Modern dairy cattle back their origin to temperate climates. Intensive selection for high milk yield has made them less resilient to changing climatic conditions. Environmentally-induced hyperthermia in dairy cows leads to significant production losses. Noticeable sequels of heat stress (HS) are reduced feed intake and a variety of metabolic reshuffles, ending up in production and health losses. HS is an evident problem throughout the world [1]. HS negatively impacts a variety of performance parameters in dairy cattle including milk yield, growth, and reproduction and therefore possesses a significant financial burden of ~$900 million/year for the dairy sector in the U.S.A. alone [2]. HS adversely affects milk production and its composition in the dairy animals, especially in the animals of high genetic merit ^[3][4][3,4].

Investigating the biological mechanisms of HS behind lower animal production is crucial in devising mitigation strategies to ameliorate production declines. This prerequisite knowledge will help to suggest genetic, management, nutrition, and allied preventive mitigation strategies to secure and enhance dairy production. Over the last many years, advancement in cooling systems has lessened the production losses in hot weather conditions [5]. Modern dairy cattle are high milk yielders and energy status is the single most important factor in this context, a lot of studies have been conducted on understanding of the energetic metabolism and its repercussions under the influence of HS. However, a few advances have focused approaches to improve physiological and metabolic mechanisms in order to increase production of the heat-stressed dairy cattle. 

2. Heat Stress Assessment and Principals of Mitigation

This title provides up to date pre-requisite information of HS to the readership, global warming is expected to increase mean temperatures. When animal fails to lose radiant heat, which is mainly through convection, it suffers from the HS [6]. When ambient temperature exceeds 25 °C, cattle experiences HS [7]. Traditionally, temperature–humidity index (THI) is used to assess HS in dairy production ^[8][9][10][8,9,10]. THI calculations are based on dry (Tdb in °C) and wet bulb temperatures (Twb in °C)/relative humidity (RH in %). Different formulas are given as: THI =0.72(Tdb+Twb)+40.6 [11] and THI = (1.8×T+32)–(0.55–0.0055×RH)×(1.8×T–26) [12]. THI has been successfully employed to asses HS in the dairy cattle at various conditions of indoor or outdoor [13] different climate and production systems ^[14][15][14,15]. The common consensus about THI scale is the upper threshold THI, upon which the cow starts to experiences signs of hyperthermia ^[7][10][7,10]. This threshold has been reported variable upon different systems, generally started from 67 [16] and 72 ^[5][17][5,17], THI above these limits initiated hyperthermia derived discomfort [15], altered physiology [11], decreased feed intake [18], and decline in milk yield and composition [19]. Besides this, THI threshold may act variably for different physiological parameter, being lower for respiration rate and higher for rectal temperature [17]. Careful THI measurement [15] combined with physiological parameters of HS assessment [20] can be useful to predict real heat load on cattle. For THI measurements climate conditions should be obtained at cow level (ambient) to evaluate the heat stress conditions that dairy cows are actually exposed to [21]. There is a need for re-ranking across the THI scale according to farming systems and different climatic conditions [14]. Therefore, surroundings microclimatic conditions along with allied physiological parameters should be taken in account to accurately predict HS in cows [15]. Development of automatic monitoring techniques makes it possible to combine THI with other physiological indexes (i.e., body temperature and activity), helping to comprehensively evaluate HS in dairy cows [20]. Individual animal temperature monitoring is of vital importance in this context. Rectal temperature [22], deep body temperature measurements [23] like, vaginal temperature [24], skin implanted thermo-loggers [25], rumen temperature [26], infrared thermography [27], and milk temperature [18] are various methods used so far for thermal monitoring of cows. Likewise, the individual cow monitoring of panting score, studies have advised that wind speed and solar radiation should also be taken in account while assessing HS through THI scale [28]. These factors are well-known to have a significant influence on the magnitude of HS. However, each aforementioned method has some advantages and drawbacks, for example, vaginal temperature is accurate, milk temperature monitoring is easy [18], and infrared thermography give best results on forehead and eye area [27]. Rapidly evolving temperature and activity monitoring technology produce big data, which can be affectively used for modeling to predict accurate HS ^[29][30][31][29,30,31] and at the same time thermo-tolerant animals can be identified for possible future breeding. Identification of thermo-tolerant cows based on the physiological exhibits [32], defining their phenotypes together with the integration of molecular techniques [33] can help to achieve thermo-tolerance breeding in the cattle. There is a necessity of boosting convective heat shedding by cows through structural engineering of barns and forced air flow because heat loss decreases with high incoming air temperatures. Evaporative cooling is the alternative, requiring partial enclosing of the barn; however, a limitation factor could be the humidity in ambient air. A better alternative approach is forced ventilation coupled with surface soaking of animals. Animals wetting can be achieved through sprinklers, foggers, and misters according to a situation which varies. The forced evaporative cooling may be useful in various parts of the dairy, the holding area for milking, the feeding lane, and the rest area [6]. These approaches vary according to barn structure, animal density, farming practices, climatic conditions, and technological adaptations. Consultation with the relevant experts is necessary for the farmers in this context, so that suitable solutions are due provided.

3. Physiological and Behavioral Modifications of the Cattle

Homoeothermic animals (depending on their physiological state) have a thermo-neutral zone where energy expenditure to maintain the normal body temperature is minimal, constant, and independent of environmental temperature [34]. Initial responses to the HS are considered homeostatic mechanisms and include increased water intake, sweating and respiration rates, reduced heart rate and feed intake [35]. If exposure to the thermal load is increased, heat acclimation (if survivable) is achieved via processes of acclamatory homeostasis [35]. However, this acclamation may not remain homeostatic if HS is prolonged and thereby the animal will initiate homeorhetic mechanisms to dissipate incremental heat load and acclimatize to stress conditions [36]. Increased heat dissipation (primarily through evaporative heat loss), reduced feed intake and milk yield and increased water intake are the typical signs of homeostatic responses in response to the HS [37]. When the temperature of the hypothalamus is above thermo-neutral zone, the heat loss mechanisms, such as vasodilatation and sweating are activated [36]. Heat-stressed cows consume less feed and consequently ruminate less, and this results in decreased buffering agents (ruminating is the primary stimulant of saliva production) entering the rumen. In addition, because of the redistribution of blood flow to the periphery (in an attempt to enhance heat dissipation) and subsequent reduction in blood delivery to the gastrointestinal track, thus disturbing the digestion process. Cows in thermal neutral conditions typically consume 12 to 15 meals per day but decrease eating frequency to 3 to 5 meals per day during heat stress [11]. The decreased frequency is accompanied by larger meals, which could have gut health consequences. High body temperature due to HS evokes a series of physiological responses. Excessive flow of energy (in the form of unabated heat) into the body, in addition to energy depletion required for lactation and growth [38], can lead to reduced reproductive efficiency [39], deteriorated living conditions, reduced welfare, and in extreme cases death [28], unless the animal can activate various adaptive mechanisms to increase the external net energy flow. Documented physiological coping strategies used by dairy cows include increased respiration rate; panting; and sweating; decreased feed intake; reduced milk yield, growth, and reproductive performance. Cattle modify their feeding and drinking behavior; take feed in cooler hours, and frequent water intake. When ambient temperature increases, cattle significantly increase heat production [40], therefore enhanced energy expenditure during HS is believed to originate from high physical adaptive activities like panting and sweating [41]. HS maintenance costs in lactating dairy cattle are estimated to increase by as much as 25% to 30% during heat stress ^[10][42][10,42]. However, due to a variety of acclamatory responses and depending on the severity and intensity of the HS, it will vary significantly. An increase in environmental temperature has a direct negative effect on the appetite center of the hypothalamus to decrease feed intake [10]. Chronic hyperthermia leading to severe or prolonged inappetence is also reported [11]. In summary, physiological responses and coping strategies under the influence of HS are surely posing extreme burden on dairy cows, which are mainly initiated and coordinated through autonomous nervous system [36]. High milk yield burden coupled with deteriorating livability principally needs adequate cooling and better feeding practices with high energy density so that a cow can withstand HS, successfully dissipate it, and at the same time maintain milk yield demands.

4. Negative Energy Balance (NEBAL) Typical to Heat Stress

Reduced feed intake caused by HS is thought to be a primary response towards decreased milk yield ^[41][43][41,54]. However, now it is known that reduced feed intake is merely responsible for about 35% of the HS induced drop in milk production ^[44][69]. Rather major effects of HS consequences are intake-independent changes in nutrients partitioning. Another study held 50% of HS induced feed intake reduction being responsible for lower lactation yield. Additional reduction causes are born by intake independent changes in post-absorptive glucose and lipid metabolism [4]. This brief introduction makes it clear that indeed decreased intake is responsible for lower production, however metabolic alterations during HS drive the additional stress and decreased milk yield. Consequences of this metabolism shift are extended beyond the production; reproduction and health are also affected. Below we will discuss the individual components of this context.

4.1. Insulin and Glucose Axis of Heat Stress

The variety of post-absorptive metabolic changes occurring in HS cow, notable ones are high insulin activity, failure of adipose tissue mobilization and thus failure to enlist glucose sparing mechanisms ^[4][44][4,69]. These changes lead to production losses to an extent which is more than that for cows with poor nutrition status. These causes and effects are also shown to be similar for growth parameters, with major part be explained by the HS-induced reduction of feed intake ^[45][70]. HS cows are shown to have increased basal insulin concentration and high insulin response to a glucose tolerance test ^[4][42][4,42]. Exact HS specific insulin increase has been long debated, but it appears to be adaptive and protective in nature towards stressors ^[46][55]. Contrastingly a review summarized that prolactin together with other involvements; support insulinemeia typical of HS, this high insulin concentration can also be contributed towards many other things, like, lipopolysaccharides (LPS), and high concentration of intracellular Ca+ ^[46][55]. HS may alter glucose uptake in many ways, it can be tissue-specific, how much part insulin-independent glucose disposal accounts during HS; needs to be investigated thoroughly, as insulin-independent glucose transporters (GLUTs) also tend to increase during in-vitro HS ^[47][71]. Conversely, when heat-stressed and cooled cows were compared in a study, cooled cows have low glucose levels, low insulin response and increased fats metabolism with relatively high milk yield ^[48][72].

4.2. Insulin and Lipids Metabolism Axis of Heat Stress

Early lactating cows and those with underfed proper diet tends to mobilize fat reserves to keep up with high energy demand of lactation ^[49][73]. Decreased nutrient intake being an utmost indicator of the HS; is generally associated with NEBAL [4], bodyweight loss ^[44][69], and elevated NEFA levels ^[50][51][74,75]. HS causes a marked increase in circulating cortisol, norepinephrine and epinephrine levels ^[52][45], catabolic signals that normally stimulate lipolysis and adipose mobilization. But this is not the case in HS dairy cows; instead, NEFA levels go down significantly ^[4][46][4,55]. An experimental study showed that HS cattle have blunted NEFA response towards epinephrine challenge ^[44][69]. Circulating NEFA and derived ketone bodies helps overcome NABAL, and this lipolysis response spares glucose from primary homeorhetic responses and instead is directed towards the lactation support ^[53][54][64,68]. There is high insulin activity during HS as described above. Insulin is also a potent anti-lipolytic hormone ^[55][62] and may explain why heat-stressed animals do not mobilize adipose tissue triglycerides. Instead of NEFA mobilization, HS has been shown to increase lipoprotein lipase, suggesting anabolism of triglycerides. Limiting adipose tissue mobilization is the key step by which heat stressed animals are prevented from employing glucose-sparing mechanisms normally enlisted to maintain milk or skeletal muscle synthesis during periods of temporary malnutrition. The lack of available NEFA to systemic tissues for oxidative purposes is coupled with the decrease of volatile fatty acids (VFAs) availability; leaving glucose and amino acids (AA) as the available oxidative substrates. Therefore, glucose is consumed as main oxidative fuel in the HS animals ^[56][76]. Now it is clear that the HS poses a sever burden on the energetic metabolic balance of the dairy cow. Glucose as preferential fuel could debilitate animal, compromise homeostatic physiological responses of heat abatement and decrease production and reproduction potential of the cows. Therefore mitigation strategies, whether physical or nutritional, should be focused on improving energetic metabolism through maximizing glucose rescue.

4.3. Protein Metabolism in Heat Stress

Amino acids of blood are known to synthesize the major components of milk protein in bovine mammary glands. Many studies indicate marked changes in circulating amino acids under catabolic conditions ^[57][77] and HS ^[58][78], because of the insulin resistance in peripheral tissues and use of AA in gluconeogenesis ^[59][79]. HS reduces milk protein content, and changes the AA profile of dairy cows, suggesting that more AA are required for maintenance (immune response and gluconeogenesis) but not for milk protein synthesis under HS ^[58][78]. Highly significant variation of Hb, PCV, plasma glucose, total protein, and albumin has been reported for the different temperature exposure ^[60][80]. Muscle anabolic and nucleic acids synthesis is also shown to be severely hampered by HS ^[61][81].

In addition to adipose tissue, skeletal muscle is also mobilized during periods of inadequate nutrient intake (in thermal neutral conditions) to support lactation. Heat-stressed cows ^[62][82] have increased plasma urea nitrogen levels. A better circulating indicator of muscle catabolism is either 3-methyl-histidine or creatine, both of which are increased in heat-stressed lactating cows ^[63][83]. HS has been reported to interfere with nitrogen metabolism and cause nitrogenous repartition in dairy cow and decreases milk protein content while increasing milk urea concentration ^[64][84]. Additional evidence suggests that HS alters protein metabolism and milk protein levels decreased in heat-stressed cows ^[44][62][69,82].

The increase in skeletal muscle protein catabolism is interesting as the role of insulin is to promote protein anabolism ^[65][85]. This change in function can be attributed to various modifications in energetic metabolisms at multi-levels. HS can increase cell membrane permeability causing Ca+ leakage that could increase protein sensitivity to HS ^[66][86]. The concentration of blood alanine, glucose, aspartate, and glycine, associated with gluconeogenesis, are shown to be significantly increased under HS ^[58][78]. Conclusively we can say that skeletal muscle catabolism may be a strategy to support gluconeogenesis ^[67][87], rather than for oxidation purposes because the efficiency of capturing ATP from amino acid oxidation is low. Oxidative stress ^[58][78], immune response and unique gluconeogenesis ^[46][55] depleted a large number of amino acids, thus decreased the availability of amino acids for milk protein synthesis in our study. Therefore, high extra-mammary protein catabolism and amino acid consumption during HS have been accounted for low milk proteins and milk yield ^[68][88].

5. Effects on Milk Production

Continual genetic selection for greater performance results in increased HS sensitivity and a resulted in a decreasing trend in the lactation curve as well as poor milk quality in dairy animals during summer seasons. HS adversely affects milk production and its composition in dairy animals, especially animals of high genetic merit ^[3][4][3,4]. The components of milk are strongly affected by HS ^[69][89]. The greater number of somatic cells counted in milk during summer also shows that the hyperthermic environment severely affects the quality of the milk ^[70][90]. Furthermore, it is found that a hyperthermal environment could also reduce the milk protein content via the reduction of casein concentration ^[71][91]. Highly producing cows have been shown to utilize majority of its glucose in mammary tissues for milk production ^[72][92]. Due to high energy demand under HS, existing energy intake would not be enough to cover the daily requirements for the milk production. Total average milk production per cow was significantly (p < 0.05) higher in the spring period (42.74 ± 4.98 L) compared to summer (39.60 ± 5.09 L) ^[73][93]. HS above critical threshold decreases DMI by 9.6% and milk production by 21%, together with lower milk fat and milk protein in the summer season ^[3][74][3,94]. Reduced nutrient intake (indirect effects of heat) accounted for only about 35% of the heat stress-induced decrease in milk synthesis ^[44][69]. Additionally, the analysis of milk protein fractions also showed a reduction in percentages of casein, lactalbumin, immunoglobulin G (IgG) and IgA; 80% of these changes were associated with loss of productivity and 20% with health issues which might be due to disruption of internal homeostasis mechanism ^[75][95]. Similarly, lipid composition of milk is also disturbed during the HS ^[76][96]. Milk levels before HS, lactation stages, and parity are positively related to the extent of milk yield decline during HS. Studies have shown 35% decline for mid-lactation ^[44][69] and 14% for early lactation cows ^[74][94]. Besides milk yield and composition HS increased the somatic cell count of the milk ^[77][97] through initiation of immune response in the mammary tissue ^[78][98]. HS tend to activate immune system, which is energy intensive phenomena; therefore, the glucose consumption in dairy cow is increased ^[79][99]. Milk yield, composition, and quality are affected by HS. Failure to rescue milk yield due to shifts in energy metabolism; protein catabolism; alterations in lipid metabolism due to endocrine alterations; and immune response due to oxidative stress and inflammation are the major factors in this context.