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Behavioral responses to heat stress

Cassandra Tucker, Davis and Karin Schutz Published on 08 June 2010

Heat stress is an economic cost for U.S. producers. It is estimated that hot weather costs dairy farmers $900 million per year in reduced milk production and reduced fertility. In addition, heat stress is an animal welfare issue, as severe heat stress can result in death.

Animal behavior can provide insights into how and when to cool dairy cows. We will review the behavioral responses to heat stress and examine results from studies looking at two common methods for cooling dairy cattle: shade and sprinklers.

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Behavioral responses to heat stress
• Shade seeking
Cows readily use shade when given access to it, and the provision of shade can alleviate negative effects of increased heat load. Not surprisingly, there is evidence that dairy cattle are motivated to use shade in warm weather. For example, we have found that dairy cows choose to stand in shade instead of lying in warm conditions (air temperature greater than 86°F) even when they were deprived of lying for the previous 12 hours. Furthermore, shade use increased with increasing air temperature and solar radiation. Indeed, there is evidence that cattle will engage in aggressive behavior to gain access to shade, especially when the heat load increases.

• Reduced feed intake
One of the most-discussed behavioral responses to heat load is reduced feed intake. For example, in one study, Ominiski and colleagues exposed cows to heat stress in an environmental chamber (90°F) and found that intake was reduced by 3.1 pounds per day, or approximately 5 percent per day. This change also resulted in a 3.8-pound reduction in milk yield. Extensively managed cattle also change grazing times and patterns to cope with heat load. For example, Kendall and others found shaded cows used shade during the day and grazed more at night compared to unshaded cows.

Increase in time spent standing
A second, but less discussed, change in behavior associated with heat load is an increase in time spent standing. In a recent experiment, we found that time spent standing increased by 10 percent (13.8 hours to 15.3 hours per day) when heat load increased by 15 percent. We suggest that cows spend more time standing to increase heat loss by increasing the amount of skin exposed to air flow or wind. A considerable amount of heat is lost from the underside of the cow, one possible reason why standing may be an important behavioral response to heat load.

• Increase time around water trough
Unshaded cattle adopt other behavioral strategies to alleviate heat load, such as increasing time around the water trough. In a study by Mader and colleagues, the percentage of cattle around the water trough was two to three times greater for unshaded groups compared to groups that had greater than 3.5 square meters shade per animal, especially when heat load was at its peak.

We do not fully understand why cows spend more time around the water trough, and there are several possible explanations. Cattle may reduce the effects of high heat load by increasing water consumption. Access to cool drinking water improved weight gain in feedlot cattle in summer. There is also the possibility that cattle spend more time around the water trough because evaporation from the trough may create a cooler microclimate compared to the rest of the enclosure.

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• Increased respiration
Respiration rate increases in response to heat load with little or no lag in time. For example, in feedlot cattle, respiration rate increased from approximately 65 breaths per minute when the temperature humidity index (THI) is less than 76 to 93 breaths per minute when THI is greater than or equal to 84. Cows increase respiration rate in order to promote heat loss via evaporation. Respiration rate can be the most practical way to identify heat stress, as flank movements are easy to count.

Behavioral responses to shade and sprinklers
• Shade
To date, it is clear that shade provides benefits for cattle in terms of behavior, physiological responses and production. There are many ways to provide shade, but little is known about the importance of various design features of shade (e.g. blockage of solar radiation, shade amount per animal, etc.).

Animal behavior, in combination with measures of heat load, such as body temperature, can provide valuable insights into when and how to cool cows. In a series of experiments, we examined how a single design feature of shade, protection from solar radiation, influenced the behavioral and physiological responses of dairy cattle in summer.

In the first experiment, we compared the behavior and body temperature of dairy cattle when they had no access to shade, or free access to shade that blocked either 25, 50 or 99 percent of solar radiation. Cows were kept in one of the four treatments in separate pastures and were observed all day.

We found that use of the shade increased with higher levels of protection from solar radiation. As average ambient solar radiation increased, total use of the shade structures increased. Cows with more protection from solar radiation had lower minimum body temperature, although the difference was relatively small.

Similarly, shade use peaked during the hottest part of the day, or when solar radiation was highest. Cows used the shade simultaneously during the middle of the day. These results highlight that cows need to be able to use shade simultaneously during periods of peak heat load. Having access to shade that blocked more solar radiation and being able to use this shade at the same time helped these cows maintain their body temperature.

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In a second experiment, we asked the cows for their opinion about the amount of solar radiation the shade blocked. We let the cows choose between two different types of shade cloth that blocked: 50 or 99 percent; 25 or 50 percent; and 25 or 99 percent of solar radiation. Each group of cows had access to four shade structures, and two structures of each type were provided to reduce competition.

We found that cows preferred shade cloth that blocked greater amounts of solar radiation in two of the combinations (99 percent versus 25 percent: 72.3% time spent in the 99% option, 50 percent versus 25 percent: 72.0 percent time spent in the 50 percent option), but showed no preference for shade cloth that blocked 50 or 99 percent of solar radiation when these two options were presented at the same time.

Together, these results demonstrate that the degree of protection from solar radiation is an important design feature of effective shade for dairy cattle. From the levels we tested, cows should be provided with shade that blocks at least 50 percent of the solar radiation.

• Cooling with water: Sprinklers
Although shade is beneficial and readily used by cattle, cooling with water can provide more effective relief from heat. For example, in an experiment we found that sprinklers reduced respiration rate by 62 percent compared to a 30 percent reduction when cows were only provided with shade. Others have found that the combination of shade and sprinklers provides the most marked reduction in respiration rate.

Indeed, there is extensive evidence that cooling with water, either as sprinklers or misters with or without fans, reduces respiration rate. Despite the overwhelming evidence that water is an effective way to cool cows, several questions remain unanswered. Namely, few researchers have explored if and when cattle will voluntarily use sprinklers. In most experiments, sprinklers are used over the feedbunk such that cows will get wet while they feed. There is some evidence that cattle may find sprinklers aversive.

In previous work, we have found that body temperature will rise when sprinklers are used on cooler days (less than 74°F), perhaps due to vasoconstriction in order to maintain core temperature. In this experiment, cows were forced to get wet in the 90 minutes before afternoon milking and these results indicate that sprinklers may cause cold stress on days when ambient temperature was below 74°F.

Others have also found that body temperature will rise shortly after sprinkling. It is likely that these differences between experiments can be explained by weather conditions, as we have shown that this short-term increase in body temperature only occurs on cooler days.

There is only limited evidence about the behavioral response to sprinklers. In winter, cows seek shelter, particularly when it is raining, and when exposed to wind and rain are five times more likely to stand with their head lowered compared to their sheltered counterparts. When cows are unable to escape wetting by sprinklers because the water is applied to the entire pen or group, cows also spend more time with the head in a lower position, possibly in an attempt to avoid getting water in sensitive areas like the ears, but also possibly because this is the only part of the body they can readily move away from the water.

Aversion is not directly measured by head position and only limited conclusions can be drawn from this behavior. Indeed, when given the choice in very warm conditions, cattle will stand under sprinklers.

Conclusions
Dairy cattle change their behavior to reduce heat load in hot weather. Cows seek shade, reduce feed intake, spend more time standing, spend more time near the water trough and increase respiration rate as ambient conditions become warmer. These behavioral changes can provide insight into when and how to cool cows. Both shade and sprinklers can effectively reduce heat load. Animal behavior is a useful tool to understand how to best use and design shade and water cooling systems: cows will tell us about how to best cool them. PD

References omitted due to space but are available upon request by sending an email to .

—Excerpts from 2009 Southwest Nutrition and Management Conference

Cassandra Tucker
Animal Science
University of California

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