Heat stress is a widely recognized health and performance risk to dairy cattle that continues to gain the attention of researchers.
Dr. Jimena Laporta has studied heat stress for most of her academic career, first at the University of Florida, and now as Assistant Professor in lactation physiology at the University of Wisconsin.
But as much as we have learned about the far-reaching effects of heat stress in cows – and even the health and performance of their in utero offspring – what about calves?
Laporta recently told the audience of an I-29 Moo University webinar that conventional wisdom would dictate that calves are less bothered by heat and humidity than their adult counterparts. After all, they have a greater surface area relative to body mass, allowing them to dissipate heat more easily. They also have underdeveloped rumens and generate less metabolic heat.
But surprisingly, those factors don’t necessarily yield an advantage for calves when it comes to coping with heat. Similar to mature cattle, calves have a thermoneutral zone of about 50-72°F. In this range, they can maintain their body temperature without having to expend additional energy.
Conditions outside of this range, whether cold or hot, will cause calves to repartition energy to regulate their body temperature. “That energy is going to be taken away from growth and immune function,” Laporta explained.
She said the precise thermoneutral temperature for calves, and the threshold at which they are affected by heat stress, is dependent on a number of factors, including age and size of the calf; wind speed; ambient temperature, and relative humidity. The Temperature-Humidity Index (THI) takes humidity into account along with ambient temperature, making it a more accurate environmental predictor of heat stress.
A THI of 68 is used as the standard threshold for heat stress in adult dairy cows. But Laporta and her team wanted to set environmental and animal-based heat stress benchmarks specific to dairy calves. They did so by evaluating THI in relation to animal-based factors including skin temperature, rectal temperature, respiratory rate and heart rate.
Further, they evaluated calves being raised in both Florida and Wisconsin, to determine whether climate and environmental factors made a difference in calf heat-stress tolerance. Their findings:
- In Florida, the THI at which calves showed abrupt changes in respiration was 65. For rectal temperature, it was 67.
- In Wisconsin, the respiratory and rectal temperature breakpoints were both at a THI of 69.
- Evaluating overall effect of THI, heat stress in calves was triggered at a THI of 65 in Florida and 69 in Wisconsin. This translates to surprisingly lower respiration of 30 and 40 breaths per minute, respectively.
So, in the 24/7 hot, humid environment of Florida, calves were actually more susceptible to environmental heat than adult cows, and began to show signs of stress at a lower THI. Given that heat-stressed calves eat less milk replacer and starter grain, and even have been shown to later have delayed conception and lower first-lactation milk production, Laporta said heat stress abatement is a probably a worthwhile investment. “This is a highly likely speculation based on the results we have seen so far,” she stated.
Laporta and her team are even researching methods of mechanically ventilating individual calf hutches using fans powered by solar panels. More than ever, both researchers and producers are realizing the long-term benefits of raising calves right.
“We are investing in our calves for life,” she declared. “Heat abatement should be one of those investments.”
