Heat Stress Mitigation through Feeding and Nutritional Interventions in Ruminants

Abstract

The livestock producers have been facing numerous challenges including feeding, management, diseases and environmental conditions. The changes in the environment, particularly heat stress, affect the comfort level that in turn affects production and reproduction. Heat stress in ruminants occurs due to an imbalance between heat dissipation rate and heat exposure from different sources. The external sources include environmental factors such as temperature, humidity, solar radiations, wind speed, wind direction and their indexes while internal sources of heat include metabolism. The high-producing ruminants consume more feed so higher metabolic rates produce more internal heat, which makes these animals prone to heat stress. Different heat stress mitigation strategies have been opted in the world. Nutritional interventions have been suitable and sustainable options. There are a number of nutrients/feed ingredients that may help in the mitigation of heat stress in ruminants. Supplementing ruminant feed with feed additives, minerals, vitamins, antioxidants and balancing the energy and protein level of feed and managing feeding patterns and feeding frequency have been taken as part of solution to provide relief from effects of heat stress. The nutritional interventions as a regular practice help in possible sustainable mitigation of heat stress in ruminants through regulating metabolic heat production level.

1. Introduction

The heat stress in livestock farming is a real factor of economic importance. The ruminants in the herd at below comfort level dispute the resources in nullifying the stress effects instead of presenting real production potential. It is necessary to mitigate the heat stress through management either by adding sprinklers and fans or through nutritional manipulation whichever fits better. The nutritional interventions present better version in term of reducing the metabolic heat production in ruminants leading to reduction in the required rate of heat dissipation and chances of heat stress. The ruminants across the globe are being reared for milk, meat and draft purposes too. Large ruminants have more requirements of food than small ruminants yet, the metabolic heat produced from fermentation and other metabolic processes both in large and small ruminants are as per the ratio of the physical structure and body capacity and the metabolic rate of these ruminants. Small ruminants have not been intensively selected for higher production at the level of genetic selection and breeding in large ruminants. The bulk of milk production in cows creates the need for a higher quantity of better composition-based feed to fulfill the protein and energy requirements of these large ruminants. These diets need higher metabolic rates for efficient conversion into valuable products, the higher metabolic rates produce more internal heat. Higher internal heat production needs more rapid dissipation of heat from the body. The external environmental temperature humidity index may influence the rate of heat dissipation. Any level of imbalance between heat exposure (from internal and external sources) and heat dissipation results in discomfort and heat stress. Heat stress in ruminants will impart negative impacts on production and reproduction, even on animal welfare. Heat stress mitigation strategies become need of hour under such conditions. These mitigation strategies have a wide range of applications starting from genetic selection to practically applying fans and cooling systems at farms. Yet, nutritional interventions for heat stress mitigation have been explored and applied for efficient and sustainable solutions. This chapter has covered various aspects of heat stress in ruminants and its mitigation through nutritional interventions.

2. Heat stress and ruminants

Animal’s health, wellbeing and performance (both productive and reproductive) are affected by the climatic stressors. The ruminants in the world face environmental pressure, yet in tropics due to high environmental temperature as compared to cold regions, these animals bear more stress from the temperature extremes, particularly higher temperature combined with humidity. Heat stress in temperate areas is also a factor affecting ruminants during summer months. However, global animal production is increasing especially in tropical and subtropical areas of the world [1]. The contribution in production is also associated with the consumption of food of ruminant origin in various regions of the world. The severe heat stress causes losses at farms in terms of morbidity, sometimes even mortality, and reduced performance resulting in heavy economic losses. The rising global temperature predictions suggest a rise in the level of heat stress in ruminants in the future.

The ruminant’s level of production and metabolic heat production rates are positively correlated. The genetically improved or selected animals for higher production performance may have chances of higher susceptibility to heat stress from higher ambient temperatures. Other factors include nocturnal temperatures, wind velocity, cloud cover along with air temperature and humidity which collectively or individually affect the animal’s capacity to dissipate heat and maintain homeostasis [2].

However, the selection for high production is desirable for traits of economic importance including milk and meat in ruminants. The climatic condition in which ruminants are being reared is critically important. The ruminants feel heat stress when there is an imbalance between external and internal temperature and disturbance in the heat loss mechanisms due to any possible factor related to living organisms. The temperature humidity index is one of these factors used to assess the thermal comfort zone of dairy cows [3, 4]. There is a thermoneutral zone for each species at which animals generally feel comfortable without presenting any sign of thermal stress. Generally, up to 25°C ambient temperature is considered normal, above which cattle start to feel heat stress. The comfortable THI has been reported to be between 67 and 72 in different studies [5]. The THI above this level will affect the ruminants in all aspects of their welfare, production, reproduction and product quality. The threshold THI for standing cows has been used as 70 and for lying cows as 65 [6]. However, the THI levels have been categorized into severe stress conditions (THI ≥ 90), mild stress (80 ≤ THI ≤ 89) and mild stress (72 ≤ THI ≤ 79) as mentioned in [7]. Furthermore, recent studies present that these above levels underestimate the true level of heat stress and submitted that the heat stress threshold level exists under or up to 68 THI [8].

The heat stress conditions can be chronic or acute. The heat stress is generally presented in terms of indexes prepared from the data of temperature and humidity, termed as temperature humidity index [THI]. The other indexes may include various relatable variables including solar radiations and wind speed. The ruminant’s capacity to cope with these acute or chronic conditions may be assessed through temperature estimations from various processes including respiration, panting, heat production and body site temperatures including skin, rectal, vaginal and cloacal temperature. Skin temperature may be recorded from different body sites. The stress’s indirect indicators can also be measured including production performances. The effects of heat stress are also evident at the molecular level in living beings [9]. These parameters in turn help to estimate the capacity of animal to dissipate the heat. Heat stress also threatens the global ecological balance [10]. Animals try to maintain their body temperature within the range of ±0.5°C [11]. The effects of heat stress on animals are evident in terms of production efficiency, reproductive performance and feed consumption [12]. The animals are offered with different feed types based on composition at different physiological stages and production stages. The heat stress in ruminants is assessed from the important variables including pulse rate, respiration rate, rectal temperature, skin temperature and sweating rate, these physiological processes help in maintaining homeostasis and heat balance in ruminants. Although these variables are important, genetic differences for these variables exist in animals at species and breed levels, besides the effects of physiological stage and age of animals. Heat stress in ruminants imparts varying level of effects from cellular to physiological stages.

3. Heat stress effects

Heat stress is the most expensive factor in livestock rearing in terms of losses [13]. The three more consistent factors predisposing animals to heat stress irrespective of production level include increasing global temperature, frequent heat waves and relocating animals to unfamiliar tropical environments, yet one more factor, extended weather extremes also increase the possibility of occurrence of heat stress. The heat stress affects not only the production and reproduction but also the product quality. Animals with a rapid rate of metabolism are considered more prone to the heat stress. The acute heat stress in ruminants (beef) before slaughtering, for example, can reduce the water holding capacity of meat, softness and pale color due to glycogenolysis [14]. While the chronic heat stress on the other hand reduces the glycogen level of muscles and resulting in dark and firm meat characteristics [15]. These effects present that heat stress at different levels and durations affects product quality in different ways.

The increased animal welfare issues are the first and foremost impact of heat stress in ruminant dairy cows [16] followed by reduced production, slow growth rates and fertility, deteriorating product quality, lowering the market price of the product, inconsistent quality and also the increasing treatment costs [17]. All these factors present the importance of prevention from heat stress and heat stress mitigation strategies. The genetic selection for higher production as compared to selection for heat tolerance and climate adaptability has been prioritized in the recent past, yet the shift to the latter will help in heat stress abatement.

The ruminants (Holstein heifers) are prone to heat stress if thermodynamics changes occur, for example, the heat production contributed from feed fermentation in the rumen increased the metabolic heat and the heat excretion from the body is not balanced due to external environmental factors [18]. Sweating is the main tool in ruminants for evaporative heat loss, yet it is more in bos indicus cattle due to short and sleek hair coats and higher blood flow to the skin as compared to bos taurus cattle. Likewise, in sheep, evaporative heat loss is a major factor for cooling the animal, yet fleece length affects the rate of evaporation and below 40 mm is considered better for thermoregulation.

Catecholamines (adrenaline and noradrenaline) mediate the heat stress response along with the autonomous nervous system in animals. The responses mediated include an increase in respiration rate, pulse rate, body temperature and also the increased blood flow toward skin instead of viscera to contribute to heat loss from the body. The catecholamines act on the β2-receptors to inhibit glucogenesis and activate the glycogenolysis through a series of reactions in muscles mediated through cyclic adenosine monophosphate, resulting in inhibition of glycogen synthase and activation of glycogen phosphorylase [19]. The heat stress also increases the concentration of plasma glucocorticoids by activation of hypothalamic-pituitary-adrenal axis. These glucocorticoids increase vasodilation, and proteolysis and alter lipid metabolism, vasodilation facilitates the heat loss mechanism [20].

The decreased feed intake is another prominent effect of heat stress. The reduction in feed conversion ratio has also been observed in steers [21]. The ruminants counter the thermal stress through reduction in feed intake to keep the metabolic heat at lowest during high ambient temperature and humidity, as a natural response [22, 23]. The feed intake reduction only presents about 50% of the effects of heat stress, rest are explained through the changes in hormonal level of animals. The water intake requirements are increased twofolds during heat stress in animals. This increased water intake is important to balance the evaporative loss in ruminants (sheep) through sweating and panting [24]. It also directly increases the cooling of the rumen, so the water shortage or scarcity or lack of access to water during heat stress may trigger more harmful responses and also the dehydration in animals.

Heat stress may also induce oxidative stress through increasing reactive oxygen species and decreasing natural antioxidants. Lipid and protein oxidation, free radical mediated reactions, cytotoxicity occur due to tissue acidosis and oxidative stress which in turn result in animal health impairment, production losses and decrease in product quality [25]. Oxidative stress can even damage the cellular contents and biological molecules including carbohydrates, cholesterol, lipids, proteins and genetic material such as DNA and RNA [25]. The heat shock proteins (HSPs) also act as indicators of heat stress in ruminants. The expression of heat shock protein increases due to heat stress [26]. The heat shock proteins are important elements of living cells helping in cell repair and maintenance and cytoskeleton modulation [27]. The increased level of HSPs alters the tenderness of beef meat [28] and HSPs also play a role in proteolysis inhibition and meat toughness in sheep [29].

The thyroid gland is associated with thermogenesis in mammals. The reduced plasma concentration of triiodothyronine (T3) and tetraiodothyronine (T4) due to heat stress reduces the metabolic heat production preventing extra heat load as an adaptive mechanism and along with more actions including energy requirement reduction and reduction in lipolysis [30]. So, the reduced plasma concentration of thyroid hormone acts as an indicator of thermal tolerance in animals. The concentration of insulin increases from basal level and insulin sensitivity also increases as a result of heat stress in animals. As lipid metabolism generates more metabolic heat as compared to carbohydrates and proteins [31], the reduction in the metabolism of adipose tissues is considered heat tolerance evolutionary mechanism. The reduction in metabolism of adipose tissue is also attributed to the reduced thyroid hormones (see Figure 1).

Figure 1.

Heat stress effects in cattle and its modulations.

Among ruminants, the feedlot cattle are more prone to heat stress [17]. There may be various factors including but not limited to rough radiant surfaces, high energy diets and less volunteering in seeking water, shade or ventilation [1]. The feed type of animals also contributes to susceptibility of heat stress, for example, increase in body temperature of cattle finished at grain is more than those fed on grasses [32]. The rapidly fermentable grains feeding to wethers (castrated male sheep) predispose them to heat stress as compared to slow-fermenting feed [33, 34]. The heat stress affects the weight gain in ruminants, two goat breeds presented different final weights after exposure to heat stress [35]. Heat stress contributes to altering the physiological mechanisms of the rumen, which may lead to metabolic disorders and related health problems [36, 37]. Changes in the metabolic mechanism in the rumen may alter the microbial population and pH, decrease rumination and rumen motility and decrease feed intake and salivary production [37]. To better understand the effects of heat stress on metabolism, it is necessary to understand the metabolism in animals at comfort level. The metabolic adaptation to heat stress in dairy cows is attributed to the endocrine system and signaling proteins associated with these systems that may alter the endocrine status and target tissue responses if there is a heat stress challenge [31]. The effects of heat stress on feeding behavior are evident in dairy animals [38]. The exposure of cattle to heat stress during early lactation has negative impacts on the biochemical indices in blood, fertility and production [39].

4. Heat stress mitigation

The heat stress mitigation strategies are based on the basic reasons of heat stress. The thermal stress may be due to factors, alone or in combination, including high THI values, higher internal heat production (due to varying feed ingredients), feeding patterns, water intake frequency, animal’s physiology, production level and farm management practices. Accordingly, mitigation strategies are devised to be implemented for efficient solutions. A basic understanding of the source of heat stress is of vital importance. The cooling aids or systems can be one of the approaches that can be applied on different production levels. The permanent solution for avoiding negative effects of heat stress, can be the genetic selection for thermo-tolerance, yet it may take time to bring in the final results and altering the genetic makeup of billions of animals particularly ruminants, however, once genetic changes have been achieved, there will be the availability of base population for future breeding and production of thermo-tolerant ruminants. As in the future, the effect of thermal stress seems to increase as a result of increasing global warming. It can be done by identification of thermotolerant animals from among the high-producing cows [40] so as to not reducing production when selecting for thermotolerance. However, it is established that selection for higher production has increased the chances of heat stress in highly producing animals. The high-yielding dairy cows, due to high energy requirement in transition period, undergo various physiological and metabolic adaptations [41]. These changes involve the increased lipolysis of adipose tissues and release of non-esterified fatty acids during third week before and up to 5th week after calving [42]. These are used for β-oxidation and in tricarboxylic acid cycle for generating energy, resulting in hepatic gluconeogenesis. However, in ruminants (dairy cattle), non-esterified fatty acids could be incompletely oxidized or re-esterified to triglycerides, resulting in a negative energy balance [43]. Different phenotypic markers, phenotypes and omic techniques may be implicated to identify the heat tolerant animals [44]. The anatomical and physiological differences of animals across the species or within species, including coat colors in animals, sweat gland density and sleek hair coat, can also be used for the selection of animals with thermotolerance or adaptability. Nutritional interventions are also of prime importance in animals for the heat stress mitigation. These interventions include different versions including balancing of energy in animal feed intake, changing the composition of the feed, energy and fiber contents of the feed offered, feeding time and feeding frequency, supplementation of vitamins and minerals and addition of yeasts and other feed additives to improve the provision of nutrients and at the same time to prevent the animal from internal metabolic impacts that possibly may contribute to the thermal stress. Such interventions lonely or in combination with managemental strategies including cooling aids and sprinkler systems may help in alleviating the negative effects of heat stress in feedlot cattle [45]. The interest in nutritional interventions to ameliorate the heat stress effects on production and health is rising [46]. The rumen degradable proteins and ruminal undegradable protein reduction in feed results in increased use of amino acid in synthesis of milk and also helps limit the catabolism in warm climate exposed cattle [47].

Supplementation of Ionophores in dairy cattle during heat stress improved the feed intake and feed efficiency [48]. Ionophores and monensin feeding also have beneficial effects in cattle experiencing heat stress [48]. The dietary cation-anion differences and bicarbonates by their buffering nature also contribute to relief from the heat stress in animals. Supplementing dietary yeast improves the condition of heat-stressed animals. Cows in summers fed with a diet containing ruminal by-pass fats and good crude protein contents maintain the production level and also have cooling effects by less metabolic heat production. Lactating dairy cows fed on a palm oil-supplemented diet have better dry matter intake and fewer signs of heat stress [49]. The heat stress has affected the nitrogen utilization, rumen function and digestibility in buffalo [50]. The negative energy balance has been improved in animals during heat stress when fed on a diet containing conjugated linoleic acids, yet it reduces the milk fat percentage. Supplementation of lipoic acid could be beneficial for alleviating heat stress in animals due to its effects on metabolism. The heat stress may also disrupt the metabolism along with effects on other physiological parameters.

The minerals, including Mn, Mo, P, Se and Zn, in animal feed improve the metabolism and health of animals. The supplementation of Zn in ruminants (dairy cows) during heat stress and mist sprays help in reducing effects of heat stress and maintaining the production level [51]. The dietary supplementation of Zn improves the intestinal health and has potent antioxidant effects. The dietary antioxidants mitigate the heat stress.

Vitamin A, which comprises retinol, retinal, and retinoic acid, presents a vital role in body functions including vision, growth and reproduction and possesses powerful antioxidant property [52]. This vitamin helps in mitigating the oxidative stress created by heat exposure. The supplementation of vitamin D₃ and Ca reduced hyperthermia in heat-stressed Holstein cows and also reduced markers of leaky gut and inflammation [53]. Vitamins including ascorbic acid, Vitamin B, Vitamin E, protected Niacin and Nicotinic acid are beneficial in thermotolerance in Holstein cows [54]. Niacin supplementation improves the metabolism and reduces the thermal stress effects in lactating dairy cows. Vitamin B has anti-heat stress effects and anti-inflammatory effects. The nicotinic acid has protective effects on intestinal integrity against negative impacts of heat stress [55]. The dietary niacin improves the immune function of beef cattle during heat stress and provides relief from heat stress [56]. The supplementation of Vitamin C and niacin, either in combination or alone, alleviates the effects of heat stress and improves the feed conversion efficiency, digestibility, rumen fermentation, feed intake and milk yield in lactating Friesian cows [57].

The supplementation of dietary betaine helps in mitigating the negative effects incurred by the heat stress in Karan Fries heifers [58]. Dietary betaine and chromium supplementation improves the betterment of heat-stressed dairy cows and Girolando cows, improving energy metabolism and production [59, 60, 61]. Chromium (Cr) has potential antioxidant effects and prevents lipid peroxidation in heat-stressed animals as heat stress primarily may cause lipid peroxidation due to oxidative stress [60]. The Cr also improves the humeral and cellular immunity in buffalo calves under heat stress. The Cr also improves the function of insulin in insulin-sensitive tissues, resulting in better growth, improved feed intake, fertility parameters, carcass quality and immune functions [60]. Chromium propionate has potential heat stress mitigating effects in animals by reducing metabolic heat production. The supplementation of Chromium propionate in mid-lactation dairy cows increases the milk yield by improving dry matter intake, antioxidant status and by decreasing rectal temperature and respiration rate in Holstein cows [62]. Propionate supplementation also yields better results as its conversion into glucose is around 30%. Chromium propionate (Cr-Pro) and calcium propionate (Ca-Pro) supplementation provide relief from heat stress in Holstein dairy cows [63].

The dietary intake of substrates as an evolutionary mechanism of adaptation produces less insulin during summer in ruminants (Holstein) as compared to spring and winter [64]. The reduction in RDP and RUP resulted in lowering insulin levels in the body which has an association with fatty acid mobilization and utilization in lactating dairy cows [47]. Nutritional interventions are recommended to effectively mitigate the heat stress effects [51]. The diet containing higher dietary fiber contents was better taken by the heat stressed Holstein calves, as compared to particle size, resulting in increased intake of dietary fibers that increases the longer lying behavior, digestion, feed intake, growth and final body weight [65]. The effects of heat stress on rumen microbiota were observed [66]. The dietary curcumin nano-micelles benefited the rumen microbiota and increased the rumen function in heat-stress animals [67]. The exploration of further products and byproducts of plant origin, microbial origin and mineral and vitamins may contribute more to mitigating the unwanted effects of heat stress.

5. Conclusion

The ruminants reared in grazing or feedlot systems have been exposed to environmental conditions. The environmental conditions in temperate and tropical climates are different. During heat stress, animal welfare, production, reproduction and behavior are affected. The mitigation of the condition through nutritional interventions becomes the need of time. The nutrients in the rumen have different fates and different metabolic activities including regulation of metabolism, enriching ruminal microbiota, improving rumen function, maintenance of pH, improving digestibility and reducing metabolic heat production. Based on scientific studies, following conclusions were drawn:

1. The supplementation of vitamins, minerals, fatty acids, yeasts, products of microbial origin, probiotics and plant extracts helps in regulating the internal heat production level.

2. The nutritional interventions bring changes in cellular response in terms of heat shock proteins, favoring suitable microbiota in the rumen.

3. The nutritional interventions improve the physiological condition in ruminants including respiration rate, lowering body temperature and reducing the need for sweating.

These responses create comfort in ruminants and help in maintaining production too. Thus, the heat stress mitigation through nutritional intervention needs to be explored more at molecular, cellular and organism levels in ruminants to improve welfare, comfort and productivity and to reduce losses. The future prediction of global warming creates the need to pay more attention to explore and provide alternative sustainable nutritional solutions for heat stress mitigation.

Conflict of interest

The authors declare no conflict of interest.