortality and hot temperatures?

We examined records on one of our farms during the summer months and presented the data in Figure 1. In this data set, the high air temperature for a given week was compared with the weekly preweaning mortality. The comparison is not precise, however, because the high temperature in a given week could be a spike during a single day, followed by cooler temperatures and not a sustained warm temperature (although it usually was). Also, the mortality for a given week is the result of deaths during the last 3-4 weeks (weaning age was about 24 days).

How hot does it get in farrowing huts in a West Texas summer?

We collected data on the air temperature outside (using a weather station) and inside insulated and non-insulated huts. During the example presented in Figure 2, the air temperature reached a high of 96 F (35.5 C), with an average daily air temperature of 78 F (25.5 C). But the temperature inside the non-insulated hut, while averaging 82 F (27.8 C), reached a peak of 103.8 F (39.8 C). The insulated hut reached a peak of 99.5 F (37.5 C) with an average of 80 F (26.6 C). Thus, we can expect the air temperature inside the non-insulated farrowing huts to exceed outside temperatures by about 4 F (2.2 C) and for the inside air temperature to exceed outside air temperature by 2 F (1.1 C) for insulated huts.

Figure 1. Relationship in West Texas between warm temperatures and preweaning mortality. Note the positive slope indicating a .36 unit increase in preweaning mortality per degree F (~.5 C) between about 55 F (12.7 C) and 105 F (40.5 C).

When managing outdoor sows during warm weather, it is very important to understand that however hot it is outside, it is a little warmer inside the farrowing huts. The reason for this is that inside the farrowing hut we have the normal air temperature plus the metabolic heat of the sow and piglets which adds to the heat load inside the hut. Fortunately, the hut does protect sows from the hot radiant heat of the sun.

There was one published report on the effects of insulation of farrowing huts on the performance of outdoor sows. The work was done in Scotland and thus, they were more concerned with warmth in the winter than cooling effects in the summer months. However, in that study conducted by Sandra Edwards et al., (1995), they showed no significant difference between insulated and noninsulated huts on sow and litter performance and piglet preweaning mortality. In that study, the numbers of pigs weaned per sow were very similar for sows in each type of hut.

Figure 2. Air temperatures (F) for outside air and inside an insulated and noninsulated farrowing hut. Note the large swing in air temperature in West Texas from a low of 66 F (18.9 C) to a high of 96 F (35.5 C) in a single, normal summer day. When the air temperature exceeds 80 F (26.6 C), proper wallow management is critical to sow survival and performance.

How Do Sows and Piglets Perceive Their EET In Summer?

During warm months, sows and piglets have different thermal needs and perceptions. As given in Table 2, sows are in thermal distress at EET's that are just right for newborn piglets. For all practical purposes, it is virtually impossible to heat stress a piglet less than one week old. But for the sow (open, pregnant or lactating) heat stress control should be implemented when the air temperature exceeds 80 F (22 C). I have given heat stress control measures in table form, by climate type (Table 3).

In a warm EET, access to the wallow is required. We have shown that the wallow is used as soon as the air temperature is above freezing. Thus in mild climates, wallows should be available to sow use year-round (or at least for those days that have an air temperature above freezing).

When the air and water temperature exceeds pig body temperatures, the wallow will not be comfortable and thus sows will not use them--or they will run in and then get out. To avoid heat stress problems, wallows should be shaded in humid climates when temperatures are hot and in dry climates when air temperatures are very hot (Table 3).

Effectiveness of Behavioral Thermoregulation

Of all the modes of heat flux used by the pig, the pig chooses to use evaporation and conduction as the primary modes of heat loss during behavioral thermoregulation. Wind currents will add to the effectiveness of evaporation heat loss. Wallowing and rooting in the cool earth are the two primary methods used by pigs to cool themselves.

When mud evaporates from the skin of the pig, it loses 800 g/h/m² (Curtis, 1983). Compared with 200 g/h/m² that cattle evaporate from their sweat, pigs are much more effective at losing heat during warm weather by behavioral thermoregulation than they would be by sweating – and they conserve body fluids and salts.

Table 2. Recommended thermal conditions for swine used in agricultural research and teaching. Adapted from the Ag Guide (FASS, 1999).

Type & weight	Preferred range^a	Lower extreme^b	Upper extreme^c
Lactating sow & litter	59-79ºF (15-26ºC) for sow; piglets have 90ºF (32ºC) minimum creep area	77ºF (25ºC) creep area; 60ºF (15ºC) sow area	90ºF (32ºC) for sow; no practical upper limit for piglets
Prenursery, 3-15 kg (7-33 lb.)	79-90ºF (26-32ºC)	59ºF (15ºC)	95ºf (35ºC)
Nursery, 15-35 kg (33-77 lb.)	64-79ºF (18-26ºC)	41ºF (5ºC)	95ºF (35ºC)
Sow or boar, >100 kg (>220 lb.)	50-77ºF (10-25ºC)	4ºF (-20ºC)	90ºF (32ºC)

^aBased on values given by NRC (1981), DeShazer & Overhults (1982), Curtis (1985), and Hahn (1985).

^b Values represent lower extremes in air temperature when pigs are held in groups. Bedding is recommended when air temperature approaches the lower extreme.

^cExcept for brief periods, above these air temperatures, cooling should be provided by means such as evaporatively cooled air for growing pigs or a water drip for lactating sows.

Table 3. Heat stress control measures for outdoor sows using shade and wallows. Sows should always have free access to water and shade.

Air Temperature	Dry Climate	Humid Climate
Marginal: less than 70ºF ( ºC)	no action needed	access to wallow required
Warm: 71-80ºF (ºC)	access to wallow required	access to wallow required
Hot: 81-95ºF (ºC)	access to wallow required	shaded wallow
Very hot: 96-115ºF (ºC)	shaded wallow	shaded wallow

^aIf the air temperature is in the given range for at least 4 hours per day, then the listed heat stress control measures should be implemented.

Wallowing behavior of sows is quite interesting to observe. When the temperature is slightly warm (above freezing), pigs will stand in the cool water. When the temperature rises, they will lay with their udders in the cool water and mud. When it gets warmer still, they will coat their skin with mud and roll from time to time from one side to the other allowing the mud to evaporate. Sometimes on hot days, if wallows are deep enough, only the sow’s snout and head will be visible above the water. When they leave the wallow on very hot days, if it is designed well, sows can be observed with a heavy coat of mud on their belly and flanks.

Most sows wallow in social groups. A collection of sows will be spread out in the wallow, usually without facing one another. Some individual sows will prefer to wallow alone or at a distant part of the wallow. The wallow must be large enough to accommodate the social needs of the pigs while behavioral thermoregulation is being expressed.

Wallow management is critical to the success of an outdoor unit. Wallows must contain water and mud, not just thick mud. The surface area must be large and generous.

The wallow is a very effective method of cooling. We sampled respiratory rates for sows indoors and outdoors during a warm day. Six sows were sampled in each of gestation and farrowing environments, both indoors and outdoors. Indoor sows had drip cooling (on 3 minutes out of 10 when the air temperature equaled or exceeded 80 F) while outdoor sows had wallows.

Indoor-housed sows have consistently higher respiratory rates (RR) than outdoor sows (Table 4). The outdoor mud wallow is a much more effective cooling substrate than is the drip cooling (which is recognized as the most effective cooling method for zone cooling indoor sows; McGlone et al., 1988).

Table 4. Respiratory rates (RR, mean, pooled SE = 9.1 breaths/min) and air temperatures for indoor and outdoor gestating and lactating sows (n=6/condition). Indoor-kept sows, both gestating and farrowing, had higher (P < .01) respiratory rates than outdoor-kept sows. Indoor sows had drip cooling while outdoor sows had mud wallows.

	Indoor		Outdoor
Item	Temp F	RR breaths/min	Temp F	RR breaths/min
Gestation	92	67.8	92	32.7
Farrowing*	82	52.2	92	37.5
Average	Na	60.0	na	35.1

* Indoor farrowing had evaporative cooling.

na = not applicable

Building a wallow. Make a hole and fill with water – the pigs will do the rest.

A proper wallow will have a larger area with fluid water on its surface, not just thick mud. A constant water supply is preferred.

Some sows prefer to wallow alone and sufficient space should be provided to accommodate their preferences.

A wallow-waterer that provides clean, fresh, cool water, by running 24 h/day. Sows can be observed sitting under the stream of water drinking fresh water.

Wallow Management

One should be aware of how the wallow cools sows. When air temperatures are hot, but below sow body temperature, sows enter the wallow and usually cover 50-75% of their body with water and mud. Their metabolically-active udders are losing heat by conduction to the cool muddy waters. As they roll, their water and mud-caked skin evaporates water, cooling the skin. During hot and very hot temperatures, sows should stay in the wallow as much as possible. They should only leave to feed and nurse.

Wallows should be inviting to sows. An effective wallow will be cool and large enough to accommodate twice as many sows as are on the paddock. By giving generous wallow space, even the submissive sows will gain access to the cooling wallow.

The wallow should have a source of fresh water. You do not want the sow who wants a drink to get out of the wallow and walk a distance to get a drink. She will not do it, she will drink the wallow water. One good idea is to continuously drip or stream fresh water over the wallow. This technique has the added benefit of keeping the water lines moving, thereby preventing the water lines from getting extremely hot.

Wallows should not have a thick mud appearance. Wallows should look like a pond, not like a thick goo. If wallows are thick, then they do not have enough water added each day.

Wallow Tips

· Make it large enough for twice the intended numbers of sows

· Wallows should not be thick mud – they should be fluid

· Fresh water should be available in the wallow

· During very hot times, the wallow should be shaded

Literature Cited

Bull, R. P. P. C. Harrison, G. L. Riskowski and H. W. Gonyou. 1997. Preference among cooling systems by gilts under heat stress. J. Animal Science 75:2078-2083.

Curtis, S. E. 1983. Environmental management in animal agriculture. Iowa State University Press. Ames, Iowa.

Edwards, S. A., I. Riddoch and C. Fordyce. 1995. Effect of outdoor farrowing hut insulation on piglet mortality and growth. Farm Building Progress. 117:33-35.

Federation of Animal Science Societies. 1999. Guide for the care and use of agricultural animals in agricultural research and teaching. Federation of Animal Science Societies, Savoy, IL, USA.

McGlone, J. J., W. F. Stansbury and L. F. Tribble. 1988. Management of lactating sows during heat stress: effects of water drip, snout coolers. Floor type and a high energy diet. J. Animal Science 66:885-891.

Omtvedt, I. T., R. E. Nelson, R. L. Edwards, D. F. Stephens and E. J. Turman. Influence of heat stress during early, mid and late pregnancy on gilts. J. Animal Science 32:312-317.

Stone, B. A. 1982. Heat induced infertility of boars: the inter-relationship between depressed sperm output and fertility and an estimation of the critical temperature above which sperm output is impaired. Animal Reproduction Science 4:283-299.

Thompkins, E. C., C. J. Heidenreich, and M. Stob. 1967. Effect of post-breeding thermal stress on embryonic mortality in swine. J. Animal Science 26:377-380.

[1] Presented at a symposium on outdoor pig production in Brazil, September, 1999.