Canadian Poultry Magazine

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Beating the Heat

Conditions that can lead to heat stress

January 11, 2008
By Jim Knisley


Conditions that can lead to heat stress and how to counter them

Canadian poultry producers, even those in our sometimes hot, humid south, don’t often have to contend with dangerous summer conditions. Seldom does even Southern Ontario soar above 30 degrees and 60, 70 or 80 per cent humidity for days on end.


Southern Ontario has had critically high temperatures in just three of the past 20 summers. Those extended summer heat waves came in 1989, 1990 and 2005, said Dr. Path Manathan DVM, ACPV.  Most summers’ average highs settle in just under the critical point. For example, the 30-year average for Windsor, the hottest spot, comes in at 27.9 degrees.

But Dr. Manathan said producers shouldn’t take too much comfort in the data. “Every year it is getting a little bit worse,” he said.
Temperatures are edging up bit by bit and it would be prudent for producers to be prepared for more years with extreme summer heat. Some climatologists are betting that extreme conditions will become more common in the future because of global warming.
If they are right, and the scientific models indicate they are, poultry producers may have to look even further south for lessons on how to help the birds through the hottest weeks.

At a recent poultry producer update in Vineland, Ont., sponsored by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), Dr. Manathan, outlined the conditions that lead to heat stress and ways to counter them.

Humidity:  It’s a Killer

When temperatures hit 27 degrees poultry comes under heat stress. At 29 degrees and up to 50 per cent humidity the birds will begin panting to try and remove heat from their bodies. To counter the effects of the heat and humidity fans, misters and evaporative cooling can stabilize conditions and prevent bird death. So long as the humidity stays below 50 per cent the temperature can rise to 41 degrees and those techniques will work.

But as the humidity rises, the problems multiply. With humidity between 60 and 80 per cent and temperatures above 29 degrees, effective ventilation will keep deaths to a minimum. But if the humidity jumps above 80 per cent deaths will occur and it would take air conditioning to control the situation.

The formula is very straightforward: high temperatures + high humidity = more harmful.

With relative humidity up to 50 per cent the birds get rid of half the excess body heat by evaporation. When the relative humidity rises to 70 per cent or higher they can no longer get rid of body heat through evaporation and they begin panting, which is a danger sign.

The consequences of heat stress are costly. Broilers don’t grow as fast, they don’t eat properly and layers produce smaller eggs with thinner shells. Reproduction is repressed, mortality jumps and for the producer there is a loss of profit.

The signs that the birds are being stressed by the heat include: less activity, they crouch near walls or drinkers looking for a cooler spot, they dig into the litter, they spread their wings in an attempt to evaporate more heat and finally they will begin panting.
The birds will also try and separate themselves from the rest of the flock as they try to get more air around themselves and get away from the body heat of the other birds.

At the metabolic level, the heat causes a series of changes in the birds. The body temperature rises, body water is depleted, electrolytes are depleted, an acid-base imbalance develops, hormonal imbalances emerge and there is tissue damage.

Methods of Heat Loss
During the summer months, when daily temperatures regularly reach the mid-20s to low 30s, it becomes critical for the birds to dissipate body heat to the surrounding environment. Poultry do not sweat and therefore must dissipate heat in other ways to maintain their body temperature at approximately 40°C. Body heat is dissipated to the surrounding environment through radiation, conduction, convection, and evaporation.

 The first three avenues are known as sensible heat loss; these methods are effective when the environmental temperature is below or within the thermal neutral zone of the bird (12 to 24ºC). The proportion of heat lost through radiation, conduction, and convection depends upon the temperature difference between the bird and its environment. The bird loses heat from surfaces such as wattles, shanks, and unfeathered areas under wings.

To maintain body temperature by sensible heat loss, the bird does not need to drastically alter its normal behavioral patterns, feed intake, or metabolism.

The purpose of poultry house ventilation is to maintain a high enough air velocity or a low enough temperature in the house that the birds can maintain body temperature by sensible heat loss.

Once the environmental temperature reaches approximately 25ºC, the method of heat loss begins shifting from sensible to evaporative heat loss. Dissipation of body heat by the evaporative process requires the bird to expend energy by panting (hyperventilation), which begins to occur at about 26ºC.

Production efficiency can be affected long before the temperature reaches a level at which survival becomes a concern. Heat stress begins when the ambient temperature climbs above 26ºC and is readily apparent above 29ºC. When a bird begins to pant, physiological changes have already started within its body to dissipate excess heat. Even before the bird reaches this point, anything that you do to help birds remain comfortable will help maintain optimum growth rates, hatchability, egg size, egg shell quality, and egg production.

Kenneth E. Anderson, Extension Poultry Specialist and Thomas A. Carter, Specialist-in-Charge, Poultry Science Extension, from North Carolina State University offer the following as a general guide to the reaction of adult poultry to various temperatures.

12° to 24°
Thermal neutral zone. The temperature range in which the bird does not need to alter its basic metabolic rate or behaviour to maintain its body temperature.

18° to 24°
Ideal temperature range.
24° to 29°:
A slight reduction in feed consumption can be expected, but if nutrient intake is adequate, production efficiency is good. Egg size may be reduced and shell
quality may suffer as temperatures reach the top of this range.

29° to 32°:
Feed consumption falls further. Weight gains are lower. Egg size and shell quality deteriorate. Egg production usually suffers. Cooling procedures should be started before this temperature range is reached.

32° to 35°:
Feed consumption continues to drop. There is some danger of heat prostration among layers, especially the heavier birds and those in full production. At these temperatures, cooling procedures must be carried out.

35° to 37°:
Heat prostration is probable. Emergency measures may be needed. Egg production and feed consumption are severely reduced. Water consumption is very high.

Over 37°:
Emergency measures are needed to cool birds. Survival is the concern at these temperatures.

To prevent and deal with these issues Dr. Manathan said ventilation is key. Moving air around the birds is essential to help with their evaporative cooling. But studies show that often producers turn up the fans later than they should or never get to the point where the volume of air being moved is adequate for the conditions.
Another technique that is common in the U.S., but uncommon in Canada is the use of circulatory fans. “Circulating fans work at the bird level,” he said.
That is essential because it is conditions at the level at which the birds live that are important, not what is going on above that.

Harry Huffman, P.Eng. and Drs. Babek Sanei and Lloyd Weber completed an extensive heat stress study in Ontario in 2003, in which  barn ventilation systems were evaluated.   Their recommendations:
Be sure to exchange sufficient air during hot weather.  For most broiler chicken facilities, this means a complete air exchange every minute.  Air volume calculations should be based on ceiling height minus 1 foot, to account for bird height.  Higher ceiling  height does not help with summer heat stress since  the heat is still kept within the room.
Therefore, exchange all of the room air every 60 seconds  regardless of ceiling height.   
Ventilation calculations can also be based on bird  weight.  As shown in the following table, minimum summer ventilation rate should range between 7 and 10 CFM per bird.
Given the recommended bird density and a standard 8-foot ceiling height, each of these per bird ventilation rates will also result in at least an air  change every minute.  Note that these rates should be increased proportionally with higher ceilings to ensure that all of the generated heat is removed continuously.   
Although the actual exhaust capacity of a particular fan can vary depending on make and model, the following table can be used as a guideline for fans required to achieve a given air exchange rate.  More specific values can be obtained from your fan’s manufacturer.
*Note: dirty exhaust fan shutters or a loose belt can reduce its output capacity by more than 30 per cent.  Therefore, it is extremely important to ensure that all exhaust fans (and even the air intake screening) are thoroughly cleaned between each group of birds.   Also ensure belt driven fans are adjusted properly.

For exhaust fans to be effective, they must have an adequate, unrestricted supply of fresh outside air.   The minimum net air intake opening needs to be at least 1.5 ft 2 for every 1000 CFM of exhaust fan capacity.  An opening as large as 2.0 ft 2 per 1000 CFM is preferred to ensure all of the studs, hood framing and screening have been accounted for.   The inlet baffle opening can be adjusted to create the static pressure (room vacuum) required to create the best airflow pattern for that specific building.
A  static pressure gauge and a seasonal smoke test  can be helpful for fine tuning the air inlet settings.  In fact, all rooms should be equipped with a static pressure gauge to assist
in obtaining the correct settings. Generally, a static pressure level between 0.04 and 0.06 inches of water is required for good summer airflow.  Higher static pressure levels tend  to prevent the airflow from circulating properly down  at bird level and force most of the air to travel rapidly  across the room near the ceiling with little benefit to  the birds.
Dr. Brian Fairchild and Czarick of the University of Georgia recommend checking static pressure on a regular basis. Tunnel ventilation is basically negative pressure ventilation, and static pressure is the difference between inside and outside atmospheric pressure which is expressed in inches of water column.
Ideally, static pressure should be measured close to the fans. Monitoring for changes in static pressure over time can be a good check of fan performance. For example , it is not good if a house static pressure is checked and it is a 0.10 but one year later has a lower pressure. This means that either the house has more air leaks than it did a year ago or that the fans are not moving as much air as they did previously.

Another summer technique, that isn’t popular with producers, is to reduce stocking levels by 25 per cent. This will allow the birds to spread out – allowing them to move away from the body heat of other birds – and reduce the amount of total heat the birds are producing. It also allows ventilation systems to work more effectively at the bird level.

Hand in hand with moving air is ensuring the birds are drinking. In very hot conditions the birds will be thirsty, but they are also lethargic and don’t want to move.
Dr. Manathan said on hot days producers should walk their barns to encourage the birds to break the lethargy and move to the water and encourage drinking.
There are also feeding techniques that apply during hot weather.
“Feed the birds during the cool periods of the day and fast them during the hot periods of the day,” he said.
Feed should be withdrawn  six to eight hours before the temperature is expected to peak.
It is advisable to replace dietary energy with dietary fat because the digestion of fat produces less heat.
Because the birds will eat les when it’s hot the nutrient density of proteins, amino acids, minerals and vitamins should be increased.
Vitamin C (ascorbic acid) will enhance the survival of birds during extreme heat and should be added to the diet at 200 to 300 g/ton of feed or one g/litre of water. The vitamin C should be administered eight hours before the predicted onset of heat stress.
To replenish the loss of electrolyte and correct the acid-base balance during heat stress potassium, sodium, chloride and bicarbonate should be added to the water. This will stimulate water intake and help prevent dehydration. Do not use Nicarbazin in feed during hot weather because it can aggravate heat stress mortality, he said.

Another presenter at the OMAFRA producer updates, Harold Meadows of W. Murray Clark Limited, said caged birds are more susceptible to heat stress because they are unable to find cooler places and there is less conductive heat loss.
The bird’s most effective method of controlling its internal heat balance is to consume more feed when it’s cold and less feed when it’s hot. “The rest is up to us,” he said.
As temperatures rise egg farmers can expect to se a one per cent drop in production for every one-degree increase in temperature.
Breeders also get hit by the heat. For every degree above 24 degrees Celsius a drop of one percentage point in production can be expected and the hens will never fully recover. Production will usually stay one or two per cent below normal after heat stress.
The high temperatures also decrease sexual appetite and decrease the hatchability of embryos.
Broilers eat less and grow slower. There is also the possibility of mortality.
In 2002, more than 600,000 broilers in Quebec and New Brunswick died from heat stress.
“Summer temperatures are getting hotter and mortality from heat stress is becoming a bigger problem every year,” he said. n

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