Preventing heat stress begins in utero

MADISON, Wis. — As temperatures begin to rise with the approach of summer, methods to keep cattle cool are employed. Unfortunately, calves and dry cows might rank low on the list for dairy producers when implementing cooling strategies. However, animals of every age endure the impacts of heat stress, and, for calves, the consequences can be lifelong.

“Heat stress does not discriminate, and it will impact cattle of all ages and physiological states,” said Jimena Laporta, assistant professor of lactation physiology at the University of Wisconsin-Madison. “Although we don’t see these immediate production losses like we do for lactating cows, youngstock are still susceptible, and we see production and economic losses later on.”

Laporta and Jennifer Van Os, assistant professor and extension specialist in animal welfare, discussed the effects of heat stress in dairy calves and ways to provide relief from warm weather during a University of Wisconsin-Madison Badger Dairy Insight webinar May 21. Both Laporta and Van Os are with the UW-Madison Division of Extension Dairy Program and UW-Madison Department of Animal and Dairy Science.

In her research, Laporta has discovered that calves are impacted by heat stress before they are even born. When examining prenatal heat stress effects, Laporta and her team focus on the last trimester of gestation which coincides with the dry period.

“This is a critical developmental window, and there are a lot of events that will be temperature sensitive for that fetus,” Laporta said. “This includes the maturation of organs, establishment of cell hierarchy, cell differentiation and cell communication.”

The fetus is reliant on maternal regulation of temperature, therefore, maternal heat stress leads to in utero heat stress, which creates thermal stress on the growing fetus, Laporta said.

“There is increasing evidence that heat stress during these early developmental windows of life have carryover effects later on in life,” Laporta said.

Over the last few years, Laporta’s team did studies in Florida and Wisconsin with two groups of cows. The first group was heat stressed during the dry period, while cows in the second group were provided with heat abatement methods like soakers and fans. Laporta followed the heifers born from these cows throughout their lifetime, comparing the in utero cooled calves to the in utero heat stressed calves.

Negative effects for calves that experienced heat stress began at birth. Their gestation length was five days shorter on average than calves cooled in utero, leaving less time for fetal development. This resulted in calves that were about 10 pounds lighter at birth. They were also smaller at weaning, weighing about 17 pounds less.

Calves experiencing heat stress in the womb also had a reduced ability to absorb immunoglobulins from colostrum, leaving them with a less robust immune system. The difference in absorption was 20% lower for these calves compared to calves whose dams were cooled during the dry period. In addition, milk and starter intake was reduced for heat stressed calves, which ate less at each visit to the automatic feeder.

Smaller body size was a concern not only at birth but across the entire pre-weaning period. Heat-stressed calves had shorter body length, shorter chest girth and shorter hip height as well as a smaller head circumference. Even after one year of age, these calves still measured smaller than their cooled counterparts.

“The effects are permanent, not transient, up until one year of age,” Laporta said.

Fewer in utero heat stress daughters survived to first calving, and those that made it had a reduced productive life, with a lifespan that averaged 12 months shorter.

“These heifers leave the herd sooner overall, and those that make it to first lactation produce less milk,” Laporta said. “They barely made 65 pounds during their first lactation, and second and third lactations look pretty much identical. At peak, these animals were producing 90 pounds but then crashed quickly.”

In utero cooled calves on the other hand were well above 65 pounds of milk in their first lactation and much higher in second and third lactations.

“We see lower milk yield and lower energy corrected milk for at least three lactations in heat stress heifers,” Laporta said. “This highlights the long-term carryover effects of heat stress which leads to producing less milk over her lifetime.”

Laporta said that in utero heat stress derails the mammary gland development. Mammary gland tissue is impacted, resulting in smaller udders, truncated ductal epithelial structures, reduced branching and lower cell proliferation.

“This is seen as soon as they are born,” Laporta said. “At puberty, we also see they now have a similar body weight and height, but they produce less estrogen. At this stage, we want the mammary gland to be growing and proliferating. They have fewer mammary gland cells, which are those that make milk, which is why they’re producing less milk.”

Once calves are on the ground, they should be monitored for heat stress just like other members of the herd.

“To prevent the onset of heat stress, we should begin monitoring calves before the temperature-humidity index is 69 and before the temperature is 69.8 degrees Fahrenheit,” Laporta said. “We can improve natural passive ventilation by elevating hutches, changing hutch orientation, or adding extra windows and rear vents.”

During studies on heat abatement for group-housed calves in Florida and Wisconsin, researchers found that calves with access to fans consumed more grain and milk — drinking more milk overall, even during the hottest times of the day. As heifers, they got pregnant sooner and produced more milk in their first lactation.

Van Os said the goal of heat abatement strategies for calves is to try to reduce heat gain and promote heat loss. When housed in a barn, a roof can provide shade. If housed in an outdoor hutch, the shade factor depends on hutch material.

“Some hutches have a greenhouse effect, causing the sun to amplify heat within the hutch,” Van Os said. “Other hutches can block out solar radiation. Adding supplemental shade via cloth or natural shade from trees can help. A shade cloth should block at least 80% of solar radiation.”

Mechanisms for promoting heat loss in a barn include mechanical or active ventilation from fans which force air to move across the animal.

“This actively cools the calves as well as gets rid of stale air and brings in fresh air,” Van Os said. “This can also be done with positive pressure tubes. In an outdoor hutch, you can use natural or passive ventilation to exchange air, reducing stale air from inside the hutch and replacing it with fresh air from outside using passive strategies; you’re not forcing air through with fans.”

Another option is to elevate hutches — either by putting them on a stand or up on cinder blocks. Openings in the hutch can also be modified to promote air exchange.

“When calves are in the hutch, they’re producing metabolic body heat and dissipating it back into the environment,” Van Os said. “When housed in pairs, two bodies in a hutch create even more heat.”

A study at UW-Madison compared two hutch environments. One hutch was unventilated with the only opening in the front of the hutch. The air speed average in this hutch was 0.4-feet per minute.

The other hutch was passively ventilated with the bedding door partially propped open with a cable tie. Port holes were also installed, which can be opened in summer to provide additional passive ventilation or closed in winter to prevent drafts. The additional airflow through the hutch created an average air speed of 43 feet per minute.

Results were measured at three different points in the calves’ lives — week four, week six and week nine. Regardless of week of life, inside the ventilated hutch had a lower THI.

“If using pair housing, it’s important to help calves relieve the additional heat load that two bodies contribute,” Van Os said. “Passive ventilation results in a cooler microclimate inside of the hutch even when two calves spend time inside. If you have calves in outdoor hutches, you can adopt these practices.”