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.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.
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 (ft/min) |
Air Speed (km/hr) |
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.
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