Higher production performance and larger feed efficiencies in modern day poultry have made them highly vulnerable to heat stress. Ambient conditions, especially high temperature in combination with high relative humidity in poultry shed leads to heat stress in poultry birds and in turn reduces their productivity and increases mortality significantly. India, being predominantly a tropical country, the day time temperatures during summer reaches as high as 45°C at many places. The capital and operational cost of conventional evaporative air cooling system is very high as compared to the investment standards of poultry management in India. Apart from this, poultry sheds are generally located in rural areas where schedule power cuts are normal affair. Hence, majority of the poultry operators do not use proper cooling system which makes them susceptible to the adverse effects of temperature on production cost and hence profits.

Heat stress remains a perpetual challenge for the poultry enterprises in tropical climate of India. It results from a negative balance between the net amount of energy flowing to its surrounding environment and the amount of heat energy produced by the layers . High temperatures, especially when coupled with high humidity, impose severe stress on Broilers & layer birds and lead to reduced performance .High environmental temperatures stimulate the hypothalamo hypophyseal-adrenocortical axis which increases corticosteroid secretion in response to stress . Higher levels of circulating corticosteroids have a catabolic effect through increase in the free radicals by altering oxidative metabolism,causing impairment of cellular functions and thus damage to the cell membrane, muscle wasting and retarded growth . During the periods of heat stress, most of the production energy is diverted to thermoregulatory adaptations which results in oxidative stress induced immunosupression, predisposing birds to various infectious diseases and high mortality rates . In Broilers & laying hens, heat stress supresses body weight, egg production, egg weight, shell quality and is generally accompanied by suppression of feed intake . Therefore, it remains a prime issue to re-evaluate the poultry management in hot weather, so that heat stress is minimized. There are various strategies to minimize heat stress in Broilers & layer hens either by changing their environmental condition or by modification of their diets . Nutritional strategy being more viable during heat period is based on diet balancing in order to cover the needs of stressed birds for amino acids (protein), energy and electrolytes . The body requirement of ascorbic acid during heat stress in poultry is greater than the amount synthesized by normal tissues and its administration to broilers during heat stress has been shown to be beneficial to the body . Non enzymatic antioxidants vitamin C and vitamin E are used and are being explored extensively in the poultry diet, because of their antioxidant effect in the neutralization of the free radicals generated during heat stress .

What is heat stress?—

Birds are ‘heat stressed’ if they have difficulty achieving a balance between body heat production and body heat loss. This can occur at all ages and in all types of poultry. birds can lose heat at a controlled rate using normal behaviour. There is no heat stress and body temperature is held constant. When conditions mean the ‘upper critical temperature’ is exceeded, birds must lose heat actively by panting. Panting is a normal response to heat and is not initially considered a welfare problem. But as temperatures increase, the rate of panting increases. If heat production becomes greater than ‘maximum heat loss’ either in intensity (acute heat stress) or over long periods (chronic heat stress), birds may die. The body temperature of the broiler must remain very close to 410 C (1060 F). If body temperature rises more than 40 C above this, the bird will die.
Effects of Heat stress —–
What happens inside of the birds —-
● Due to panting CO2 excretion increases and results in a higher pH value of the blood
● Mineral secrection in the urine goes up -creating a shortage of these minerals
● Due to high CO2 output (panting), shortage of carbonates (CO3) can occur
● More corticosterone is produced

Heat stressed poultry ——-
Negative effects: —
● Lower feedintake, lower growth
● More fat, less meat
● Lower egg production
● Higher mortality
● Worse egg shell quality, due to panting, more CO2 production, thus less CO3 availability for formation of eggshell (CaCO3)

Strategies to prevent production losses —–
1. Reduce heat production
2.Increase the removal of produced heat
3. Increase dietary nutrient levels
4. Reduce the concentration of corticosterone
5. Prevent the induction of respiratory alkalosis

How is body heat produced?
Heat is produced by metabolism within the body, which includes maintenance, growth and egg production. Heat production is affected by body weight, species and breed, level of production, level of feed intake, feed quality and, to a lesser extent, by the amount of activity and exercise.

What are other heat sources within houses? —-

Excluding temperature of the air ventilating the house, heat is also added from the roof and walls. Much of the heat from working litter is used to evaporate moisture and dry the litter. However, in hot weather, damp litter will make heat stressed birds feel much more uncomfortable than dry litter. In dry litter, birds will attempt to dust bathe more readily to aid cooling. The heat of electric lights and motors is a very small fraction of that produced by the body metabolism (normally less than 1%). Body heat is the dominant source of heat.

How do birds lose heat?—–

Heat can be lost in a variety of ways. Three normal methods of heat loss are listed below. Birds modify their behaviour to stay in the ‘thermoneutral zone’.

● Radiation – Heat will be lost from the body by radiation if the surrounding surfaces are below bird surface temperature. Conversely hot walls and roofs may radiate heat to the bird surfaces.
● Convection – heat loss will occur from the natural rise of warm air from around a hot body. Providing moving air can assist convection, but only if the air moves fast enough to break down the boundary layer of still air that surrounds the body.
● Conduction – heat will transfer from one surface in contact with another surface, for example, if the birds are seated on litter that is cooler than their bodies. However, the litter immediately under the birds soon assumes a temperature close to that of the body. After a bird can no longer maintain its body heat balance by one of these three methods (upper critical temperature), it must use “evaporative heat loss”, or panting. Evaporative heat loss, whilst essential to the bird, does not contribute to heating the house.
● Evaporation – This is very important at high temperatures as poultry do not sweat but depend on panting. This is only effective if the humidity is not too high. Hot, humid conditions are therefore much more stressful than hot dry conditions. How is heat lost from houses? Heat is lost via ventilated air and by conduction through the roof and walls when not in direct sunlight. In hot weather, the air change is the dominant route.

How do birds respond to increasing temperature?—-

Birds will try to re-establish their heat balance with the surrounding by changing their normal behaviour. Birds may:
● Try to move away from other birds. ● Move against cooler surfaces, such as the block walls or into moving air streams. ● Lift their wings away from their bodies to reduce insulation and expose any areas of skin that have no feathers. ● Elect to pant slowly. ● Rest to reduce heat generated by activity. ● Reduce feed intake. ● Increase water consumption. ● Divert blood from internal organs to the skin, which darkens skin colour. ● Begin fast panting.

What are the consequences of panting? —–

● Heat is lost as moisture is evaporated from airways in the birds. ● Panting requires muscle activity, requiring energy use that generates some additional heat. The heat lost by evaporation must be greater than the additional heat generated by panting. ● Slow panting is a normal activity and can be sustained for extended periods of time. ● Respiration rate can increase by as much as 10 times the resting rate. Heavy panting can tire birds, reducing their ability to cope with extended periods of hot weather. ● High relative humidity reduces the effectiveness of evaporative heat loss. ● Increased respiration rate results in loss of carbon dioxide and a rise in blood plasma pH (called respiratory alkalosis). Blood potassium and phosphates are depleted, sodium and chloride levels increase. ● Growth rate or egg production will reduce.

Can poultry acclimatise to high temperatures? —–

Adult birds take about five days to acclimatise to high temperatures. Birds are more susceptible to sudden, large changes in temperature. The first very hot days after a cool spring often result in increased incidence of heat stress. Some of this will be due to poor acclimatisation, but some will be due to managers being less well prepared than later in the summer.

Does reducing feed intake or feed removal help in heat stress relief?——

Birds in hot environments reduce feed consumption naturally, reducing heat from metabolism. Feed conversion efficiency and growth rate is also reduced. Techniques that increase activity or stimulate food consumption may be counterproductive. Feed removal prior to the hottest part of the day has been shown to be beneficial in reducing mortality. The advantages are reduced metabolic heat output, and where feeds systems can be lifted, increased floor space and improved air distribution over the floor. Problems can occur, however, on the reintroduction of food – the stimulus to eat again may result in a surge of activity. If the birds have not fully recovered from heat stress during the day, the sudden activity can prove fatal. There is an argument to leave the feed available for broilers, since they do not habitually store food in the crop, and are better able to restrict their own food intake. Stockmen must be very careful about the timing of reintroducing the food soon after the heat of the day, noting bird behaviour.

Can cool water alleviate heat stress? ——-

Water is lost from the lungs when birds pant and so more water needs to be drunk to prevent dehydration. Cool water stimulates water intake. Reducing the body temperature of the bird is beneficial and has been associated with reduced mortality.

Can dietary adjustments make a difference? ——-

Where feed consumption is decreased due to spells of hot weather, dietary adjustments can help maintain a good supply of nutrients. Protein contributes more metabolic heat than fats and carbohydrates, so a correct energy: protein ratio is important. Dietary vitamins and minerals can be reformulated

Prevention of Heat stress in poultry —–

1) Providing ventilation with cool air—

Well located and well laid out poultry farms, with trees and lawns around and getting good breeze will have lesser problems in summer season. In addition, roof insulation plus increased ventilation by fans would help to withstand moderate hot weather. Care should be taken to avoid overcrowding and to provide shade to overhead water tanks and pipe system.
In extremely hot weather, however, the above measures are not sufficient and evaporative cooling systems -sprinklers, foggers, or pad cooling-are necessary. These systems work very well especially if relative humidity is low and temperature inside houses can be brought down by 10 to 15C
Use of fans without evaporative cooling system may be harmful in extreme hot weather since it is only hot air that is blown in to the house.

2) Daily management to prevent heat stress :—–

In very hot season, the following measures are advised.
i) Feed withdrawal from 9am to 4.30 pm.: This is very effective in reducing heat stress mortality. Feed intake and digestion produce nearly 7% additional heat in the body which is maximum 4 to 5 hours after feed intake. This should not coincide with hottest part of the day (2 to 3 pm) Birds fasted in the day compensate intake in the night time and it should be seen that lights do not go off in night.
If biosecurity is poor, feed withdrawal may lead to coccidiosis, necrotic enteritis. Necessary medicines may be kept at hand in case such eventuality arises.
ii) Drinking Water: Cool Water (at 10-12 C) should be supplied continuously to birds. Watering space should be doubled. Over head tanks and pipe system should be properly covered to keep the water cool. Birds reject warm water and that accentuates heat stress.
Providing fresh cool water (5c) in noon time is very effective for internal cooling of body (heat sinks) and reducing symptoms of stress.

3) Preventive treatment through drinking water:—

a) In moderate hot weather
Ascorbic acid….62.5 mg/litre
+ Acetylsalicylic acid 62.5 mg/litre
+ Sodium bi carbonate 75 mg/litre
+ Potassium chloride ( KCl) 125 mg/Litre
b) In heat stress
Vit C 400 mg/ L
+ Electrolytes
+ Acetyl salicylic acid ( Disprin 1 tablet/5 L)
+ Sodium bicarbonate 1gr/Litre…….may be used .

4 ) Use of summer feed formula ——–

In summer, feed should be made more dense with nutrients, Vitamins and minerals to compensate for reduced intake. Thus as the hot season progresses it may be necessary to fine tune feed formula again in mid summer In addition following features, which have bearing on heat stress control, should be included in summer feed formula:
a) Crude Proteins. Crude Protein level in feed should not be increased and protein from only vegetable source should be used. Proteins in general and those from animal sources in particular have higher heat increment values i.e. produce more internal heat in the body .
Secondly vegetable proteins (Soya, Sesame, Sun flower) are rich in Arginine. Under heat stress arginine absorption is low and that results in plasma aminoacid imbalance, leading to increased catabolism of amino acids adding to body heat Hence vegetable proteins are preferred in summer.
While keeping protein levels same, crucial amino acids may be increased to compensate for reduced feed intake.
b) Fats: fat should be increased by 2 to 3% at the cost of carbohydrates without changing ME. Fats are good in summer because their heat increment value is lowest give better cooling effect in body because of higher water content and fat stimulates feed consumption
c) Vitamin C: Because of the release of corticosteroids in heat stress, there is increased demand for Vit C by adrenal glands for controlled production of hormones needed for gluconeogenesis. In heat stress there is also reduced synthesis and partial depletion of Vit C. Inclusion of Vit C at 150 – 200-400 Gr/ton of feed is recommended in summer months. If included in feed , there is no need to give ;in water again.
d) Sodium bi carbonate: It has positive effect of increasing water intake, and also reducing systemic acidosis. It can also be given through feed instead of through drinking water. In vegetarian feed, inclusion of Sodabicarb at 0.4% (4 kg/ton) + Sodium chloride0.25%( 2.5kg/ton) would properly balance sodium and chloride levels in feed.
e) Betain ( osmolyte) in feed
Betain (0.5 to 1 gr/ton) helps in maintaining water balance in the body cells against extra cellular osmotic gradient. Electrolytes also have similar action and are better given through water.
f) Anti oxidants: In heat stress there is excessive oxidative metabolism and release of free radicals in the body. Free radicals damage all types of biological molecules, and cells of vital organs more so of immune system VitE captures and neutralizes free radicals and its inclusion at 50 to 100 ppm in feed is advisable. Vitamin C also has antioxidant property. Similarly antioxidants namely BHA ,BHT and EQ will protect fats and oils in feed from oxidative rancidity .
g) Other recommendations:
i) Virginiamycin 15 to 20 ppm in feed apart from being growth promoter, reduces metabolic heat production, alleviates heat stress and stimulates immune responses.
j) Anticoccidials: Nicarbazine and Monensin are contraindicated in summer. The former decreases tolerance to heat and the latter depresses water intake.
k) Biotin supplementation at 150 micrograms /Kg feed is recommended.
l) Vit K supplementation is recommended particularly at time of debeaking or if there is threat of coccidiosis because in heat stress blood clotting time is prolonged.
m) Toxin binders : In wet summer, there is rapid growth and toxin formation in feed. Good quality toxn binders at higher dose should be used in feed.

Change in feeding practices—-

Under hot and humid conditions, feed should not be stored for longer than a week.
The bird’s body temperature increases after feed ingestion due to the thermogenic processes of digestion and metabolism. With morning feeding, the thermogenic effect coincides with the rising environmental temperature, aggravating heat stress, The thermogenic effect lasts for 8-10 hours at 35°C, compared to just 2 hours at 20°C. Metabolic heat production is 20-70% less in starved birds than in fed birds. Therefore, during hot weather, birds should be deprived of feed while the temperature is reaching and at its peak. Feeding during early and late hours of the day will help to minimize growth checks and mortality in broilers. Intermittent feeding, i.e. providing the light for 30 minutes followed by 3 hours dark, may also reduce the activity (heat production) of the bird but 20-30% more feeder and waterier space will be required. For layers, feeding during later part of the day will ensure sufficient calcium is available for optimum shell calcification,
Low feed intake is the main cause of poor performance at high temperatures. The following practices can help to raise feed consumption and may be worthwhile considering:
• Wet mash feeding
• pellet or crumble form of feed
• Low-calcium diets with choice feeding of calcium sources
• Frequent feeding and stirring of feed in the feeder
• Addition of fat or molasses so to increase feed palatability.

Other associated problems in summer season—-

In summer season, apart from hot weather related ones, other associated problems could arise and following precautions are necessary to address them:
i) Regular sanitization of drinking water, periodic cleaning of water tanks and flushing pipe system. This is necessary since bacterial counts in water increases in summer due to water table going down.
ii) Use of Litter conditioner: In summer, since there is increased intake and excretion of water by birds there is wet litter and ammonia problems in poultry houses. Hence it is important to maintain litter dry.
iii) Vaccination: Strict cold chain should be maintained during transport storage and administration of vaccines. Vaccination should be carried out during cool hours.
v) Immunostimulants should be given for three days following each vaccination..

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