Monitoring heat stress in dairy cows: Part 1 – thermal indices, physiological and behavioral traits

Heat stress related production loss, compromised welfare and cattle mortality are global concerns, which are increasing in the context of climate change and increase productivity of the cows. Cattle response to heat stress varies, based on individuality and thermal environment. In this article I intend to describe heat stress monitoring, in individual and herd basis, making use of thermal indices, as well as physiological and behavioral changes occurring in the heat stressed cow.

Thermal indices are diagnostic quantity of ambient parameters (air temperature, relative humidity, solar radiation, wind speed etc.) to assess animal thermal comfort level in its environment. Air temperature as a single indicator omits other important ambient factors that influence thermal comfort of animals. The temperature humidity index (THI) and the heat load index (HLI) are the most common thermal indices, due to their widespread use today in the dairy and beef industries.

Temperature and humidity-based indices - Indices based on air temperature (dry and/or wet bulb), black globe temperature and relative air humidity can estimate the thermal comfort level of humans, cattle and other animals. Temperature humidity index (THI) is based on the sum of dry and wet bulb temperatures (°C) and is the most widely used index of heat stress in livestock, combining ambient temperature and relative humidity. This index has been widely used, as most weather stations readily collect the required climatic input variables. However, there have been many modifications in THI including simple conversions of temperature measurement unit from °C to °F or vice versa and differential relative weightings on temperature and humidity. A THI value of 72 as the upper threshold of cattle thermal comfort has been suggested by earlier literature, but recent research suggests a lower threshold of 65 (as lowest daily temperature) and 68 (as average daily temperature), for high producing cattle. However, evaluation of seven different THI equations found that THI thresholds can vary, based on geographical location and the equation used. In this regard, higher weighting on humidity were effective in humid climates whereas higher weights on dry bulb temperature were more suitable for dry climate. The THI as an index of thermal comfort has some limitations. The important climatic variables impacting heat gain or heat loss such as wind speed and solar radiation and individual animal factors including health, genotype, coat characteristics were not accounted for in THI methods. THI modifications and thresholds did not consider animal or management factors that can impact the individual heat-response of cattle under similar thermal conditions. The most common way to characterize “climate region” today is to sum the number of hours per day and days per year, where THI is greater than 68.

Heat load index (HLI) - To overcome the limitations of THI, the heat load index (HLI) was developed, incorporating the climatic parameters like solar radiation and wind speed, in addition to temperature and humidity. This index takes in account also animal parameters (genotype, coat characteristics, health, acclimatization, etc.) and management practices. The HLI model incorporate black globe temperature, to account for the effect of temperature and solar radiation, relative humidity and wind speed; and adjustments were suggested to make the model dynamic. The HLI index is mostly in use in grazing systems and much less in cows under full confinement.

Importance of individualized approach - Most of the thermal indices and model-based estimates of thermal comfort are herd-level predictions of the average animal response.

These indices present a potential risk to individuals that do not fall within the set threshold for the population. Several studies indicate contrasting ability of cattle breeds to cope with heat. European breeds for example, have greater respiration rates than Bos indicus breeds, even at lower temperatures. In general, breeds of tropical origin are more heat tolerant than temperate climate breeds, and therefore, may require differential mitigation measures under similar thermal conditions.

Animals mostly alter their physiology and/or behavior in response to stress. Behavior is a powerful tool to evaluate how an animal is coping with stressors in its environment, as it is inter-linked with intrinsic animal factors and provides a non-invasive visual indicator of wellbeing.

Cattle need to maintain their body temperature within a narrow range to allow body function optimally. Besides the heat produced internally, cattle also take in additional heat from solar radiation, reflected radiation from the ground and other surrounding physical structures, and from the air itself, if the air temperature is greater than the animal’s body surface temperature. Cattle temperature normally ranges between 38 and 39 °C with a diurnal fluctuation of ± 0.5 °C depending on environmental temperature, peaking in the early evening and reaching a minimum in the early morning.

During excessively hot conditions animal body temperature rises and may lead to prolonged elevations above tolerance levels that may cause damage of body tissues and organs and even morbidity. In this regard, a body temperature greater than 41 °C can be lethal. The body temperature (either core or surface temperature) of cattle can be used as an indicator of heat stress. Measurement of cattle body temperature is impacted by the anatomical location of the site, including the tympanic membrane, rectum, vagina, reticulo-rumen and skin, as well as method of measurement, namely manual thermometer, infrared thermography, radio telemetry or temperature data logger.

Traditionally, rectal temperature has been considered as a robust indicator of cow’s body temperature but measuring it continuously is very limited. Vaginal temperature is highly associated with rectal temperature and empty controlled internal drug release (CIDR) devices with attached temperature data loggers have been used for short term body temperature monitoring, being of course limited to be used in female cattle.

The measurement of cow body temperature by an implanted temperature data logger with transmitter revealed that a change in the temperature is dependent on ambient conditions and lagged ambient temperature by 1–5 h and therefore, it seems that at any point of time it is indicative of heat stress in 1–5 h before that measurement. The variability between individuals in heat-tolerance and heat-susceptibility may play a significant role in the interpretation of recorded data. The problem with monitoring body core or surface temperature is that current technologies are not practical or suitable for constant monitoring of individual cattle in large herds over a long duration due to the limitations of memory to store the data, battery life, short range of communication and cost. Moreover, most body temperature monitoring cannot not be performed in real time and mostly suitable in these days for research purpose, with limited availability for commercial farms.

The respiration rate and panting behavior of cattle are predominantly associated with ambient conditions and there are genetic-specific temperature thresholds above which they increase. The respiration rate of cattle is a key indicator of thermal stress as it is impacted by different temperature-humidity categories. Research has demonstrated that it increases within 4 h before to 4 h after the hottest part of the day in an ambient temperatures ranging between 21 and 25 °C. There are also varying recommendations as to the threshold above which heat stress mitigation measures are needed. Less than 40 breaths per min is suggested as the normal, although a slightly higher value of approximately 60 has also been recommended. Under severe heat stress conditions, the respiration rate of dairy cattle may be greater than 150 breaths per minute and in some circumstances it may be reduced at severe stress levels due to respiratory phase shifts between “rapid-shallow” and “slow-deep” breathing.

For large numbers of animals, the visual assessment of respiration rate is time consuming and it is challenging to maintain accuracy from the considerable distance needed to minimize animal disturbance. Moreover, respiration rate does not incorporate respiratory dynamics such as drooling and open mouth panting associated with increasing heat stress. The respiratory dynamics of cattle can be assessed as a panting score which accounts for the visual changes in the respiratory behaviors.

However, panting score of cattle varies by genotype and individual circumstances, and not all the animals within the same group respond equally to a specific heat load event; therefore, decisions based on mean panting score may not account for individual variability. Furthermore, a panting score represents a point in time and lacks continuity.

Studies have shown heart rate together with internal body temperature can indicate heat stress. As heart rate and respiration rate are positively correlated, therefore, heart rate can be potential indicators of heat stress taking into consideration the individual variation in panting ability and can be used for the assessment of a short-term heat response, while additional parameters like respiration rate and body temperature may be required for prolonged heat exposure.

Monitoring metabolic and endocrine changes can help detect stress events as heat stress responses at the blood level may precede visible behavioral or physiological changes. Plasma cortisol concentration increase significantly from the baseline value in cows exposed to high temperatures. A reduction in plasma somatotropin, triiodothyronine and thyroxine occurs in cows exposed to high ambient temperatures.

Reduced concentrations of insulin-like growth factor-I (IGF-I), plasma glucose, plasma vitamin C, and non-esterified fatty acids (NEFA) have been attributed to heat stress.

There are contrasting opinions as to whether the changes in blood metabolic profile are the direct effect of heat stress or the indirect effect due to reduced dry matter intake.

Cattle metabolic response to heat stress varies also due to physiological condition of the animal, and caution is required when utilizing it in commercial practice. Practical limitations such as cost and the sophistication of technology may also restrict their use only to experimental conditions, unless biosensors will be developed, for low-cost continuous monitoring.

Cattle under heat stressed conditions have altered behavior as compared with those in thermos neutral conditions. Basic characteristics of behavior such as bout length, transition frequencies between activities, resting, and variability in behavior can be affected due to heat stress. Heat stressed cattle drink more to maintain evaporative water loss, and increase standing bouts, which is thought to enhance cooling by exposing more surface area to the environment. Decreased lying bouts and eating are associated with heat stress. Eating time is shifted with heat stress, where eating frequency per day was reduced from 15 to 3 times with greater meal size. Animals usually avoid eating during the hotter parts of the day and eat more during the early morning and late afternoon hours including nighttime. In a study with Holstein lactating cows, total daily activity was increased and total daytime and nighttime rumination durations were decreased with increasing temperature humidity index (THI). Similarly, daily rumination time during summer was negatively associated with daily maximum THI (>72) and there was a clear shift in rumination pattern with more than 60% of total daily rumination occurring at night. Behavioral responses to heat stress are associated with animal factors including breed, coat color, body weight, condition score, sex, temperament, physiological condition, acclimation, and other individual characteristics.