Heat stress breakpoints for non-lactating cattle will vary by climate, heat abatement, and maturity.
By Dr. Bethany Dado-Senn, Dr. Veronique Ouellet, and Dr. Jimena Laporta It’s no secret that this summer has been a challenging one for dairy cows. It seems like nearly every week in most parts of the U.S., bulk tank averages and milk components struggle as dairy farmers combat heat stress in their lactating cows. Notably, there are other cattle in the herd that don’t contribute to the bulk tank but still struggle with production, welfare, and health under elevated ambient temperatures. These include non-lactating, dry dairy cows and young stock. In a recent presentation at the 2022 Four State Dairy Nutrition conference, Dr. Jimena Laporta discussed some of the consequences and benchmarks for heat stress detection and prevention in dry cows and pre-weaned dairy calves. These benchmarks were established based on research conducted by our group in both the Southeastern and Midwestern U.S. regions. The Southeastern U.S. is a subtropical climate characterized by sustained and severe levels of heat stress across the entire summer, whereas the Midwest is a temperate, continental climate characterized by substantial diurnal and season temperature variations. Therefore, establishing customized benchmarks for each region deems necessary and will allow for more accurate and timely heat stress detection and intervention on-farm. Dry period heat stress - When dry cows are exposed to heat stress in the late stages of gestation, they make less milk in subsequent lactations, and their calf is born earlier, smaller, and more immunocompromised. Further, the in utero heat-stressed heifer calf will have a slower developing mammary gland that will make less milk at maturity almost two years later.
In a recent study, we determined thresholds at which dry cows begin to display signs of heat stress in a subtropical, Southeastern U.S. climate (Ouellet et al. 2021). We analyzed the thermoregulatory and environmental records of 218 dry cows experimentally exposed to either heat stress or heat abatement (i.e., cooled via fans and water soakers) during the entire dry period (Figure 1). Using segmented regression models, we assessed environmental “breakpoints” at which respiration rate or rectal temperature began to abruptly rise in both heat-stressed and cooled dry cows. At a temperature-humidity index (THI) of 77, heat-stressed dry cows had a sharp rise in respiration rate and rectal temperatures (Figure 2). Meanwhile, respiration rates began to abruptly rise at a THI of 75 in dry-cooled cows, yet the overall respiratory rates never rose above 60 breaths per minute (Figure 2). Research in both dry and lactating cows suggests that 60 breaths per minute is the target respiration rate benchmark at which these cows begin to experience heat stress (Toledo et al. 2020).
Pre-weaned heat stress - Although dairy calves are more thermotolerant relative to mature cows, they are still susceptible to welfare and productive impairments under heat stress. Under elevated ambient temperatures, pre-weaned calves may have reduced feed intakes, impaired growth, altered thermoregulatory behaviors, and increased stress responses. Identifying heat stress in young stock can be challenging, as calf productive and vital/thermoregulatory responses are rarely measured on-farm. By developing environmental thresholds for pre-weaned calves, we can aid dairy farmers in early detection and monitoring of heat stress to prompt further monitoring and implementation of heat abetment strategies. We conducted two separate studies in a subtropical (i.e., Florida) and temperate/continental climate (i.e., Wisconsin) where we collected summer environmental and thermoregulatory responses in pre-weaned dairy calves to establish environmental breakpoints.
In the summer of 2018, we evaluated 48 group-housed auto-feeder calves under natural ventilation (heat-stressed) or active ventilation via fans (cooled, Figure 3) in Florida. We measured respiration rates and rectal temperatures thrice daily between 21 and 42 days of age, and milk intakes were collected daily (Dado-Senn et al. 2020). Through correlation analysis, we found that THI was the optimal environmental indicator in a subtropical climate to monitor the onset of calf heat stress. We implemented segmented regression to determine breakpoints. We found that heat-stressed calves abruptly increased respiration rate and rectal temperatures at a THI of 65 and 67 respectively, while cooled calves saw a rise in respiration rate at a THI of 69 and saw no breakpoint for rectal temperature (Figure 3). Interestingly, we also found that there was a sudden drop in milk replacer intake for heat-stressed calves at a THI of 82 (Figure 3). To better understand the nuances of calf heat stress responses in the more variable, temperate Midwestern climate, we conducted a similar study in the summer of 2021 in Wisconsin (Dado-Senn et al. in review). We assessed thermoregulatory responses from 63 hutch-housed calves twice daily between 14 to 42 days of age, but in this study, we did not provide any active cooling (Figure 4). Correlation analysis in this experiment identified ambient, dry bulb temperature to be the optimal environmental indicator instead of THI. This finding makes sense, as the humidity contribution of the THI equation is not as constant or severe in a temperate relative to subtropical climate. We found a sharp rise in calf respiration rate and rectal temperate when THI reaches 69 or when dry bulb temperature reaches 21 to 21.5 °C (Figure 4).
