Heat stress in cattle: reduction and management

Management of heat stress in cattle

Although all heat abatement methods require an initial capital investment, fans, and water spray also involve ongoing expenses for energy and water, and producers must decide when to activate these devices. The economic benefits of providing heat abatement to reduce losses to production, reproduction, and survival are easily recognized. To optimize animal welfare, however, earlier intervention may be required to help cattle cope before obvious problems occur; understanding this rationale re- quires a more detailed exploration of the definition of heat stress in cows.

Ideal temperature for dairy cow

Heat stress in cattle and thermal comfort in homeothermic animals are often conceptualized about ambient conditions, metabolic heat production, and outcomes such as milk yield. Homeothermy refers to core body temperature remaining within the normal range, and hypothermy and hyperthermy are deviations below and above this range, respectively. Thermoneutrality refers to a narrower range of conditions (bounded by lower and upper critical ambient temperatures) in which metabolic heat production re- mains stable (Fig. 1). Definitions of heat stress and thermal comfort, however, vary in the literature. This author favors the 50-year-old interpretation by Bianca, who described thermal comfort within an even narrower range in which the physiologic and behavioral defense mechanisms associated with heat exchange (see previous discussion) are not yet activated (see Fig. 1). Based on this concept, cattle can be identified as beginning to experience heat stress based on vasodilation occurring; followed by sweating, increasing respiratory rate, and panting; behavioral changes to reduce heat gain and increase heat dissipation;

and, finally, by behavioral strategies such as reduced feed intake to lower heat production in the subsequent decades, however, many investigators have equated thermal comfort with thermoneutrality and have defined heat stress in cattle as occurring above the up-per critical temperature 34 or when production losses begin to appear. 35 Many thermal indexes have been developed from models describing the relationships between environmental conditions and cattle responses, primarily in terms of production losses. The most common is the temperature-humidity index (THI), which combines air temperature and relative humidity. 36 Many studies have aimed to identify breakpoints or thresholds at which increases in THI result in marked changes in body temperature, milk yield, milk quality (ie, somatic cell score), 39 or mortality.5 Based on such studies, THI values of 72 or, more recently, 68 are frequently used as thresholds for heat stress in cattle Defining heat stress in cows with THI thresholds has several limitations.

Fig. 1. Heat stress in cattle can be conceptualized as occurring when the animal’s behavioral and physiologic defense mechanisms against overheating are activated. These responses begin before the animal’s core body temperature has begun to increase and before changes in metabolic heat production occur (while both homeothermy and thermoneutrality, respectively, are maintained). The blue and orange lines represent core body temperature and metabolic heat production, respectively. As heat stress begins, metabolic heat production may initially decrease as a result of adaptive behavioral responses (eg, reduced feed intake) and will eventually increase, along with core body temperature, when the defense mechanisms become insufficient. Responses to cold environmental conditions are not depicted.

Defining heat stress in cows with THI thresholds has several limitations.First, environmental variables besides temperature and humidity also contribute to heat exchange, and indexes have been developed evaluating a more comprehensive suite of environmental variables (eg, heat load index, which adds air speed and black globe temperature to incorporate the effects of radiation) for their effects on body temperature. Second, when ambient heat increases, there is a lag before body temperature increases and production declines. More recent models incorporate increased respiration rate and skin temperature (eg, equivalent temperature index of cattle), which precede elevated body temperature and are more sensitive indicators of the need for heat abatement. Third, cattle may begin to feel discomfort and experience negative effects in conditions cooler than predicted by THI thresholds for production losses. Consistent with Bianca’s9 earlier interpretation of heat stress, cows seek cooling under thermoneutral conditions: they make some voluntary use of spray overnight and show clear preferences for water spray starting in the morning, several hours before the daily peak in ambient temperature, as well as on days with mild conditions.

Cooling down with water

As heat stress in cattle begins, metabolic heat production may initially decrease as a result of adaptive behavioral responses (eg, reduced feed intake) and will eventually increase, along with core body temperature, when the defense mechanisms become insufficient. Responses to cold environmental conditions are not depicted. (Data from Bianca W. Thermoregulation. In Hafez ESE, editor. Adaptation of domestic – animals. Philadelphia: Lea & Febiger; 1968. p. 97-118.).

production declines. More recent models incorporate increased respiration rate and skin temperature (eg, equivalent temperature index of cattle), 41 which precede elevated body temperature and are more sensitive indicators of the need for heat abatement. Third, cattle may begin to feel discomfort and experience negative effects in conditions cooler than predicted by THI thresholds for production losses. Consistent with Bianca’s earlier interpretation of heat stress, cows seek cooling under the- neutral conditions: they make some voluntary use of spray overnight21,42 and show clear preferences for water spray starting in the morning, several hours before the daily peak in ambient temperature, as well as on days with mild conditions In commercial dairy operations, water-based cooling systems are commonly activated either manually or based on a single air temperature sensor, usually at or around 21° to 24°C (70°-75°F), which may not capture the variety of microclimates that cow experience. Ideally, the criteria for activating heat abatement should be adjusted b observing the animals on a given farm. Measuring animal responses is important Considerations for Cooling Dairy Cows with Water not only because farms vary in their facilities and management but also because individual cattle respond differently within the same environment depending on breed, milk production, pregnancy or health status, the plane of nutrition, and coat characteristics, along with factors such as social status, which could affect access to drinking water, cooler microclimates, and heat abatement. Nonetheless, it can be challenging for producers to identify cows experiencing heat stress and in need of additional intervention, and for animal welfare assessment programs to establish criteria for verifying the effectiveness of a farm’s heat abatement strategies. For example, the Welfare Quality program in Europe designates thermal comfort as a requirement for good cattle housing; however, no measure has been established to assess this principle.

Conspicuous, noninvasive indicators (Box 2) to identify heat stress in cows include signs of panting, such as drool, an extended tongue, or an open mouth. 45,46 However, panting is associated with markedly elevated respiration rates (100 breaths per minute on average),45,46 and identifying an increase in respiration rate earlier would be a more sensitive indicator. Recording respiration rate can be labor-intensive, however, because this measure needs to be observed at least every 90 minutes accurately determine changes over time.

46 In addition, for a continuous variable, establishing precise cutoffs for intervention is challenging. In summer, cows with 24-hour access to cooling had average respiration rates of 50 breaths per minute,21,42 whereas cows given a choice after being deprived of cooling began to show preferences for high-output sprinklers when their respiration rates reached approximately 60 breaths per minute. 43 In an experiment testing respiration rate monitors in a tunnel-ventilated barn with evaporative cooling pads, a THI of 68 corresponded to approximately 60 breaths per minute. Based on these patterns, along with suggestions in the older literature, 47 a threshold of 60 breaths per minute could be used as the minimum respiration rate for cooling intervention. Shortly, technological advancements may allow for more automated detection of heat stress in cattle using physiologic and behavioral indicators, and for automatic activation and adjustment of heat abatement resources.