Heat stress management in poultry
Introduction
The poultry industry has become one of the important economic growth sectors in India. The sustainable growth in poultry production over the last decades to meet the highly growing need to provide food security calls for a reduction in the negative impact of heat stress, one of the most challenging economic factors. India has highly variable climatic conditions ranging from tropical in the south to temperate in the northern Himalayas. India can be divided into six climatic zones, namely, hot and dry, warm and humid, moderate, cold and cloudy, cold and sunny, and composite (Bansal & Minke, 1988). A hot & humid environment is one of the major stressors in poultry production that results in substantial economic loss. The resultant heat stress comes from the interactions among air temperature, humidity, radiant heat, and air speed.
Heat stress is a condition where chickens are unable to maintain a balance between body heat production and heat loss. In the present scenario, where heat has touched around 53ºC – 55ºC in most of the states in India this summer, the high genetic potential for better production performance and feed conversion efficiency in chickens makes them more susceptible to heat stress. The normal body temperature of the chicken is around 41°C to 42ºC, and the normal range of normothermia zone (comfortable ambient temperature) i.e., 14.5ºC to 25.5ºC (Freeman,1966), 22°C to 28ºC and 18°C to 24ºC (Holik,2009) is established for poultry. Any temperature higher than 25ºC exerts heat stress in birds and an increase in humidity further worsens the problem.
Heat stress causes several physiological changes, which lead to reduced feed intake, feed efficiency, body weight, egg production, increased susceptibility to diseases and mortality. Additionally, it affects meat and egg quality. A meta-analysis including 12 studies based on the impact of heat stress in broiler birds shows that in birds affected with heat stress average weight was reduced by 150 gm, feed intake was reduced by 100 gm, mortality increased by 3.74% and FCR altered from 0.12 to 0.17 points compared to birds reared in thermoneutral zone (Liu et al., 2020).
Clinical Signs of Heat Stress:
Gasping and/or panting, spreading wings, Lethargic and droopy, extremely pale combs and wattles, closed eyes, lying down, drop in egg production, reduced egg size, egg weight, and poor shell quality, increased thirst, decreased appetite, loss in body weight, increased cannibalism, etc.
Effect of Heat Stress:
Water is the main content of the body tissues as well as an egg. Production of eggs requires a lot of fluids, so keeping the flock hydrated and comfortable is important (Boissy et al., 2007). Layers also require large amounts of calcium to avoid soft eggshells. In hot conditions, feed intake is reduced, so calcium consumption is low. When the upper critical temperature (temperature above normothermia) is exceeded in the environment, birds must lose heat actively. Panting is a normal response to heat and is not considered beneficial. But as 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.
To manage heat stress effectively, modifications in the housing and nutritional management of birds are required.
A. Housing Management
1. Housing Design:
To minimize heat-stress-related problems during hot weather, it is always beneficial to insulate poultry-house roofs or ceilings. A properly designed roof overhang helps to reduce the possibility of both direct and indirect sunlight entering a house during hot weather. There are a variety of methods of insulating a poultry house – ceiling, dropped ceiling, rigid board insulation, spray polyurethane insulation, etc. Similarly, reflective roof paints have been shown to reduce roof temperature substantially (from 5°C to 32°C,) thus dramatically reducing heat gain through roof surfaces (Bucklin et al.,1993).
2. Ventilation / Air Movement:
During the summer when the temperature and humidity are high, proper poultry house ventilation is vital to ensure the necessary removal of heat and the continued productivity of the flock. Increasing the amount of air movement over a bird is one of the most effective methods that can increase heat removal from birds during hot weather.
3. Stocking Density:
In high-density stocked barns, the radiant heat between the birds accumulates and the temperature increases. Reducing the bird density in the summer will give more floor space per bird and allow more heat to escape from underneath their bodies and from the litter. It also helps in the easy movement of birds to the nearest drinking point. In poultry houses with less efficient ventilation equipment, it is sound practice to reduce stocking densities in the summer.
B. Nutritional Management
1) Feeding practices:
Any management technique that increases nutrient intake during heat stress will reduce the drop in production efficiency. Three easy ways to increase nutrient consumption are to increase nutrient density, feeding time, and adjust ventilation fans to provide more cooling during the evening. In addition to this, the replacement of energy sources with fats in the diets of broilers suffering from heat stress helps improve feed intake and performance, because of the lower heat increment of fat compared to other energy sources such as carbohydrates or proteins. Balancing of proteins should be done accordingly.
2) Feeding time:
Feeding at the right time of the day is very important to help the birds deal with heat stress. Feed the birds at the time of day when feed consumption is highest. If birds are fed during the cooler part of the day, feed consumption will be higher. Birds should not be fed during the afternoon in periods of hot weather since this will increase the amount of body heat that they must dissipate and thus increase the potential for heat exhaustion. One-third of the daily feed ration should be given in the morning and two-thirds in the late afternoon. Night feeding may be practised.
3) Drinking water :
During heat stress, the bird tries to maintain its body temperature by increased respiration, i.e., evaporation of metabolic water, which may considerably increase the water requirement. So, birds must consume more & cool water during hot, dry weather to prevent dehydration. Overhead tanks and pipe systems must be properly covered to keep the water cool. Providing fresh cool water at noon is very effective for internal cooling of the body and reducing the impact of stress.
Feed Additives & Supplements
Incorporation of certain additives (alone or in combination) in diet may be required to effectively manage or prevent heat stress. Some of the widely used ingredients are listed below:
a) Electrolytes:
Heat stress causes increased loss of several minerals including potassium, sodium, phosphorus, magnesium and zinc. Electrolytes like potassium chloride (0.6%) and sodium or calcium carbonates can be added to correct respiratory alkalosis.
b) Phytogenic:
Phytogenic like amla, and lemon provides a natural antioxidant effect, maintains acid-base equilibrium & corrects respiratory alkalosis. Bael possess properties (antidiarrheal, antimicrobial & ulcer healing, etc.) which improves digestion and reduces loose droppings.
c) Minerals and vitamins:
Incorporation of additional minerals like chromium is also suggested due to their increased excretion during heat stress. Synthesis of vitamin C and absorption of vitamin A and E gets disturbed in heat stress so additional vitamin supplementation may be suggested.
d) Others:
Feed additives like betaine, a popular osmolyte is being extensively used in poultry diets due to their strong osmoregulatory properties. It increases water retention in the body thus reducing dehydration and ultimately mortality. Tri methyl amine in synthetic betaine is highly variable as are dose and efficacy.
A comprehensive solution like Thermogard (an anti-heat stress product manufactured by Regen Biocorps) includes Natural antioxidants, phytogenics, electrolytes, minerals, vitamin C & betaine (found in Bael, Amla, Lemon and other herbs) which perform better at lesser doses.
This type of solution can prove to be highly economical and less cumbersome in providing relief from heat stress to birds.
Conclusion
Management of heat stress requires a lot of attention from housing to management level and with the advancement of applied science and technology, farmers have adopted some of them (if not all). Still, it is a critical challenge in the Indian Sub-Continent, owing to diverse agroclimatic and socio-economic patterns. It is demonstrated that micro-ingredient supplementation plays a critical role in overcoming the challenges of heat stress. As presented in this review, much information has been published on the effects of heat stress on productivity and how to manage heat stress effectively. However, our understanding of basic mechanisms associated with the reported effects, as well as those related to the behaviour and welfare of the birds under heat stress conditions, is scarce.
