Dairy Cow Thermal State

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This entry is adapted from the peer-reviewed paper 10.3390/vetsci9080416

Heat stress in cattle is broadly defined as a physiological condition in which body temperature rises, and the animals are no longer able to adequately dissipate body heat to maintain thermal equilibrium due to environmental factors. Dairy cattle are particularly sensitive to heat stress because of the higher metabolic rate needed for milk production. Due to global warming and the expected growth of milk production in warmer regions, an increase in the occurrence of heat stress can only be avoided with the use of environmental control systems.

dairy cow heat stress cow thermal state

1. Introduction

Heat stress in cattle is broadly defined as a physiological condition during which the animal is no longer able to regulate its internal temperature within a comfortable level because of an increase in body heat storage ^[1]. This physiological condition can lead to impaired health and immunity ^[2]^[3], deteriorated living conditions and even mortality during extreme events ^[4]^[5], especially in younger cattle ^[6]^[7]. Due to global warming, the number of heat stress events is expected to increase (2000 h in Central Europe and Mediterranean regions ^[8]) causing vast economic losses (e.g., the dairy industry loses approximately USD 900 million/year only in the U. S. ^[3]). Furthermore, the conventional genetic selection of dairy breeds to increase dry matter intake (DMI) and milk yield ^[9]^[10] resulted in cows with elevated internal heat loads (due to high milk production) that may lead to heat stress much earlier than their lower-producing counterparts ^[11]^[12]^[13]. For instance, the loss of milk production due to heat stress is expected to increase at a rate of 174 ± 7 kg/cow/decade in the 21st century ^[14]. Thus, the impact of this syndrome on animals cannot be neglected in any way as the future dairy industry faces the difficult challenge of increasing milk production in warmer environmental conditions ^[8], while preserving the welfare of dairy animals.

Several strategies are reported in the literature to mitigate heat stress, from cattle housing design to shifting feeding times to cooler periods and shade seeking ^[3]^[15]^[16], but only cooling and forced ventilation systems are effective above certain temperatures and humidity conditions ^[16]. Further, these systems must be connected in time to prevent the development of heat stress in cattle but also disconnected when they are no longer needed, as their use raises major concerns regarding energy and water consumption, which may be economically and environmentally unsustainable in the near future ^[17]^[18]^[19].

2. Heat Stressed Dairy Cows and the Available Technological Solutions to Detect and Mitigate Heat Load

2.1. Heat Stress Indicators

Heat stress has an enormous impact on animal health, biological functioning, and welfare. One of the most noticeable consequences of heat stress in dairy cows is the reduction of DMI, which causes a drop in milk production by decreasing the availability of nutrients used for milk synthesis ^[20]^[21]. Moreover, heat stress compromises cattle welfare by changing or inhibiting social and eating behavior ^[22], increasing susceptibility to disease ^[23], and causing stress and discomfort ^[24]. Under harsh ambient conditions, animals show physiological or behavioral responses or, most often, a combination of both. For instance, in the early stage of heat exposure, the animal body quickly responds to maintain homeostasis. As the amount of heat load increases, the physiological response becomes more evident, triggering an increase in both respiratory (RR; ^[25]^[26]) and heart rates (HR; ^[27]^[28]). Therefore, cows change their behavior ^[3]^[24], e.g., water ingestion, reducing movement and seeking shaded areas to minimize heat load, leading to the subsequent decrease in milk production, and finally a decline in fertility ^[29]. Although interdependency exists, physiological, behavioral, and production indicators were examined separately to facilitate the analyses ^[30]^[31].

2.1.1. Physiological Indicators

The core body temperature in cattle indicates the temperature of the most important organs of the body such as the heart, liver, and brain ^[32]. It is often used as an indicator of heat stress and typical values of 38.0 to 39.3 °C are observed in non-heat-stressed cattle ^[27]. Rectal or vaginal temperature is used as a conventional “gold standard” measure of core body temperature ^[33]. Yan et al. ^[34] studied the rectal temperature of dairy cows exposed to several heat stress conditions, reporting an increase of 1 °C in rectal temperature (from ~38.5 to ~39.5 °C) from neutral to heat stress conditions. The rectal temperature is the predominant method employed to measure the internal temperature of heat-stressed cows, but other internal regions of the cow body, such as the vagina ^[35]^[36], the rumen ^[16], and the tympanum ^[37], have also been studied and correlated with typical behavioral and ambient conditions of heat stress. Nordlund et al. ^[35] monitored the vaginal temperature of 20 high-producing cows, observing a temperature increase during lying bouts and a temperature decrease when the cows were standing in pens (both free stall and milking center holding pens). Another example, reported by Curtis et al. ^[16], is the delay in the rumen temperature increase of 2 to 5 h when compared to ambient temperature, probably due to the thermal inertia caused by the cow’s substantial body mass. One possible approach to identifying animals affected by heat stress is by monitoring the external temperature of the cow’s body. Several authors have been studying the temperature of eyes, limbs, and udder and have correlated it with core temperature ^[28]^[38].

In some situations, the variation of other physiological indicators is more significant than the core body temperature. For example, in moderate heat stress conditions, Vizzoto et al. ^[27], compared shaded and non-shaded cows at 1300 h (GMT−0200 h) and observed a significant increase in the respiratory and heart rates, but no significant differences in the cow’s body temperature. Cows with access to shade had a lower respiratory rate of 5.9 breaths per minute and a lower heart rate of 20.5 beats per minute when compared with those without access to shade. However, a significant increase in the core temperature of not-shaded cows was observed later at 1700 h. Respiratory rate increase followed by a body temperature increase was also reported by Ferrazza et al. ^[25] in Holstein cows exposed to intense and prolonged heat stress conditions. Moreover, Brown-Brandl et al. ^[26] concluded that the respiration rate is a good physiological indicator of heat stress because there was little or no lag associated with it and it was consistently affected in all the categories of the daily maximum temperature–humidity index (THI; i.e., it measures the risk of the animal suffering from heat stress; see Section 3.1).

The endocrine response to heat stress is mainly reflected by elevated glucocorticoids (cortisol), aldosterone, antidiuretic hormone, thyroxine, prolactin, and growth hormone. However, their use as heat stress indicators would imply frequent blood sampling, an invasive procedure that requires the handling of animals. A less invasive measurement is milk cortisol (MC) concentration as it is highly correlated to plasma cortisol concentrations sampled at the same time ^[39]. Glucocorticoid metabolites (produced by the liver and excreted into the gut via the bile), but not the native glucocorticoids, can be detected in feces ^[40]. These fecal Cortisol Metabolites (FCM) can thus provide an integrated measure of stress over several hours, whereas MC provides a measure of stress occurring in the previous minutes. Despite being recognized as useful indicators of the occurrence of heat stress in cattle, their use in a heat stress study has several limitations besides laboratory costs. In the case of MC, its measurement as short-term indicators would imply milking cows outside milking hours, which could be a stressful event for the cows.

2.1.2. Behavioural Indicators

Under heat stress conditions, cows tend to adapt their body posture ^[41]^[42], spending more time standing to increase the body surface area exposed to air, thus dissipating more heat. Allen et al. ^[43] showed a direct correlation between the core temperature and the duration of the standing period of lactating dairy cows experiencing mild to moderate heat stress. When a cow’s vaginal temperature exceeded 38.93 °C, there was a 50% likelihood that the cows would be standing. Furthermore, Yan et al. ^[34] correlated the rectal temperature and temperature–humidity index of cows with different postures (i.e., laying or standing). They observed that recumbent cows showed higher rectal temperature for lower values of THI, which indicates how the laying posture negatively affects the cows’ thermal state.

Heat stress also affects the cow’s disposition to display natural estrous behavior, reducing both the duration and intensity of estrous expression ^[44]^[45] and is responsible for a decrease of 20 to 30% in conception rates ^[46]. Furthermore, the decrease in conception rates during summer is also explained by oocyte quality reduction, early embryonic death, endometrium dysfunction, and reduced spermatogenesis in bulls ^[47]^[48].

Although less documented in the literature due to difficult quantification, it is undeniable the important changes in the emotional state, with evident signs of malaise, disorientation and frustration in animals affected by heat stress ^[3].

Other changes in the social behavior of heat-stressed animals are also reported, such as higher levels of aggression near water drinkers or competition for shade seeking ^[22]^[49]^[50]. Furthermore, the eating and drinking behavior of cows also changes ^[50]. For example, the increase in heat load is accompanied by an increase in water intake and a significant reduction in dry matter intake (DMI). This reduction in feed intake occurs in all mammalian species under heat stress conditions. However, the extent of this reduction depends not only on environmental conditions but also on the production level ^[30]. In heat-stressed lactating cows, feed intake may be reduced by as much as 40%, according to the National Research Council of the United States of America ^[51]. This causes a decline in milk production by decreasing the availability of nutrients used for milk synthesis. Likewise, a change in milk composition is to be expected such as a decline in total protein or fat content ^[52]^[53] but also in fat composition ^[54]. These factors related to the yield and composition of milk are often used as indicators of performance, as described in the following subsection.

2.1.3. Performance-Related Indicators

Under heat stress conditions, the activation of the cow’s thermoregulatory system can increase metabolic maintenance requirements by 7 to 25% ^[55], exacerbating both the existing metabolic stress and the reduction of milk production ^[3]^[30]. However, the drop in milk production has a time delay associated with it, being clearly noticed a few days after the first day of exposure to a high-temperature environment. For example, Van Laer et al. ^[56] reported a significant decrease in milk yield after a lag period of two days. Nevertheless, as the heat load decreases to thermoneutral conditions, the milk yield progressively returns to values obtained in thermoneutral cows ^[30].

Another relevant performance-related indicator that might be affected by heat stress is the milk composition which typically presents a reduction of lactose, protein, and/or fat with increasing values of bioclimatic indexes ^[53]^[56]. Nonetheless, the decrease in milk production is not always accompanied by a change in the composition of the milk ^[30].

2.2. Methods to Detect Heat-Stressed Cows

The complexity of cow physiological mechanisms and their interdependency with its behavior and with environmental conditions recommend the measuring of indicators at various levels: cow’s body, performance, behavior, and environment. The record of the latter is straightforward, and the parameters often studied are the environment temperature, humidity, air velocity, and solar radiation ^[57]^[58]. Several types of equipment are available in the market such as temperature and humidity sensors for indoor/outdoor conditions ^[23]^[59], anemometers for measuring wind direction and velocity ^[8]^[42]^[60], and solar radiation sensors ^[57]^[61]. However, a broader range of methods may be used to assess animal health and welfare ^[30], from equipment with different frequencies of data acquisition (e.g., real-time vs. hourly measurements) to continuous or scan observations. Thus, to facilitate the analyses of the methods used to study the animal physiological, behavioral, and performance-related indicators, the methods are described in the following sections by these categories: equipment measurement frequency and observations.

2.2.1. Equipment with Low Data Acquisition Frequency

Over the last decades, manual detection has been the most used method for measuring physiological parameters such as heart rate, sweating rate or rectal temperature ^[57]. However, these methods can be classified as invasive or, at least, disturbing to the animal, since the interaction with the animal, e.g., by touching or inserting portable equipment in the animal body ^[33], can add extra stress and consequently influence the measurements. These methods are also time-consuming and may involve significant additional labor costs. The advantage of these type of methods is that it often requires cheap and user-friendly equipment, e.g., a thermometer for rectal temperature measurement ^[1]^[25] or a stethoscope for heart rate measurement ^[25]^[27].

Sensors coupled with data loggers are also often used to record the measurements over discrete time points ^[2]^[57]^[62]. An evaporimeter was used by Rungruang et al. ^[63] to measure, four times a day, the sweating rate at the skin or hair-end level. In order to avoid the displacement of cattle from their social group for restrainment in a crush, the surface body temperature is often measured with thermal data loggers, e.g., a neck collar attachment for skin temperature measurement ^[2].

Another rapidly developing promising alternative is infrared thermography ^[2]. Thermal imaging cameras for Infrared Thermography (IRT) offer a remote, non-contact method for recording surface temperature and have been explored as a proxy for core body temperature measurements ^[28]. However, the use of such systems presents important operational challenges since the successful determination of IRT of external body surfaces depends on the effective minimization of confounding factors, e.g., the animal skin and hair, or external factors such as ambient temperature, sunlight or wind ^[2]^[64]. In a study that used infrared thermographic images to predict heat stress events in feedlot cattle, Unruh et al. ^[65] concluded that it was an objective method to monitor beef calves for heat stress in research settings, but at the time, they also noted that thermographic data was of little predictive benefit compared, e.g., with forecasted weather conditions. Like Idris et al. ^[2], these authors refer that the use of infrared thermography as a diagnostic tool to monitor heat stress in cattle requires further research ^[65].

The research and development of monitoring technology have been accompanying the modernization of the farm sector in the direction of a more efficient and automatized system (i.e., livestock precision farming). To ensure a prompt response, the new monitoring equipment should provide the continuous monitoring of the process (i.e., herd thermal state and environment conditions), ideally in real-time.

2.2.2. Equipment with High Data Acquisition Frequency

Automated temperature monitoring devices can have several applications in livestock, either to monitor animal health or to support scientists and farmers for precision farming and remote monitoring. In 2018, Koltes et al. ^[66] analyzed different automated body temperature monitoring technologies and discussed their use to develop new strategies to control potential animal health problems. The authors highlighted that the measurement at the animal level would be useful to manage heat stress and disease, despite the associated investment costs. Furthermore, they pointed out a need for developing software for complex data storage and treatments. A similar conclusion was drawn by Sellier et al. ^[62]. In order to avoid displacement of cattle from their social group and restraint in a crush, thermal data loggers can be used to measure body temperature by, e.g., a neck collar attachment (for skin temperature), a rumen bolus or a vaginal insert, recording temperature at pre-determined time intervals or having a telemetric option to transmit recorded data in real-time to a user-defined receiver ^[2]. As an alternative, the core body temperature can be monitored in real-time using implantable biosensors ^[60].

Rumen boluses provide prompt means of measuring rumen (or reticulum) temperature, but measurements may be influenced by the intake of fluids. One of the challenges in using rumen temperature is the acute impact of water intake ^[58]. Ingestion of large volumes (10 to 15% rumen volume) of cold water (0 to 8 °C) provokes a sudden drop in rumen temperature (up to 10 °C) within 15 min of water consumption and required approximately 2 to 3 h to return to baseline ^[67]. However, as water intake is directly related to feed intake, it may also be reduced in heat-stressed animals compared to thermoneutral controls ^[68]. Both feed and water intake can be monitored through several methods: neck-mounted activity collars, ear tags and/or leg data logger, coupled with micro-electro-mechanical accelerometers. This can provide important data regarding feeding behavior as head movements toward the feed bunk are recorded while a ruminal bolus can provide temperature data ^[69].

Sensors coupled with data loggers have also been used for high-frequency (minutes) measures that can be considered continuous. Polsky et al. ^[36] used temperature-recording data loggers coupled to an intra-vaginal progesterone implant for vaginal temperature measurement with a 10 min frequency. Respiration sensors coupled with a micro-computer have also been used to record the respiration rate with high incidence ^[61]^[70]. On the downside, battery problems or malfunctions (e.g., sensor displacement or removal) are usually only revealed at the end of the trial.

2.2.3. Observations

Respiration rate and panting serve as early indicators of increasing heat stress ^[24] and provide an easy method for the non-invasive and distant assessment of heat stress response ^[48]^[71], unless cattle are more than approximately 30 m away (at this distance it is difficult to visualize the cow’s behavior ^[72]). The panting score is often assigned on a scale of 0 to 4, in which zero is no panting and four is severe panting ^[27]^[28]. The ethogram for panting scoring can be helpful to assess an individual or a group of animals’ response to heat stress ^[37]^[72]. However, human observation adds uncertainty to the collected data and the outcomes are extremely dependent on the researcher or farmer’s experience ^[2]. Additionally, it entails a substantial rise in labor costs.

The major limitation of these animal-based methods is the moment at which the heat stress is detected. The methods identify the signs of heat stress and not the conditions that potentially lead to heat stress (pre-heat stress conditions), so it fails to prevent the deterioration of the animal’s health.

2.3. Cooling Technologies

A wide range of alternative strategies to reduce heat load is used, from shifting feeding times to cooler periods ^[3] to nutrition adaptation ^[73]. However, above certain temperature and humidity conditions, only evaporative cooling and forced ventilation systems are effective (e.g., fans ^[74] and sprinklers ^[20]). Several environmental control systems for dairy cows exist based on the principles of convection, conduction, radiation, and evaporation and most can mitigate or even avoid heat overload episodes ^[57]^[75]. Fan installations, which facilitate air movement and increase convection, can decrease both respiratory rate and rectal temperature and promote DMI ^[76]. Similar outcomes are achievable with other forms of evaporative cooling systems that make use of high-pressure mist injected into fans or large water droplets from low-pressure sprinkler systems that completely soak the cow’s hair coat ^[20]. Despite advances in cooling technologies, primary concerns, regarding energy and water consumption, arise with the use of these systems, namely sprinkler systems. Depending on herd size, large volumes of water are needed for cooling, reaching levels of water consumption (215 to 454 L per cow per day) that may become economically and environmentally unsustainable in the near future ^[17]^[18]. Furthermore, one should not forget that these systems generate equally large amounts of wastewater that must be managed. Along with drinking water and water needed during the milking routine, water for evaporative cooling is one of the main three uses of potable water in commercial dairies and decreasing water usage and contamination is critical to the sustainability of the industry ^[19]. Therefore, these cooling systems must be turned on in time to provide immediate thermal relief for dairy cows but also turned off when unnecessary, to avoid wasting energy and water.

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