Dog days of summer

Hello all. I am new here and following in the footsteps of Dr. Jim Bennett for the Veterinary Wisdom column. I am one of six doctors at Northern Valley Livestock Services practicing primarily dairy production medicine. I appreciate the honor of continuing the column and hope to provide information that is valuable for all readers.

I don’t know about you, but I love the warm weather of summer. Sometimes the humidity is a tad much, but I much prefer it over the cold winter months. However, cows and calves do not feel the same. Heat stress in adult cows has been well-researched, but calves can be forgotten when it comes to heat and humidity. In Jim Quigley’s most recent Calf Note, he discussed that calves experience a relative loss of water during heat stress in addition to the other detrimental side effects.

There are several important factors to consider regarding heat stress in calves. The first is the thermoneutral zone. This is the temperature range where the calf does not have to expend energy to raise or lower body temperature. That’s right, it takes actual energy to lower one’s body temperature. This means that fewer calories will be utilized for growth and immune function. Depending on the reference, the thermoneutral zone for a calf will range from 50-77 degrees Fahrenheit. Another factor to consider is the effective ambient temperature, which accounts for aspects like sunlight or shade. This is the temperature that you, the calf and I experience, but not necessarily the same as the number on the thermometer. Then there is also the temperature humidity index, which considers humidity. Various resources suggest that at a THI of 78 degrees, calves experience moderate heat stress and begin to experience severe heat stress at a THI of 88 degrees.   

So how do calves compensate for hot days? Calves lose heat through several behavioral and metabolic mechanisms. These include changing positions, reducing feed intake, redirecting blood flow, respiratory loss (panting) and sweating. In fact, sweating accounts for approximately 80% of heat loss. Cattle accomplish sweating through the many apocrine glands in their skin. From Quigley’s review of research published on water loss in calves during heat stress, he found that calves primarily lose water through respiratory losses and sweating.  Take a 110-pound calf for example. At 66 degrees, the respiratory loss of water was about 1 liter and the loss through sweating was about 0.5 liters. However, the water loss for the same calf at 86 degrees was much greater. The respiratory loss was approximately 1.7 liters and the sweat loss was potentially another 4.4 liters of water.  

So, if we add together the loss from panting and sweating, we can estimate how much water a calf might lose on a hot day. On a 90-degree day, like today, some calves may lose up to 6 liters of water. How can we help the calf cope with this loss?

First, always have clean free choice water available for all ages of calves, including very young calves. Milk should not be considered a water replacement for calves. Then consider offering diluted electrolytes in additional feedings, such as a midday feeding. This can be fed via bottle or bucket. This provides both water and electrolytes that may be lost through sweating. Diluting the electrolytes can make it more economical. Consider shade to help reduce heat stress. One way to accomplish this is to put a shade cloth over calf hutches. Last, explore ways to better passively ventilate the calf hutch or pen. Open all windows available in the hutch. Prop open the rear bedding door. Elevate the back of the hutch with a brick or similar object.  If you’re lucky enough to be in the market for some new calf hutches, look for styles that have additional port holes or windows for passive ventilation.

Enjoy the dog days of summer and don’t forget to help the future of your herd stay cool.

Megan Weisenbeck is one of six veterinarians at Northern Valley Livestock Services in Plainview, MN. She practices primarily dairy production medicine in Minnesota and Wisconsin. Megan can be reached at [email protected].