Negative Effects of Heat Stress on Calves – Dr. Ana Paula Monteiro and Dr. Sha Tao, University of Georgia

By Dr. Ana Paula Monteiro, research scientist, and Dr. Sha Tao, Department of Animal and Dairy Science, University of Georgia
Although heat stress affects a cow at all stages of her life cycle, farmers usually focus on heat-stressed lactating cows as the decrease in milk production is easily noticed. However, studies have shown that dry cows are also negatively affected by heat stress, leading to a lower subsequent milk production.

Heat stress effects in utero
Additionally, research demonstrate that heifers born to dam that are heat-stressed during the dry period are up to 11 pounds lighter at birth and remain lighter during the preweaning period compared with heifers born to cooled dams. Heifers heat-stressed in utero also have decreased efficiency of immunoglobulin absorption, thereby increasing the risk of failure of passive immune transfer.  Passive immunity is important for newborn calves as they rely on the colostrum immunoglobulins to build up their immunity and resistance to diseases. Indeed, heifers heat-stressed in utero have greater morbidity and lower survival rates during the first year of life.

In utero heat stress may also alter calves’ metabolism, making them more likely to store energy as fat, which would be translated into lower future reproductive and lactation performance. A recent study demonstrated that heifers from heat-stressed dams produce 10 pounds per day less milk during the first 35 weeks of their first lactation compared with heifers born to cooled dams.

These studies not only illustrate the importance of cooling dry cows, but also provide an example of how a cow’s thermobalance during a short period can affect the overall performance of the cow and her offspring.

Heat stress effects in preweaned calves
In addition to prenatal heat stress, producers need to recognize the importance of heat stress management for preweaned calves.

Although more tolerant to high ambient temperatures, newborn calves also need extra attention during the summer as the hot weather negatively affects calf growth and increases mortality rate. However, good management starting at birth can help calves cope with heat stress and ensure a good start of their life for better future performance.

After birth, calves need to be moved to a clean, dry place and receive 1 gallon of high quality colostrum during the first hours of life. Regardless of its source, bedding should be dry and clean with good drainage, which will decrease incidence of navel infection and improve calf performance.

Older calves can drink up to 6 gallons of water during the summer. Thus, it is important to make sure they are getting plenty of fresh water as it will help them to cope with dehydration.

Providing fresh grain daily is also important and will avoid mold, increase intake and improve transition into weaning.

Calves suffering from heat stress usually have a decrease in grain intake and a shift of dietary energy use from growth to heat dissipation. Thus, feeding more milk replacer is a good strategy to increase total energy intake during summer months, which will go toward growth. Calves with scours should be closely monitored and electrolytes should be provided until full recovery.

Ventilation
Ventilation is another key to helping calves cope with heat stress. Proper ventilation will provide fresh air and remove some of the heat load without creating a direct air draft on the animals. Among different housing options, research shows that wire mesh pens in nursery barns allow better ventilation compared with hutches.

Forced ventilation, such as cooling calves by fans during the summer, decreases calf’s respiration rate and improves ADG and feed efficiency before weaning. If fans can’t be provided, shade alone can successfully reduce heat load, but caution should be taken not to use low-roof structures as it will block natural ventilation and possibly increase pneumonia incidence.

Another way to improve ventilation and air quality inside hutches is to elevate the rear of hutches, which has been shown to decrease ambient temperature and airborne bacteria concentration inside the hutches, and also decrease respiration rate in calves.

The crew working with calves should be trained to recognize when a calf needs administration of IV fluids. Additionally, it is important to perform stressful activities, such as vaccination, castration and dehorning, early in the morning when the ambient temperature is low.