Controlling Heat Stress in Swine

The imapcts of heat stress on pigs and how to prevent it are explained by Robert Chambers of the Swine & Sheep Housing & Equipment group of the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) in that organisation's latest newsletter.
calendar icon 22 August 2012
clock icon 9 minute read

Happy swine are essential to having a productive and profitable herd. Animals perform at their peak when they are in their thermo neutral zone. When they experience heat stress, there is a corresponding loss in production.

Heat stress occurs when the environmental temperature rises above the point where the animal is producing more heat from digestion and/or receiving more heat from its surroundings than it is releasing to the surrounding environment. Its first reaction to this situation is the blood vessels in the skin surface enlarge so as to increase blood flow and the skin’s surface temperature. This increases the heat transfer rate to the environment. Sweating and increased respiration functions to increase the water vapour output and consequently latent heat output. This is the point of 'Heat Stress Alert'.

The upper critical temperature is the limit to a radical change in heat production. The animal is in 'Heat Stress Danger'. At this point, the respiration increases in intensity (panting) and the animal reduces its feed intake to slow the internal heat of digestion (sensible heat) being produced. This causes reduced growth in feeder pigs, or reduced milk production in lactating sows.

As the animal’s internal temperature increases, it reaches a point where it can no longer increase moisture loss through mainly increased respiration and sweating. This is the 'Heat Stress Emergency' point. The ability for the animal to remove more heat than it is producing and/or receiving is at the maximum out. The animal may pant harder but the evaporation rate is almost constant. Hot, high humidity weather is detrimental to animals and humans, because the principle heat transfer method to cool the body, evaporation, is reduced by this type of weather (see Figure 1). The core temperature begins to rise. This rise in body temperature triggers increased biochemical reactions which further increases heat production (the van't Hoff effect). Without relief, the cycle leads to death, although if occurring for only a few hours, it causes no lasting harm in most animals.

Figure 1. Heat Stress Index for grow-finish swine
Source: Iowa State University

Swine gain and lose heat to the their surrounding environmental temperature in four ways: conduction, thermal radiation, convection and evaporation in order to maintain their ideal core body temperature of 39ºC.

Under heat stress conditions, the goal is to minimize heat coming into the animal from the surroundings and maximize heat transfer out of the animal. By doing this, we lower the animal’s 'Effective Temperature', that is, even though the animal has the potential to be in a heat stress condition due to the ambient temperature, effectively the animal 'feels' comfortable because its core temperature is near normal as incoming heat transfer in minimized and heat transfer out of its body is maximized.

Conduction is heat transfer between contacting bodies at different temperatures, the higher the temperature differential, the more rapid the conduction. Heat transfer from the body core to the skin surface occurs by conduction through the body tissue and also by convection associated with blood flow. Slatted floor systems work well in hot weather as the animal is in direct contact with the floor surface when lying down and, therefore, maximizing conduction. Feeder and pen space should also be increased if possible so that the animals can eat and lay down without having to touch one another.

Anybody that has travelled on public transit during rush hour in a major city during a heat wave can relate to this.

Producers using bedded packs can aid in maximizing conduction heat losses by ensuring that the manure pack is kept to a minimum depth, and dry. Deep, damp manure packs start to compost and release heat and moisture. As the stomach is the heat generation centre, giving the animals minimal bedding allows them to transfer heat to the cooler floor below when they lay down.

Radiation heat transfer for swine is mainly an issue from the sun shining on the metal roofs of single story barns. A dark-coloured or tarnished roof can rise as much as 15ºC or more above the ambient air temperature. If the underside of the roof is uninsulated, this heat can radiate down in the barn space or heat an unvented attic that then radiates down. By adding insulation (R5 minimum) to the underside of the steel roofing, the radiant heat load is lowered considerably.

Enclosed attic spaces should have a minimum ventilation of 1 square foot of total opening for 300 square feet of ceiling area. Half of this area is the exhaust located on the peak and/or gable ends and half along the roof eave and/or soffit areas. If we have a barn that is 120 feet by 60 feet, flat bottom truss and insulated ceiling, the roof vent exhaust total minimum area would be 12 square feet, distributed along the peak and the soffit vent continuous slot minimum opening of 0.6 inches on each side. Gable end vents are only recommended for buildings less than 50 feet long unless used in combination with ridge vents. With naturally vented barns if the eve overhangs are short the sun can shine into the barn warming both the animals and the barn space.

Convective heat is transferred to or from the animal by the mass movement of fluid, in this case by air. Air moving past an animal provides a cooling effect, sometimes referred as the wind chill effect. Table 1 shows the air speed effects on effective temperature in swine. Animals under heat stress temperatures can benefit from natural or mechanical induced wind speed. Naturally ventilated barns should have minimum opening of five per cent of the floor area on each side of the building. For example, for a barn 60 feet by 120 feet with a eight-foot sidewall, the minimum clear opening should be three feet and rough opening of at least four feet to account for the curtain folding. Many barns have even larger openings to take full advantage of breezes on the hot days. Naturally ventilated barns are located with the ridge-line on a south-east to north-west axis so as to be perpendicular to the summer breezes that come predominately out of the south, south-west and west during the summer in southern Ontario. Barns should be sited so that they are not in the wind shadow of trees or other buildings. This shadow distance extends 10 times the height of the obstacle downwind. For example a tree line that has a height of 40 feet would have a wind shadow extending 400 feet from it.

Table 1. Air-speed effects on effective temperature for swine
Air Speed
Air Speed
Effective Decrease in Temperature (ºC)
30 0.55 -4
90 1.65 -7
300 5.5 -10
Source: McFarlane,J.M. 2000. How Do Your Pigs Really Feel? Effective Temperature for Pigs. Animal Environment Specialists, Inc., Marysville, Ohio.

Producers with mechanically ventilated barns should have a ventilation system that delivers 1.5 to 2.0 air exchanges per minute during hot weather with the exception of maternity and weaner barns. These should only have one air exchange per minute so as not to chill the animals. The system will reduce the heat build-up from the animals and will keep the temperature of the barn within 1ºC higher than the outside temperature. Air speed exhausting the fully open inlets can be in the range of 100 to 150 feet per minute.

Producers can also enhance the system by adding circulating fans to provide the wind chill effect. Naturally ventilated barns that have hot weather issues due to location or design can also benefit from the additional air movement that circulating fans provide. Figure 2 shows the proper placement of basket or panel fans in a barn. These systems must be on a thermostat so they slow down and shut off when not required to ensure animal comfort and electrical efficiency. Younger animals can be chilled out at higher air speeds even in warm temperatures. Ensure that there are refuge areas in the pen so that those animals who are feeling chilled can escape the breeze.

Figure 2. Placement of panel or basket fans
Source: Dairy Housing and Equipment Systems, Managing and Planning for Profitability, Natural Resource, Agriculture and Engineering Service, NRAES-129.

Latent heat transfer or moisture removal through respiration and the skin surface is the most important means of heat transfer in hot weather for swine. Every pound of water that is evaporated requires 1,000 BTUs of energy from the animal. More importantly, as the temperature warms up, the ability of the animal to transfer energy out of its body through sensible heat transfer is reduced. The latent heat transfer rate approximately triples as the environment warms up along with a corresponding increase in water consumption. By providing cool, fresh, readily available drinking water drinking, producers can ensure that this important cooling system is fully functional.

There are two main approaches to cooling with water. The most common is to sprinkle the animals with a sprinkler, misting or for lactating sows, a drip system. This system is used in combination with the air circulation to evaporate the water from the skin surface thereby cooling the animal. These systems must be used with controls so as to only activate at higher temperatures and intermittently, as it is the evaporation, not the water itself, that is cooling the animal.

The second approach is to add water to the air using a pad or high pressure fogging system. As the water evaporates, the air is cooled. The main downside is that the relative humidity increases and if the air is already humid, there is little room to add moisture. The goal is not to increase the relative humidity of the air greater than 80 per cent as the animals and the operators find this uncomfortable even at lower temperatures.

Table 2. Maximum air temperature drop due to vaporizing water
Ambient Temperature and Relative Humidity Potential Temperature Drop Caused by Vaporizing Water
10% RH Increase in Room 20% RH Increase in Room 30% RH Increase in Room
30°C and 40% RH 2.2°C 4.3°C 6.0°C
30°C and 50% RH 2.0°C 3.8°C 5.4°C
30°C and 60% RH 1.8°C 3.4°C Room RH too high
30°C and 70% RH 1.6°C Room RH too high Room RH too high
Source: Psychrometric Chart, Normal Temperatures, SI Metric Units, American Society of Agricultural and Biological Engineers, ASAE D271.2 APR1979 (R2005).

In summary, by increasing the animal's heat transfer rate from its body and reducing the amount of environmental heat transferring into the animal from its surrounding environment we can reduce its effective temperature. That is, even though the animal should be in heat stress, the animal's heat balance is such that it 'feels' cooler. This allows the animal to continue eating and to maintain production.

August 2012

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