by-Dr. Dinesh Kumar
INTRODUCTION
In tropical countries, heat stress is one of the major challenges in Poultry Industry which makes it difficult to maintain the egg production and liveliness of the birds during summer. Researchers have been working to develop techniques to improve the sustainability of birds during stress conditions. Situations worsen when high temperature is accompanied by high humidity. Due to the continuous increase in global warming, it is time to develop better managemental practices for birds during heat stress conditions.
STRESS
Any internal or external stimuli that leads to change in normal homeostasis or equilibrium of an organism is called as stress. There is a thermoneutral zone for birds (19ºC to 22ºC for laying hens) which describes the balance between the amount of heat produced and the amount of heat lost from body without any stress conditions. Although, this zone depends upon many factors like feed intake, housing conditions, etc., whenever temperature goes beyond this range, birds feel stressed and try to maintain body temperature through various mechanisms.
MECHANISM OF BODY HEAT REGULATION IN POULTRY
All metabolic activities involved in maintenance, growth and egg production lead to heat production in birds and are influenced by many factors like species, breed, body weight, level of production, feed intake, feed quality and to an extent by the amount of activity. Generally, when birds are in comfortable environment, a balance in heat produced to the amount of heat dissipated from body occurs. The regulation of body temperature in birds mainly occurs through five mechanisms as radiation, conduction, convection, evaporation and excretion.
Heat lost through the process of radiation does not require any medium and it is transferred from a body with high temperature to the body with comparatively lower temperature, i.e. from bird to the environment (walls/ cages). Hence, birds tend to raise their wings to let the heat dissipate from poorly feathered areas such as parts under the wings.
Heat released via conduction is a way in which heat is transferred from a body with higher temperature to a body with lower temperature by direct contact. Bird tries to get in contact with cages or other birds to lose the excess heat produced. This is not a very good method because cage temperature also increases as environmental temperature increase. Foggers and fans play vital role in this situation.
Body heat lost through convection happens by the removal of heat through temperature gradient with the help of a medium. Blowing cool air over the cages is an effective way to reduce heat, but it aids only when air is moving fast enough to agitate the air surrounding the birds.
When the environmental temperature increases to an extent that birds are not able to maintain the balance of heat exchange, the mucus membrane in nasal cavity acts as a medium which helps to get rid of excess body heat through respiration. That’s why panting is considered as most obvious clinical sign of heat stress in poultry. However, panting is effective only if humidity is not too high. Therefore, hot and humid conditions are more stressful than the hot dry conditions.
Excretion is another method to lose body heat, wherein, birds increase their water intake up to double during hot periods and excrete the heat through urine and wet faeces. There are many other behavioral, physiological, hormonal and molecular changes that occur in birds during heat stress to maintain their body temperature. These signs may be temporary or may persist for longer duration if the problem of heat stress continues.
MANAGEMENT OF HEAT STRESS
There are many strategies farmers follow these days to compensate the increasing environmental temperature. Some of them are summarized below.
1. FARM MANAGEMENTAL APPROACH
Rising temperature and humidity makes feed intake difficult for the birds. Fans are used in this case to maintain air circulation in sheds (Figure-1). During conditions of high temperature and normal humidity, a fine mist of water is sprayed on birds during day time (Figure-2). It must be ensured that the mist is fine because larger water droplets make feed and litter wet which may lead to fly growth. The process needs to be repeated at regular intervals for a short period depending upon the environmental temperature. Farmers often use wet curtains on the sides of sheds so that the air entering sheds becomes cooler (Figure-3). These curtains should be removed when there is no air flow because it will increase the humidity inside sheds and may lead to increased mortality.
Covering water tanks with materials like gunny bags can help in cooling the water (Figure-4). Placing water tank inside the shed can also help to keep the water cool during the day times, but it is possible only for smaller sheds. Use of water sprinklers on the roof (Figure-5) lets the environment inside shed remain cooler. Farmers cover the roof of sheds with the insulating material, depending upon the availability of material (Figure-6). Whitewashing the roof area to reduce the heat absorption and increase reflection is also an option.
Birds don’t prefer to drink warm water during day time. Hence, regular flushing of water pipelines is done so that birds get relatively cooler water. Before flushing, farmers follow the practice of giving electrolytes and other medicines to the birds. The process of regular flushing reduces mortality due to heat stroke and dehydration. Maintaining sufficient housing space for birds in summer decreases the heat stress, as birds need more area for physical activities to lose body heat. Manure should be removed from the sheds because the heat produced from decomposition of manure leads to increase in temperature inside the shed and poor ventilation.
Feed intake during day time leads to production of heat and cause stress. Hence, birds tend to eat in cooler hours of day. Feed should be given either early morning or in late evening. Some farmers also follow the practice of feeding at night. Providing feed and water at proper time is the key to save the birds in such conditions.
2 NUTRITIONAL APPROACH
ENERGY
The heat increment produced by protein metabolism is higher than that of carbohydrates, which in turn is higher than fat. This is the reason to increase dietary energy levels by inclusion of oil and reduce protein level under heat stress situations. Also, emulsifier needs to be added in diet for better digestion and absorption of oil.
PROTEIN
The protein requirement during heat stress is decreased because of reduced production performance. It has been observed that protein synthesis is more affected than breakdown leading to reduced protein formation. The decreased protein synthesis cannot be compensated by giving high dietary protein. The high dietary protein level at high temperature reduces feed efficiency, increases stress and mortality, reduces body weight and production performance of birds. Hence, nutritionists reduce crude protein (CP) percentage in feed to cope up with this situation.
MINERALS
The percentage of mineral mixture is increased in diet depending upon the reduction of feed intake. Normally 15-20% increased mineral diet is provided to birds, so that it helps to reduce production and mortality during heat stress. Chromium has proved to be one of the best nutrient during heat stress conditions to control increased glucose levels in blood, thereby increasing glucose uptake into the cells making it available for various body functions. The action of insulin is strengthened by chromium in vivo and in vitro. There are two theories about the mechanism of action of chromium, i.e., it increases the number of insulin receptors on cells and, it enhances the capacity of already existing insulin receptors to get attached with more glucose molecules to increase its inflow.
VITAMINS
As feed intake reduces during summer, the amount of vitamin premix is increased in the feed to meet the requirement of birds. Oxidation of feed leads to production of free radicals and it is an auto catalytic process. The production of peroxides during the process of oxidation, further interact with air to produce hyperoxides. These compounds are unstable and produce more free radicals generating a chain reaction. This damage the cells disrupting the normal functioning and reduce absorption. Birds can synthesize Vitamin-C in general, but this amount is insufficient under heat stress conditions, high relative humidity, high productive rate and parasitic infestation. The antioxidant activity of Vitamin-C protects against the damage caused by free radicals. Also, Vitamin-E supplementation is beneficial for birds in egg production, associated with an increase in feed intake, egg yolk and albumin solids. Vitamin-E should be added before, after and during the heat stress conditions for better production.
BETAINE
Betaine is an osmoregulator which maintains the movement of water molecules between intercellular and intracellular space. Hence, it helps in maintaining water balance in the body and ultimately protects birds from dehydration especially during heat stress.
CONCLUSION
Heat stress is one of the challenging period for poultry and it has negative impacts on bird’s performance in terms of reduced growth and production, and increased mortality. Many studies have been done in this regard and the focus on management, nutrient interventions, addition of electrolytes, essential oils, prebiotics and probiotics have gained importance. Recently, two innovative approaches have been explored by scientists, that is early-life conditioning (perinatal heat acclimation) and genetic selection of breeds with increased capacity of coping with heat stress conditions (increased heat tolerance). These opportunities, although promising especially for poultry production in tropical regions, still requires more research and development.
