Summer Stress Management in Livestock
Dr. Anjali Kumari
Khalsa College of Veterinary and Animal Sciences
Livestock endure umpteen stresses affecting their general physiology, health, production as well as reproduction. Stress has been defined as any force acting external to the body and able to displace the system from its resting state (Yousef, 1985). Nowadays, environment induced heat stress is a supreme concern because of the progressive warming of the Earth due to climate change and the detrimental impacts posed. In India, the summers are hot and the temperature exceeds 45°C. This is around 18° C above the upper critical temperature of dairy cattle. Besides temperature, humidity also plays a significant role in aggravating heat stress by preventing evaporative cooling through sweating.
Livestock species are homoeothermic animals and have a thermo neutral zone where normal body temperature is maintained, and energy expenditure is minimal. Thermoregulation is the means by which an animal maintains its body temperature and involves a balance between heat gain and heat loss. Heat stress or summer stress manifests when thermoregulatory mechanisms are disrupted due to high temperature, high humidity and low wind speed causing an imbalance between body heat production and its dissipation. Apart from environment, there are other factors which also influence the severity of summer stress in livestock such as,
- Species
- Breed and coat colour and thickness.
- Age
- Sex
- Physiological status
- Nutritional status
- Production level of the animal
- Type and amount of feed consumption
- Availability of cooling mechanisms
Different species of livestock have different heat tolerance levels. Goats and sheep tend to be less susceptible to heat stress than swine and cattle. Wool protects sheep from extreme heat as well as extreme cold. A thick fleece is mostly immune to temperature changes due to its insulating properties. Dark-coloured animals are more susceptible to heat stress than light coloured ones. Indigenous breed tolerate heat better than the exotic and cross bred counterparts. Young animals lack a developed thermoregulatory mechanism leading to low heat tolerance. Similarly geriatric animals have poor tolerance to heat. Females usually handle heat better than males. Fat animals are more predisposed to summer stress. In fact, any animal with a poor nutritional status or compromised immunity will be more susceptible to environmental extremes. High producing animals are profoundly affected and their production level declines along with the growth performance. Consumption of more protein rich concentrate has also lead to increased vulnerability to heat stress owing to the higher heat increment on metabolism.
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How to measure heat stress?
The temperature-humidity index (THI) takes into account both temperature and humidity to estimate the level of heat stress based on environmental conditions. A THI of 72 is mostly considered comfortable for the animals.
| Heat stress level | Temperature humidity index (THI) |
| No heat stress | Less than 68 |
| Mild | 68-71 |
| Mild to moderate | 72-79 |
| Moderate to severe | 80-90 |
| Severe | 90-99 |
Effect of heat stress on livestock
Under heat stress, animals show a number of physiological and behavioural responses varying in intensity and duration. To counter the ill effects and enable the animal to adapt, all physical, physiological and biochemical processes set in involving integrated action of various systems of the body. Body responses indicative of heat stress are
- Reduced appetite due to the release of two adipokines namely leptin and adiponectin which stimulate the hypothalamic axis and lead to reduced feed intake.
- Increased consumption of water
- Increase in body temperature
- Increased evaporative loss though sweating, panting etc.
- Change in blood hormone profile and increased production of stress hormones like cortisol
- Changes in metabolism such as reduced lipid and carbohydrate metabolism leading to negative energy balance.
- Reproductive changes such as delay in oestrus or silent oestrus
These are the changes seen in acute stress whereas chronic exposure to high temperature and humidity leads to
- Reduced growth performance esp. in meat due to reduced protein synthesis and nitrogen retention.
- Reduction in production performance eg: reduced milk yield, reduced egg production
- Development in carcinomas or skin cancer esp. in light coloured breeds
- Infertility and reduced conception in females
- Hormonal changes such as hyperinsulinemia
- Poor semen quality and reduced production in females
- Reduction in immunity and more susceptibility to diseases
Management strategies to counter heat stress in Livestock
Three basic management schemes for reducing the effect of thermal stress have been suggested
- Modification of the physical environment
- Production of resistant breeds by genetic modifications
- Nutritional management
Modification of physical environment
- Provision of shade in the animal shed can greatly reduce the heat load on the animals. Trees provide a cool environment for the animals and also absorb the harmful gases.
- Water consumption is increased by 50% in hot weather. Availability of ad libitum cool, clean and palatable drinking water is crucial at all times in summers.
- The orientation of the long axis of animal sheds should be east west to maintain a cooler environment whereas north south orientation is better suited to temperate areas.
- Open sheds are preferred over closed ones. Closed sheds tend to have higher humidity and higher mean temperature as compared to open ones.
- Animal should have access to a shaded resting area of about 30-40 square feet/ animal. In hot dry areas, providing a shaded area more than the minimum floor space requirement for different species of livestock can be beneficial.
- Height of the shed should be more otherwise it interferes with proper ventilation and heat loss by convection. Minimum roof height for a cattle shed 3-5m and a height less than that is undesirable. For a goat shed, height of the roof should be 3 m at the periphery and 3.5 m at the centre for proper ventilation.
- ‘A’ shaped roof is better than flat roof in hot climate as the shadow cast by one half saves the other from direct solar radiation.
- Thatched hay roof provides the best roofing material in hot dry climate by preventing heat gain by conduction. It may require frequent replacement in humid areas. Asbestos sheet is also a desirable roofing material.
- Roof should be painted white outside and black inside as the reflectivity of white colour is around 75%. The reflectivity of the underneath surface should be less as it determines the quantity of incidental energy from the ground which will be reflected back down to the animals, hence black colour is preferred.
- Painting of side walls white from outside reduces the surface temperature of the walls inside as compared to unpainted walls
- Use of water as cooling agent either directly on animal body as spray, mist etc. at repeated intervals or cooling the shelter micro environment by coolers, fans etc. is a widely accepted practice. Fans and coolers increase the rate of evaporative cooling in animals.
- Water can be used for spraying the floor and roof of shelter periodically or continuously during peak hot hours which lowers the inside temperature.
- Provision of wallowing tanks esp. for buffaloes and pigs helps to counter stress in these animals as they have reduced sweat glands and ineffective heat dissipating mechanisms.
- Water sprinklers can also be used to cool the animals but sprinkling all the time wastes water, adds water to the manure system, and does not help cool the cows. Cows are cooled when the sprinklers are off and the water evaporates. Hence they should be used on and off to allow for evaporative cooling.
- Grass screens on sides of shelter when sprayed with water considerably cool down the air passing through them.
Production of resistant breeds by genetic modifications
A heat-tolerant animal is capable of maintaining homeothermy in hot conditions by balancing thermogenesis and heat dissipation. Breeds originating in warm climates show adaptive superiority to heat stress over the breeds of temperate climate.
When the production system is well established and sound with facilities for adequate feeding, management, heat mitigation, proper parasite and pathogen control, selection for heat tolerance within productive breeds is likely to offer far more advantages than improving local breeds. On the contrary, if the production system lacks those amenities and heat mitigation is not realizable, crossbreeding local and selected breeds and selection for productivity and heat tolerance offers far better opportunities. Antagonism between productivity and heat tolerance and lack of individual heat tolerance records might serve as impediments to this approach and require progressive and effective phenotyping.
When genomic breeding values were used to predict heat tolerant dairy cattle in a stimulated heat wave, those predicted to be heat tolerant showed reduced decline in milk production and reduced increase in core body temperature in comparison to the cows predicted to be heat susceptible. Thus, genomic selection for heat tolerance could prove to be a cost effective tool for selection of livestock for heat tolerance.
Nutritional management
- Avoid overfeeding as it interferes with rumen function.
- In hot areas animals should be fed early morning and evening time i.e. cooler hours of the day with the provision of adequate feeding space and plenty of cool water.
- Modification in ration can help to minimize the drop in milk production, decreasing forage to concentrate ratio, result in more digestible rations. The logic is that less fibre (less bulk) will encourage intake, while more concentrates make the diet energy dense. Do not exceed 55 to 60% concentrate in rations and maintenance of adequate particle size is a must. Attention to fibre quality in hot weather diets is critical, and feeding high-quality fibre is preferred over minimal fibre diets.
- Do not overfeed highly degradable protein during hot weather. Its level of incorporation should be 18% or less. It is because of the heat increment associated with protein metabolism which increases thermal stress in animals reared in hot environment. A higher quality protein can be used instead to satisfy the dietary requirement for production as well as not to increase the heat load in animals.
- Feeding buffers such as sodium bicarbonate and magnesium oxide is also beneficial.
- Hot weather increases the need of certain minerals. Cows sweat just like other mammals, but their sweat contains a large amount of potassium (K). Consequently, K requirements increase during summer. In addition, cows need more Na during summer, and magnesium needs to be boosted when high dietary potassium is fed.
- Supplemental fat can be added in ration to increase energy intake and to make up for the lowered feed intake due to summer stress.
- Supplementing “non-nutritive” additives have the potential to improve performance during hot weather and can have an added advantage for increasing productivity even in environment extremes.
Conclusion
Environment and Climatic factors are crucial factors limiting animal production. Despite many challenges faced and the accentuation of the heat stress by global warming, key management strategies exist which can be employed effectively to alleviate heat stress. Further research is going on in areas of genomic selection and molecular and cellular responses to heat stress which shall enable to improve accuracy of selection of heat tolerant animals. Hence the opportunity to enhance heat tolerance exists and shall be soon exploited in near future.
