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Heat Stress? No sweat with new management insights!

Heat stress and heat stress management were the topics for the first block of the Production, Management, and Environment session at this year’s American Dairy Science Association Virtual Meeting with Jen Heguy serving as the moderator. Heat stress is one of the leading causes of decreased production in dairy cattle during the summer months and can become apparent due to decreased milk production and decreased breeding cycles. Although heat stress is unavoidable, researchers are studying methods to minimize the effects, whether that be through better genetics and housing to assessing new methodologies for detection.

First, researchers from the University of Florida assessed the effects of heat stress on the time budget of nulliparous Holstein heifers. Toledo et al. housed 25 pregnant Holstein heifers in freestall barns. Heifers either received shade as the heat stress treatment or shade, soakers, and fans as the cooling treatment. Leg and neck tags were used to record steps, standing, eating, rumination, and lying time. It was found that heat stressed animals spent more time eating but less time ruminating during the prepartum period. However, during the postpartum period, heat stressed heifers, while still eating more and larger evening meals, did not differ from non-heat stressed animals when it came to rumination time. Heat stressed heifers also took more steps during the postpartum period. This suggests that heat stress does affect the daily time budget during the pre- and postpartum periods. Although animals were eating, they were attempting to mitigate internal body heat, hence less time ruminating. Management strategies could be developed from this information to help producers mitigate cow heat load.
Chaalia et al. from the Universitat Autonoma de Barcelona switched things up by identifying metabolic and productive characteristics in dairy sheep that were either sensitive to heat or thermotolerant. Twenty-four Manchega dairy ewes in late lactation underwent a short-term heat challenge in a temperature-controlled room. Rectal temperature (RT) and respiratory rate (RR) were measured, with ewes classified by the RT/ RR change ratio (CR). Ten ewes from the original 24 differing in CR were then used in a crossover study with two periods (3 weeks each) and two climatic conditions (thermoneutral and heat stress), with RT, RR, milk yield, and intake being measured. Heat stressed ewes had greater prolactin concentrations and decreased milk fat and protein. No noticeable differences were noted with other measurements. This suggests that although there were slight differences between the two treatment groups, Manchega ewes, overall, were relatively tolerant to heat stress. Stage of lactation or phenotype of animals may contribute to overall sensitivity to heat.
Toledo et al. presented once again and discussed various management challenges regarding the relationship of pasture housing and heat stress in late pregnant heifers. In this study, Holstein heifers were housed on pasture or in a freestall barn. Heifers were either exposed to a heat stress challenge or cooled with shade, soakers, and fans. Temperature-humidity index and black globe temperature were measured as well lying time, steps, eating, and rumination time. Pasture heifers spent less time lying down, less time ruminating, and more time walking when compared to heifers confined to a freestall barn. This indicates that heat stress does affect the daily time budget of pregnant heifers, especially if they are on pasture. It may give insight into developing new management strategies to mitigate heat stress.
Finally, Ouellet et al. from the University of Florida discussed new methods for assessing chronic heat stress in dairy calves in a subtropical environment. Heat stress in dairy calves has received less attention because it is believed that they are more tolerant to heat stress. In this study, researchers measured respiration rate, rectal temperature, and skin temperature in dairy calves exposed to a heat stress challenge or kept cool. Environmental conditions, such as ambient temperature, humidity, THI, and wind were also measured. A positive correlation was noted between animal-based indicators, ambient temperature, and THI. Results indicate that dairy calves can undergo heat stress with THI being the best environmental indicator of heat stress. Practically, heat mitigation should be applied when THI reaches 65 if calves are exposed to chronic heat stress; in a subtropical environment, calves should be monitored between a THI of 65 and 69. This means that producers should adapt their management strategies to account for heat stress for every member of their herd.
Miriam Snider is a PhD candidate at the University of Vermont under Dr. Sabrina Greenwood. She earned her B.S. in Animal Science / Biology from Southeast Missouri State University and her M.S. from the University of Kentucky. Her current research explores forage and pasture quality/ energetics and its relationship with rumen fermentation and methane production.

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