Bird performs well within a certain range of environmental temperatures. As the temperature gets higher than the range, the bird falls on stress generally called heat stress. Heat stress is the most significant stressors influencing poultry productivity and causing considerable economic losses in the poultry industry. This induces several negative effects on physiological response of bird. Reducing feed consumption, body weight gain, feed efficiency, egg production and immune response are of important kinds easily observed. Fortunately, there are several scientifically proven ways and practical approaches found beneficial to combat or ameliorate the negative impact of heat stress. Proper understanding of environment the bird grow on and the physiological changes happening to the environment may help to use these ways and approaches to combat this potential loss factor, Heat Stress.
Heat Stress and the consequence of occurrences
Heat stress induced from the difficulty of achieving a balance between body’s heat production and heat loss. In general, birds successfully can compensate for the imbalance when the environmental temperature is in between 18-25 °C, called the thermoneutral zone. When the temperature exceeds 25 °C, the upper critical temperature, the bird has to lose heat actively by panting. Initially panting is not a problem but as the temperature increases, 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 due to failure of normal body temperature, 42 °C.
The ways of thermoregulation in birds
Birds usually do maintain their normal temperature in a heat challenge environment through several ways like Radiation: Electromagnetic waves transfer body heat through the air towards cooler objects. The higher the difference in temperature between body and the cooler distance surface, the more heat is lost from the surface of the body. Convection: Heat from body parts such as the comb, wattles and wings is naturally lost to cooler surrounding air. Conduction: In direct contact, heat is transferred from the body to a cooler surface when both are in direct contact. Birds can for example sit on litter or lean against cage wire to lose body heat. Evaporation: Panting is shallow open-mouth breathing which allows heat loss via evaporation of water from the mouth & respiratory tract. Excretion: Heat also losses with water in faces.
All these processes are highly correlated with the combination of environmental temperature and humidity. And this is the core determiner for a bird's comfort and degree of stress in hot conditions. Figure-4 is an example of temperature and humidity relationship (Heat stress index) for commercial layers. Birds, specially high yielding poultry species, can not tolerate high temperature coupled with high relative humidity. Death due to heat exhaustion will occur very quickly, especially in heavier birds, if both temperature and humidity are very high.
Effects of heat stress in poultry
Heat stress is detrimental for bird’s health in the area of endocrine regulation, blood homeostasis, immune responses, nutrients digestibility, gut performance and cellular redox balance in broilers. All these effects produce different physiological and biochemical changes in the bird. Panting for example, the respiratory system gives way to a ventilatory pattern that heat is lost with the moisture that evaporates from airways in the birds. It requires increased muscle activity and can tire birds due to energy loss. Another important consequence of high respiration rate is the corresponding decrease in the levels of blood carbon dioxide, which results in respiratory alkalosis. In the condition of respiratory alkalosis potassium and phosphates in blood are depleted whereas the sodium and chloride levels become high. Thus, to cope with heat stress the bird loses its performance by means of low weight gain, poor egg production & egg quality.
Birds kept in hot environments reduce their feed consumption as a part of their physiological adaptation to heat stress. Less feed intake results in a decrease of nutrients supply. On the other hand, water intake increases much, which leads to ion loss, increased house humidity from wet litter and so on. Exposure to heat stress the high body temperature & metabolic rates lead to raise free radicals particularly ROS concentration. Free radicals attract cells that are consequently related to deteriorating cell membrane integrity and cellular function resulting in oxidative stress. Enterocytes as highly proliferative cells are most vulnerable to these effects. In addition, during heat stress peripheral blood flow is increased to facilitate heat loss which causes cell death by ischemia and hypoxia leads to loss of intestinal barrier permeability and develop the leaky gut syndrome. Leaky gut increases the absorption of toxins and translocation of pathogenic bacteria into the body. Moreover, Endocrine responses are disrupted in response to high temperature leads to serious molecular, cellular and immune dysfunction.
Strategies to cope up the impacts
The strategies to combat the stressors are basically the combination of farming and nutrition management practices best set to the situation. Modification in management practices like housing, ventilation and cooling systems and dietary adjustment to support birds are the core part in strategies to fight against heat stress in birds. The dietary adjustment involved both the feed formulation and feeding practice.
Consequently, during periods of prolonged heat stress there is an urgent need to look at other possible adjustment flexibility to the entire system.
1. Keeping the house cool as much as possible: Proper ventilation, water spraying, humidity monitoring, increase air flow, change in lighting schedule are the main checklists that need to be given high priority to keep the house cool during hot weather. Setting the house in the proper direction can facilitate ventilation and a grass cover on the grounds around the farm can reduce hot air entrance into the farm. Use of quality litter material prevents the house air getting humid. Fan and exhauster helps to keep the air flow normal in the house.
2. Helping birds comfort & ease: Cool water with easy access, providing more space, avoiding feeding at hot times, managing feed sets and creating wind chill effects can make birds comfortable and potent enough against heat stress. Keeping the water tank 80% full, painting the tank with white color, covering the tank with jute bags and keeping ice in the tank are very helpful to keep the water cool on supply. Uniformity of birds in a flock is very important along with increasing the floor space. Increasing feeder number per bird, increasing frequency of feeding and allowing feed at cooler parts of the day are the major aspects in feeding management practice during heat stress. Birds are basically air-cooled. That is, air moving over the birds picks up their body heat and transfers it to the environment. Moreover, fast moving air over the birds creates a wind chill effect that is comfortable specially for larger birds. On extreme heat stress handling of bird & vaccination should be avoided.
3. Paying attention in Nutrition: Energy balance in feed, proper use of amino acid profile and DEB balance are some important aspects that must be considered carefully in the case of heat stress probabilities in birds. Few studies suggest that the concentration of energy can be increased 10% during heat stress to compensate for additional energy loss, while the concentration of other nutrients should be increased by 25%. Increasing fat at the cost of carbohydrates without changing metabolizable energy is the best option to adjust energy in feed. A low protein diet with balanced critical amino acids is beneficial than a diet high in total protein during summer. Raw materials which have digestive coefficients more than 85% can be helpful.
4. Focus on blood homeostasis: During heat stress birds are exposed to multiple biochemical changes to preserve their homeostasis status. It is scientifically proven that the thermoregulatory system of farm birds might be modulated by supplemental electrolytes. Thus the electrolyte balance is very important to minimize the negative impacts of heat stress in poultry. Moreover, dietary sodium bicarbonate helps in enhanced eggshell quality in layers and growth performance in broilers during heat stress conditions. Addition of Na+, K+ and Cl−salts by 1.5% more for each 1 °C rise in temperature above 20 °C reduce the negative impacts of heat stress by preserving the electrolyte balance.
5. Support to compensate the damage: Antioxidants like tocopherols, ascorbic acid, carotenoids and some minerals like Se, Zn, Cu, Mn act as antioxidants and collaborate effectively together to preserve the optimum status of redox balance, leading to protect against the harmful impacts of free radicals. Proper understanding of ROS generation and antioxidant induction pathways is the prerequisite to have these benefits in maximum. Vitamin A, D, E, C and folic acid reduce the effect of heat stress by supporting bird’s physiological strength. Betaine also helpful in preventing cell shrinkage by maintaining cellular osmotic balance.
6. Keeping close looks on opportunistic: Pathogen for Necrotic enteritis, coccidiosis and other common infectious agents are some opportunistic during heat stress. On planning, there should be an option for immediate action for these hazards.
However, these facts can only be a helpful checklist in making a good plan. Considering the depth of the problem and the potential situation, good results will depend on being able to plan and act accordingly.
Disclaimer: The views expressed in this article are based on Author’s knowledge gathered from attending different seminars, webinars, symposiums, free access journals and different articles published in internet media on heat stress in poultry. Author will be pleased to deliver the sources of information upon due request.