RECENT ADVANCES FOR MANAGEMENT OF HEAT STRESS IN DAIRY ANIMALS

RECENT ADVANCES FOR MANAGEMENT OF HEAT STRESS IN DAIRY ANIMALS

K.P. Singh*¹ and Praneeta Singh²

Government Veterinary Hospital, Deoranian, Bareilly,

Department of Animal Husbandry, Uttar Pradesh, India

 

1: Veterinary Officer, Government Veterinary Hospital, Deoranian, Bareilly, Uttar Pradesh Email: drkpsvet@rediffmail.com

2: Assistant Professor, Department of Livestock Products Technology, C.V.A.Sc., GBPUAT, Pantnagar, U.S.Nagar, Uttrakhand Email: vet_praneeta12@rediffmail.com

*Corresponding Author: Veterinary Officer, Government Veterinary Hospital, Deoranian, Bareilly, Uttar Pradesh Email: drkpsvet@rediffmail.com

ABSTRACT

The present article is designed to highlight the impact of heat stress on livestock and to suggest a method for alleviation of heat stress. Heat stress occurs in animals when heat gain from the environment and metabolism surpasses heat loss by radiation, convection, evaporation and conduction.  Enhance of respiration rates and panting during heat stress losses the CO2 resulting in altered blood acid-base chemistry and respiratory alkalosis. Heat stress induces alteration in rumen motility and microflora affects the feed digestibility and rumen fermentation.  Heat stress responsible for the reduction in conception rate due to decrease of luteinizing hormone, estradiol and gonadotrophins which disturb the normal estrous cyclicity and depress follicular development. Parenteral heat stress suppresses embryonic development by the impaired placenta which results in hypoxia and malnutrition.  It also retards the growth, immunity and future milk production of new born calves. Based on above challenges, we attempted to describe the possible causes and impacts of heat stress on the growth, health, digestibility, reproduction and production of animals. We also proposed different strategies for ameliorating heat stress consequences. Genetic development and reproductive measures such as gene selection and embryo transfer are likely more long-term approaches to enhance heat tolerance. The physical modification of the environment such as shades and sprinkle systems are the most common and easily implemented measure to alleviate heat stress. Nutritional management is another key approach for maintain homeostasis and prevent nutritional deficiencies in animals due to heat stress.

Key Words: Heat stress; livestock; production and reproduction performance

 

INTRODUCTION

India is first in milk production in the world production with 198 million tonnes in 2020 from 125.43 million milch cows and buffaloes and the projected milk production increases to 208 million tonnes in 2021. Milk production in India has increased from 146.3 million tonnes in 2014-15 to 198.4 million tonnes in 2019-20.  In India summer temperature goes beyond 45 0C which is 18 0C above the upper critical temperature of dairy cattle when the temperature exceeds 27 0C even with low humidity, the temperature is above the comfort zone for high producing dairy cows. Humidity plays a significant role in heat stress. The most common index of heat (temperature-humidity index) is calculated from the temperature and relative humidity.  At high ambient temperature, the animal wastes their feed energy in panting and sweating nature’s way of colling of animals by evaporation. During summer the milk production is reduced to extent of 50 %.

Heat stress is a form of hyperthermia (elevated body temperature) in which, the physiological systems of the body fail to regulate the body temperature within a normal range. Heat stress in animals is considered to be a violation of animal welfare and rights. Stress is a reflex reaction of animals in harsh environments and causes unfavourable consequences to range from discomfort to death. Climate change is one of the major threats to the survival of various species, ecosystems and the sustainability of livestock production systems across the world, especially in tropical and temperate countries. The thermoneutral zone (TNZ) of dairy animals ranges from 16°C to 25°C, within which they maintained a physiological body temperature of 38.4-39.1°C (Yousef, 1986). However, air temperatures above 20-25°C in the temperate climate and 25-37°C in a tropical climate like in India, enhance heat gain beyond that lost from the body and induces Heat Stress (Kumar et al., 2011).  As a result, body surface temperature, respiration rate (RR), heart rate and rectal temperature (RT) increase which in turn affects feed intake, production and reproductive efficiency of animals. Rectal Temperature >39.0°C and Respiration Rate >60/min indicated cows were undergoing heat stress sufficient to affect milk yield and fertility (Kadokawa et al., 2012). However, the animal being homeotherms can resist heat stress to some extent depending on species, breed and productivity (Collier et al., 2012).

Causes of Heat Stress

Animals may be prone to heat stress due to higher environmental temperature-humidity index (THI), solar radiation and lower wind speed in the summer season. Heat exhaustion may be attributed to several etiologies occurring singly or in multifactorial forms. Environmental factors like:

(1) Recent rainfall and subsequent rise in humidity leading to decreased sweating and breathing for heat dissipation.

(2) A high ongoing minimum and maximum circulating temperature and or high environmental humidity.

(3) A high solar radiation level due to the absence of natural cloud cover may predispose animals to heat stroke.

(4) Extended period (> 5 days) having minimal air circulation leading to consistently harsh environment.

(5) Warm cloudy nights might also increase the risk of heat stroke as the animals dissipate the heat energy during the night.

(6) A sudden shift to adverse climatic conditions might also predispose animals to heat stress.

(7) Husbandry practices and farm layout may sometimes facilitate the onset of heat stress in animals.

Some characteristics of animals might put the individual at a greater risk of heat stroke. Following attributes might be responsible for higher incidence of heat stroke in conjunction with some environmental factors:

(1) Breed: Indigenous breeds (Bos indicus e.g. Sahiwal) are more heat tolerant than exotic breeds (Bos Taurus e.g. Ayrshire).

(2) Genetic variation: Variations attributable to the phenotypes of individual animal breeds.

(3) Coat color and type: Animals with lighter coat color (e.g. cattle vs buffalo) tend to be more tolerant of heat. Animals having coarse hair type as coat may be more prone to heat stress (e.g. sheep vs goat).

(4) Body condition: Obese and emaciated animals tend to be more susceptible to heat stroke.

(5) Age: Animals that are recently weaned or aged animals are more prone to heat stress.

(6) Adaptation: Indigenous animals might adapt to local climatic conditions, provided the temperature change is gradual.

(7) Disease: Animals having ailment of any etiology might not be able to adapt to changes in the weather.

(8) Physiological State: Lactating animals or peri-parturient animals might be at higher risks of getting affected by temperature or humidity rise due to their hormonal profiles.

(9) Vector Density: Some flies and mosquitoes might cause a nuisance to animals, leading to overcrowding/ huddling towards vector-free areas.

 

Symptoms of Heat Stress

Increased breathing, heart and sweating rates may be recorded. Initially, symptoms like increased panting, hypersalivation, gastrointestinal problems (vomiting, diarrhea) are common symptoms of heat stress. Behavioural changes like seeking shade, crowding towards shady areas, orientation avoiding contact with solar radiation, standing in or next to a water source might be observed in animals having the onset of heat stroke. Productivity parameters like milk production, egg production, weight gain, FCR (feed conversion rate), milk and meat composition (decline in protein and fat contents) might show abrupt changes in heat-stressed animals. A drop in routine feed consumption and a sudden increase in water intake might signal the onset of heat stress. The later stages of heat-stressed animals might exhibit symptoms like dry and hyper congested mucosal surfaces, listlessness/lethargy, staggering gait, general weakness and electrolyte loss from the body. Neurological symptoms might include irritability, delusions, hallucinations, seizures and coma.

Effect of heat stress on the health of livestock

Heat stress affects the health of dairy animals by imposing direct or indirect effects on normal physiology, metabolism, hormonal, and immunity system.

Feed Intake

An increase in environmental temperature has a direct negative effect on the appetite centre of the hypothalamus to decreases feed intake (Baile and Forbes, 1974). Feed intake begins to decline at air temperatures of 25-26°C in lactating cows and reduces more rapidly above 30°C in temperate climatic condition and at 40°C it may decline by as much as 40% (Rhoads et al., 2013), 22-35% in dairy goats (Hamazaowi et al., 2012) or 8-10% in buffalo heifers (Hoda and Singh, 2010).

 Rumen Physiology

Increased environmental temperature alters the basic physiological mechanisms of rumen which negatively affects the ruminants with increased risk of metabolic disorders and health problems (Sorajni et al., 2013).

Acid-base balance

Animal under Heat Stress has increased respiration rate and sweating which results in increased body fluid loss that lifts maintenance requirements to control dehydration and blood homeostasis. As respiration rate increases, the expiration of CO2 via the lungs increases. This results in respiratory alkalosis, as blood carbonic acid concentration decreases (Benjamin, 1978). Therefore, the animal needs to compensate for higher blood pH by excreting bicarbonate in the urine to maintain the carbonic acid: bicarbonate ratio (Schneider et al., 1989).

Oxidative stress

Oxidative stress results in to increase in reactive oxygen species (ROS) in different cells and tissues of Heat Stress animals that have negative impacts on normal physiology and body metabolism.

Immune system

The immune system is the major body defense system to protect and cope against environmental stressors. Primary indicators of immunity response include white blood cells (WBCs), red blood cells (RBCs), hemoglobin (Hb), packed cell volume (PCV), glucose and protein concentration in the blood get altered on thermal stress. WBC (leukocytes) count increase by 21-26% (Abdel-Samee, 1987) and RBC count decrease by 12-20% (Habeeb, 1987) in thermally stressed cattle that could be due to thyromolymphatic involution or destruction of erythrocytes.

Effect of heat stress on milk production of livestock

Heat Stress adversely affects milk production and its composition in dairy animals, especially animals of high genetic merit (Wheelock et al., 2010). The stage of lactation is an important factor for the severity of imposed heat stress and animals that were in mid-lactation are mostly heat-sensitive compared to early and late lactating counterparts (Bernabucci et al., 2014). The decline in milk production due to Heat Stress was 14% in early lactation and 35% in mid-lactation (Bernabucci et al., 2014).

A hot and humid environment not only affects milk yield but also affects milk quality. Heat Stress significantly reduces the production of milk, the percentage of milk fat and the percentage of proteins, but that it does not affect the content of lactose in milk. Continual genetic selection for greater performance results in increased Heat Stress sensitivity and a decreasing trend in lactation curve as well as poor milk quality in dairy animals during summer seasons.

Effect of heat stress on reproductive performance of livestock

High air temperature and humidity affect cellular functions by direct alteration and impairment of various tissues or organs of the reproductive system in both the sexes of the animal.

Effect of heat stress on female reproductive performance

Estrous period and follicular growth

Heat Stress reduces the length and intensity of estrus besides increases the incidence of anestrous and silent heat in farm animals (Singh et al., 2013). It increases ACTH and cortisol secretion (Singh et al., 2013) and blocks estradiol-induced sexual behavior (Hein and Allrich, 1992).  Roth et al. (2000) reported that developed follicles damage and become non-viable when the body temperature exceeds 40°C. When female goats are exposed to 36.8°C and 70% relative humidity for 48 h follicular growth to ovulation suppresses, accompanied by decreased LH receptor level and follicular estradiol synthesis activity (Ozawa et al., 2005). Reduced granulosa cells aromatase activity and viability also contributed to poor estradiol secretion. Low estradiol secretion suppresses signs of estrus, gonadotropin surge, ovulation, transport of gametes and ultimately reduced fertilization (Bernabucci et al., 2014). A temperature rises of more than 2°C in unabated buffaloes may cause negative impacts due to low or desynchronized endocrine activities particularly pineal-hypothalamo-hypophyseal-gonadal axis altering respective hormone functions. Low estradiol levels on the day of estrus during the summer period may be the likely factor for the poor expression of heat in Indian buffaloes (Upadhyay et al., 2009).

Fertility

Multifactorial mechanisms are involved in reducing the fertility of dairy animals depending on the magnitude of heat stress. Heat Stress reduces oocyte development by affecting its growth and maturation]. It increases circulating prolactin levels in animal’s results in acyclicity and infertility (Alamer, 2017). Moreover, 80% of estrus may be unnoticeable during summer (Rutledge, 2001) which further reduces fertility. Oocytes of cows exposed to thermal stress lose their competence for fertilization and development to the blastocyst stage (Gendelman and Roth, 2012).  Heat Stress decreases fertility by diminishing the quality of oocytes and embryos through direct and indirect effects.

Embryonic growth and development

Embryonic growth and survival are also affected during thermal stress in dairy animals. Heat Stress causes embryonic death by interfering with protein synthesis (Edward and Hansen, 1996), oxidative cell damage (Welfenson et al., 2000), reducing interferon-tau production for signaling pregnancy recognition (Bilby, 2008) and expression of stress-related genes associated with apoptosis (Feer and Hansen, 2011). Low progesterone secretion limits endometrial function and embryo development (Khodaei-Motlagh et al., 2011). Exposure of lactating cows to Heat Stress on the 1st day after estrus reduced the proportion of embryos that developed to the blastocyst stage on the day 8th after estrus. Further, exposure of post-implantation embryos (early organogenesis) and fetus to heat stress also leads to various teratologies (Welfenson et al., 2000).

Effects on male reproductive performance

Bull is recognizing as more than half of the herd and hence, bull’s fertility is equally or more important for fertilization of oocyte to produce a good, viable and genetically potential conceptus. It is well known that bull testes must be 2-6°C cooler than core body temperature for fertile sperm to be produced. Therefore, increased testicular temperature results from thermal stress could change in seminal and biochemical parameters leads to infertility problems in bulls. Heat stress significantly lowers conception as well as fertility rates per insemination of male and subsequently reduces male’s fitness.

Treatment of Heat Stress

If your animals are showing signs of heat stress the following actions can be taken to cool them down:

• Move them to the shade immediately, preferably somewhere with a breeze. If animals are too stressed to move, pick them up and move them or provide shade where they are.
• Offer plenty of cool clean water but encourage them to drink small amounts often. Spray them with cool water, especially on the legs and feet, or stand them in water. Use sprinklers or hoses for cattle, pigs and horses. Lay wet towels over them.
• Increase air movement around them. This can be done with fans, ventilation, or wind movement.
• Decrease stocking rates to allow animals room to lie down.

Heat stress might cause serious damage to muscles/ internal organs of affected animals, indicating the need for emergency medical care. Animals should be made to move away from direct sunlight to cool areas. The continuous application of cool water therapy is highly indicated to manage a heat stressed animal. The treatment goal is to manage the hyperthermia, provide cardiovascular support and prevent complications associated with hyperthermia. Fluid, electrolyte and Vitamin C therapy could be applied to counteract the ill effects of heat stroke, especially in farm animals. The intravenous (i/v) corticosteroid therapy may help reduce myocardial stress. Anti-microbial therapy may be indicated to reduce chances of secondary infection

Prevention of Heat Stress

Physical modification of the environment

The most common approach to ameliorate Heat Stress is to alter the cow’s environment through the provision of house or shade (along with feed and drinking water), evaporative cooling system with water in the form of fog, mist, or sprinkling with natural or forced air movement, and possibly cooling ponds (Atrian and Shahryar, 2012). Modification of microenvironment to enhance heat dissipation mechanism to relieve heat stress is one of the most important measures to be considered in a hot environment. Cooling ponds and sprinklers can also be used to cool the environment. Dairy cattle allowed access to sprinklers (with and without forced ventilation) have increased milk production, improved reproduction and improved conversion of feed to milk (Wolfenson, 2009). Shading is one of the cheapest ways to modify an animal’s environment during hot weather.

For outdoors animals, the provision of shade (natural or artificial) is one of the simplest and cost-effective methods to minimize heat from solar radiation. Trees are very effective and natural shading materials providing shade to the animals combined with beneficial cooling as moisture evaporates from the leaves. Artificial shades can be used to protect from the effects of solar radiation in absence of natural shade. Various types of roofing materials can be used from metal to synthetic materials for shade structures among which a white galvanized or aluminum roof is considered best.

Nutritional Management

Heat stressed animals are more likely to have lower reproductive and productive performance. Feeding high-quality forages and balanced rations will decrease some of the effects of heat stress and will boost the performance of the animals. Some nutritional management tips to manage heat stress are:

• Provide high quality feeds like total mixed rations
• Increase the frequency of feedings
• Feed during cooler times of the day
• Keep feed fresh as much as possible
• Provide high-quality forage
• Provide adequate fiber
• The use of bypass proteins can enhance the milk yield and protein content.
• Intake of sufficient cool water is probably the most important strategy for animals to undertake during heat stress.
• Nutritional modifications could help animals to maintain homeostasis or prevent nutrient deficiencies that result from heat stress. Rations should be >18% protein on a dry basis as overfeeding requires more energy to excrete excess nitrogen than urea. Optimizing ruminally undegraded protein improves milk yield in hot climates (West, 1999). Increasing dietary fat content enhanced milk production efficiency and yield in the warm season (Linnet et al., 2004). Feed containing low fiber rations during hot weather is logical since heat production is highly associated with the metabolism of acetate compared with propionate. Heat Stress causes oxidative damage which could be minimized through supplementation of vitamins C, E and A and also mineral such as zinc (Mc Dowell, 1989). The use of vitamin C along with electrolyte supplementation was found to relieve the animals of oxidative stress and boosts cell-mediated immunity in buffaloes (Kumar et al., 2010).

Genetic selection

Advances in environmental modifications and nutritional management in part to alleviate the impact of thermal stress on animal performance during the hotter seasons. However, long-term strategies have to be evolved for adaptation to climate change. Differences in thermal tolerance exist between livestock species provide clues or tools to select thermotolerant animals using genetic tools. The identification of heat-tolerant animals within high-producing breeds will be useful only if these animals can maintain high productivity and survivability when exposed to heat stress conditions. Cattle with shorter hair, hair of greater diameter and lighter coat color are more adapted to hot environments than those with longer hair coats and darker colors (Bernabucci et al., 2014) There is heat shock gene related to thermotolerance that was identified and being used as a marker in marker-assisted selection and genome-wide selection to the developed thermotolerant bull that is used in the breeding program. Genetic Selection of animals based on specific molecular genetic markers for heat tolerance will be a boon to alleviate heat stress in cattle and buffaloes by identifying the heat-tolerant animals.

The following other strategies might be helpful in preventing heat stroke cases in dairy, meat and working animals:

• Ensure plenty of fresh and clean water. Place water in a shady area to prevent evaporative loss. In extreme summer situations, the supply of cold water may be necessary.
• Avoid physical exercise on part of the animal during the hottest hours of the day, especially in the summer months. Try to utilize early morning and evening hours in case of draught animals.
• The rain may cause a sharp rise in humidity and it should be kept in mind that humid days can often be just as bad or worse than very hot sunny days.
• If animals are supposed to work for a longer period, ensure rest and drink water at regular intervals.
• Animals that are being transported for sale/ purchase/ slaughter/ competition must have plenty of water available for drinking. Apart from this, it may be suggested to add some vitamin/mineral complexes as animals might voluntarily reduce their feed intakes during long travels.
• Animals kept outdoors must have adequate shade to shelter from the sun during summers.
• At farm levels, ketotic, mastitis or milk fever affected animals must be infused with electrolytes (Intravenously) to prevent the onset of heat stress/ hyperthermia.
• Allow the animals to wander around so they can pick a cool spot during the day.
• Management of heat stress on the part of buildings and infrastructure is very crucial. The principle of construction should allow maximum airflow throughout the farm avoiding direct exposure to solar radiation during day time.
• Devices monitoring the thermal-humidity index, air velocity might be used at the farm level to minimize the chances of heat stroke incidence in animals.

Conclusion

Based on available information’s in the literature, we can conclude that heat stress induced alteration in rumen motility and rumen microflora leads to changes in feed digestibility and production of rumen fermentation. Heat stress reduces the duration and intensity of estrus, decreases follicular development by the regulation of reproductive hormones and suppresses the embryonic development by the impaired placenta, growth and immunity of offspring. The effect of heat stress might be reduced by appropriate farm layout, close monitoring, adopting appropriate preventive measures, adequate management and planning before the onset of environmental temperature and the humidity rise. Heat stress in dairy animals can challenge the reproductive and production potential of the animals. Implementing proper breeding programs, cooling strategies at the farm with better feeding programs can help to minimize some of the negative effects of heat stress. The loss of electrolytes via skin secretions has to be minimized by the improvement of housing and cooling of the animals. Standardization of mineral supplements to control acid-base balance should be considered in an animal under different levels of thermal stress. Genetic development and reproductive measures, physical modification of the environment and nutritional management are the major strategies to ameliorate heat stress.

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