Poultry: Heat Stress in Caged Layers
Table of Contents
A caged layer, eating a normal ration and laying at a rate of 80%, will produce about 180 kilocalories of heat every day. Thus 10,000 layers will produce as much heat in a day as a furnace burning 231 litres of fuel oil. This means that on a warm summer day, a ventilation failure could result in a rise in the temperature, within the building, of 16°C in one hour. When the hen is in a comfortable environmental temperature (21-25°C) she will lose most of that heat by sensible means. Sensible heat loss is by three pathways:
When the laying hen is in a very warm environment (28-35°C) she must work at losing the heat she produces. She does so by raising and spreading her wings and separating herself from other birds if possible. Despite her best efforts, however, heat loss by sensible means decreases and loss by latent means (evaporation) increases. One reason is that, as the environmental temperature increases, the difference between the temperature of the hen's body (41°C) and the surrounding air, equipment and walls becomes very small. Thus she cannot readily lose heat by sensible means and must do so by evaporation. A second reason is that the evaporation of water uses a lot of heat - it is an effective way for her to keep cool.
How does she go about evaporating water since she does not have sweat glands? She does it by panting, similar to the dog. The hen can easily increase her respiration rate to 10 times normal and, in addition, indulges in gular (throat) flutter to aid in evaporation. One can readily see the rapid throat movement in panting birds. This throat flutter moves air in and out of the throat area and increases evaporation without such air entering the lungs. During a hot, dry day this is very efficient but on a hot, humid day the hen must pant more to keep cool. This evaporation means that a readily available supply of clean, fresh water is vital. The relative importance of sensible and latent heat loss is illustrated in Figure1.
The heat from the laying hen, either in sensible or latent form, must be removed from the building. What can be done to facilitate heat removal from birds and buildings in hot weather?
Ventilation does the following:
To adequately handle a heat wave, the ventilation system should have:
The exhaust fan capacity of a laying house is the total capacity of all exhaust fans. To obtain information on the capacities of fans, take the sizes, make and model numbers of the fans and call the supplier or your local agricultural engineer. When discussing fans with a supplier, always obtain the capacity at 0.10 in. water gauge (W.G.) static pressure.
If certified capacity ratings are unavailable for the exhaust fans in your laying house or for fans you plan to purchase, then use Table 1 as a guide.
Mount fans in solid frames and seal all cracks around the frames.
Make sure that the fan blades on all fans with sharp-edged orifices are mounted on the shaft so that one-third of the blade width is outside the orifice and two-thirds of the blade width is inside.
1 Most good quality fans properly installed will meet or exceed these ratings.
The opening through the wall or over the plate must be of adequate size.
If not, the flow of fresh air into the house will be restricted when the
maximum ventilation rate is required. The required width of air-intake
opening is listed in Table 2 for various numbers of
birds per m and per ft. of intake.
First calculate the number of birds per m or foot of intake for your house by dividing the total number of birds by the total intake length.
For example: 20,000 birds are housed in four 250 ft. long cage rows. There is air intake on both sides. The number of birds per foot of intake is 20,000/500 or 40. From Table 2 the required air intake opening width is 6.7 inches. If constructing the air intake over the plate, make sure that the width of screened opening in the soffit is at least 6.7 inches and also that there is at least a 6.7 inch wide opening through the ceiling.
Use the test indicated in Table 2 also for existing caged layer houses . If you are seriously short of intake opening area, add an additional continuous intake or reconstruct existing intakes to a larger size.
Ventilation System Management
Air-inlet baffles must control the inlet air to provide proper air movement, even distribution and proper house static pressure. Automatic baffle controllers using a static pressure sensor are recommended. A typical installation is shown in Figure 2.
Without automatic baffle control, adjust inlet baffles to keep the house static pressure within an acceptable range as fans turn on and off. In hot weather, aim for 0.05 in. W.G. In cold weather, 0.08 in. W.G.
A heat stress problem can occur any time during the year if the ventilation system fails to work properly. In winter when large fans are covered and outside doors on air-intakes are closed, it is important to ensure that there is adequate back-up fan capacity to cool the barn. An equipment failure at this time is just as serious as in the summer. Check the high temperature warning system regularly year round and be especially sure that winter fans are in working order.
What Does Heat Stress Do To Laying Hens?
While laying hens may occasionally die as a result of heat stress in Ontario, this is not the major economic consequence. Most laying hens adapt to the stress, but in the process, many aspects of their lives, internally and externally, undergo radical change. We will concentrate here on the external, visible changes.
Hens continually adjust their feed intake according to environmental temperature. Up to about 27°C this fluctuation in feed intake does not affect production unless an important nutrient is marginally deficient. Above 27°C, the bird's body temperature rises and a much more dramatic reduction in feed intake can be anticipated. The data below (Table 3) were collected in different parts of the same house, where different average maximum temperatures were recorded.
As feed intake declines, the first production trait to respond is egg
weight. The changes shown in
Source: D.R. Sloan & R.H. Harms, 1984. Univ. of Florida, Poultry Science, Volume 63, Supplement, page 38.
As temperatures rise above 32-35°C, egg production levels may also decline, as total nutrient intake is insufficient to support normal rates of lay.
Perhaps the most frequent result of heat stress is a decline in egg shell quality. Occasionally, this may be due to inadequate calcium intake, but this is rare, since most commercial layer feeds provide enough calcium to support shell formation even at low feed intakes. The maximum daily calcium intake recommended for most layers is 4.0 g. This would be obtained if the hen consumed 90 g of a feed containing 4.4% calcium, or a feed with 3.6% calcium plus 20 kg oyster shell supplement per tonne.
Most of the shell quality problems arising from heat stress are not the result of dietary calcium deficiency. Rather, they result from extremely complex metabolic changes within the hen. As the bird pants to keep cool, excess carbon dioxide is exhaled. This causes the blood to become more alkaline, and reduces its ability to hold and carry calcium for shell formation. Such a situation cannot be remedied by increasing dietary calcium.
Various other blood minerals are affected by heat stress. One of the most important is phosphorus, and the requirement for this element is increased at high temperatures. Marginal phosphorus levels, when combined with heat stress, can lead to increased mortality rates, particularly among older birds.
As a result of the increased water intake. moisture content of droppings will escalate in hot weather. This can provoke difficulties in the handling and storage of manure where the system depends on dry manure. It can also lead to increased soiling of egg shells, either directly from soiled feathers, or from splashing in shallow manure pits.
There is considerable evidence that the immune system of the bird is adversely affected by heat stress. Therefore, administration of vaccines to pullets or layers is not usually recommended during very hot weather.
Furthermore, where vaccines are spray administered, the closing down of the ventilation system to permit proper vaccine distribution is impractical in extremely hot conditions. If vaccinations cannot be avoided, they should be restricted to the coolest time of the day, when the adverse effects of reduced ventilation are at a minimum.
High temperatures may also result in less efficient utilization of vitamins, and certain vitamins in feed are themselves less stable in these circumstances.
The effects of heat stress will all be aggravated by other environmental factors such as increased bird density, feed and/or water deprivation, inadequate ventilation, vaccine reaction and the presence of diseases or parasites.
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