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Combating heat stress to keep birds productive

Extremely high temperatures have a much greater effect on the faster growing, higher yielding broiler of today and also in laying chickens. When it's hot outside, it is essential to create an environment inside the poultry house that allows the birds to dissipate excess body heat and remain comfortable. In these challenging conditions it is essential that key management techniques are applied in order to gain maximum performance. Making fat digestible in nutrition is such a measure.

By K.V. Chandrasekar, Kemin AgriFoods, Singapore
 
Chickens, unlike most other animals, do not possess sweat glands to aid in heat loss in order to maintain a constant body temperature. The chicken removes excess body heat in four ways. Body heat can be lost by radiation from the skin surface through the air to another object (i.e., another bird). Heat can be directly transferred by conduction to cooler objects with which the bird is in contact, such as the cage, litter or slats. Body heat is also lost to the surrounding air by convection.
 
When the environmental temperatures are between 28°C and 35°C (82°F and 95°F), radiation, conduction, and convection heat losses are usually adequate to maintain the bird’s body temperature. The bird dilates the blood vessels in its skin, wattles and comb to bring the internal body heat to the skin surface to facilitate conductive, convective and radiation heat loss. Floor birds will search for cool places in the house and dig into the litter to increase the conductive and convective heat loss. Drooping of the wings promotes convective heat loss by increasing the surface area of the body.
 
Caged birds are more susceptible to heat stress because they are unable to seek a cooler place and there is less conductive heat loss in cages. As the environmental temperature approaches the body temperature of the bird, 41°C (106°F), the efficiency of these heat loss mechanisms diminishes.
 
 
 
Effects of heat stress
About 75% of the metabolisable energy consumed by the bird will be converted to body heat and require being lost to the environment. Thus reduction in feed intake is an important physiological safety mechanism to reduce heat stress. As temperatures rise, the ability to lose heat by conduction, convection and radiation decreases. At this point, the bird will then try to loose heat by panting, which assists the evaporation of water from the moist linings of the respiratory system.
 
This evaporative cooling initially involves the process of passing air rapidly in and out of the mouth and is the principal means of regulating body temperature in heat stress. However, at higher temperatures water also evaporates from the air sacs within the lungs during panting, lowering the levels of blood carbon dioxide and inducing a process called respiratory alkalosis. This condition can have a serious impact on bird performance particularly when accompanied by decreased feed intake, due to the decreased potassium and minerals balance. In general, birds with higher metabolic loads e.g. male broilers and laying birds will be more sensitive to heat stress.
 
When heat production exceeds the bird’s ability to dissipate heat, birds will lie prostrate and gasping on the floor, which results in them becoming more weak and susceptible to dying from respiratory, circulatory and or metabolic imbalance.One of the primary effects of high environmental temperatures on a flock is reduced feed intake. A reduction in appetite is the birds’ effort to reduce energy intake in response to the increase in the energy in the environment, thereby reducing the energy needed from the feed. Birds may use body fat as a source of energy that produces less heat than digestion or metabolism of proteins or carbohydrates in the feed.
 
The reduced feed intake and subsequent loss of needed nutrients quickly affect the productivity of the flock. Growth retardation occurs in growing birds. Laying flocks typically have a reduction in egg size, followed by lowered egg production, and reduced egg shell quality. In breeders, high environmental temperatures decrease the hatchability of embryos and the fertility of roosters.
 
 
 
Nutritional management of heat stress
Increasing nutrient intake during heat stress, by changing the feed specification, may have an adverse effect on survivability, but increasing the digestibility of nutrients and the use of specialist micro ingredients has been shown to have benefits.The principle nutrients to consider are:
 
Proteins and amino acids: nutrient digestibility should be increased rather than nutrient density, minimise excess protein and balance amino acids and minimise the crude protein level in the diet.
 
Vitamins and minerals: certain vitamins are known to have a positive effect on the birds’ response to heat stress including Vitamin E, D, A, C, B2 and nicotinic acid. Under no circumstances should vitamins be withdrawn from the diet.
 
Energy: the diet should be supplemented with fat rather than carbohydrate. Increasing the energy density of the diet will increase growth rate but will also increase heat output.
 
As temperatures rise, the bird has to maintain the balance between heat production and heat loss, and so will reduce its feed intake. Trials indicate that feed intake is reduced by 5% for every 1SDgrC rise in temperature between 32-38SDgrC. Reduced feed intake is the main cause of poor performance. As a general rule, for each 2.5°C (5°F) increase in house temperature above 29°C (85°F), the energy content of the feed should be reduced about 22kcal/kg (10 kcal/lb).
 
The feed energy content can be decreased because more of the energy requirement of the bird can be met by the increase in environmental temperature. As the total amount of energy in the feed is decreased, the proportion of the total feed energy provided by added fat should be increased. The addition of fat may, in certain instances, be as high as 4.5% of the ration. This may require the use of low-energy feed ingredients such as wheat middlings and/or soy mill feed (soy hulls). A by-product of the digestion or metabolism of feed is the production of body heat (heat increment).
 
It is widely recognised that fat has the lowest heat increment of the energy nutrients — i.e., carbohydrate, protein, and fat. In comparison to proteins and carbohydrates, the digestion of fat results in less production of body heat per calorie of feed energy. The heat load of the bird can be reduced by replacing other dietary energy with dietary fat. Generally the energy content of the feed should be reduced gradually in increments of 22-33 kcal/kg (10-15 kcal/lb). Calorie reductions of this magnitude can be made at least twice each week.
 
 
 
When the nutrient density of the formula is increased to compensate for the reduction of feed intake, the protein content of the feed may, in some instances, be reduced by about 0.50% below the calculated value. If this is done, the intake of the needed amino acid can be optimised by providing increased quantities of synthetic amino acid such as methionine and lysine.
 
Adjusting the intake of protein is important because the body heat produced by protein digestion or metabolism is, as noted earlier, the greatest among the energy nutrients — i.e., carbohydrate, fat, and protein. Restrict the intake of feed about three hours before temperatures are expected to exceed 36°C (95°F) for more than three hours. Adjust the lighting schedule to encourage the consumption of feed in the night and early morning. A midnight feeding or an intermittent lighting programme can encourage feed consumption at night. Vitamin C in the ration (50-300 gm/tonne of feed) can protect birds from the effects of heat stress and enhance the survival of birds exposed to acute heat stress.
 
Increasing fat absorption
One of the nutritional management practices as mentioned above is to supplement more fat in diet as source of energy rather than carbohydrate and protein since the latter can result in more heat increment and therefore can increase heat stress and death. Fat as a source of energy produces less heat than digestion or metabolism of proteins or carbohydrates in the feed . If the total amount of energy in the feed is decreased, the proportion of the total feed energy provided by added fat should be increased.
 
Therefore addition of fat in broiler diets during hot weather conditions can go more than 4.5% and also added in higher quantities than normal in layer diets in order to compensate the energy requirements during low feed intake. During such circumstances the fat has to be utilised and absorbed to the maximum extent in order to compensate the energy requirements. However the digestion of fat results in less production of body heat per calorie of feed energy and therefore maximum utilisation of the same will not result in excess heat production when compared to proteins and carbohydrates.
 
It is clearly understood that fat as a source of energy is a key tactic to combat heat stress in poultry in hot weather climates but however it is important to note that the fat added has to be utilised in order to maintain performance of animals. Broilers and pullets are young chickens and therefore the bile produced is not sufficient to utilise the higher levels of fat added in the diet and therefore it will not be effectively absorbed, as a result the performance of animals will drop. Therefore addition of exogenous biosurfactant technology (such as Lysoforte dry) that is more powerful than bile can help in utilisation of fat to maximum extent and provide energy with less heat increment and can combat heat stress.
 
 
 
Maintaining performance
Lysoforte is a natural biosurfactant which increases the absorption of fat and fat soluble vitamins and maximises performance in animals by formation of smaller emulsion droplets, and also by formation of more number of micelles and thereby rapid absorption of fatty acids. It can also increase the permeability of cell membranes and can increase ion transport and can help in maintaining electrolyte balance during heat stress.
Previous results from a metabolic trial done in India have demonstrated that Lysoforte improves the digestibility of the feed fats and does not affect the performance during heat stress. From the study it became clear that Lysoforte had improved the fat digestibility by 9% (Figure 1).
 
 
 
Recent studies in Massey University in New Zealand have also confirmed the positive energy sparing effect of Lysoforte on diets with reduced ME values. The diets were formulated with different fat sources as shown in Table 1. Lysoforte addition produced a significant increase in Apparent Metabolisable Energy (AME) for all the treatments resulting in a recovery of metabolisable energy of the reduced ME diet through increased absorption of fat, containing different fat sources as indicated in Table 2.
 
To conclude, the improved fat utilisation and sparing effect of Lysoforte during hot weather conditions will not alter the performance of birds, because of less production of body heat per calorie of feed energy.
 
* References are available from the author
 
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K.V. Chandrasekar

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