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JUNEAU - Keeping dry a cow cool and comfortable is beneficial to her, her calf and a dairyman's bottom line.

Summer heat is no picnic for dairy cows, Dr. Geoffrey Dahl said during “Dry Cow Cooling”, the first of a two-part World Class Webinar on abating weather-related stress being presented by Professional Dairy Producers of Wisconsin.

Dahl, University of Florida, detailed what heat stress can do to mammary growth, metabolism and immune function, and how these effects set the stage for a more challenging transition and result in lower yield in the next lactation.

Dahl's research found cooling dry cows increased milk for 40 weeks after calving. Yields from cows cooled during the dry period were 8 to 10 pounds a day more than uncooled cows, he reported, despite zero differences in how the animals were treated after calving.

A review of 13 published studies concurred.

"Across the board, they all show the same thing. Animals cooled when dry make more milk in their next lactation," Dahl said.

Biopsies revealed that cooling dry cows has a direct impact on their mammary cells.

The difference is a effect on the proliferation, or growth, of these cells. There are a lot more in cooled cows," Dahl observed.

Cooling dry cows increases body weight prepartum, but decreases body weight postpartum. Dahl explained that cooled animals actually gained weight during their dry period and, because they are making a lot more milk after calving, they're metabolizing more body tissue.

Research also found that cooling dry cows has positive effects on their immune function, including lymphocyte proliferation and increased neutrophil action postpartum.

Dahl noted the effects on acquired immunity and antibody production could be important to vaccination profiles.

Effect on calves

Heat stress on the cow also has impacts on the unborn calf, both early in life and when she begins lactating.

Dahl confirmed that cooling the cow increases her calf's birth weight.

"We found the difference persists into weaning, as does the persistence of lower birth weights of hot cattle," he said, citing research that found in-utero heat stress of about six weeks in length reduced calf body weight and height at weaning.

"Cooled calves were heavier and taller," Dahl reported.

Cooling also improves immunity, measured by the higher circulating IgG.

"In fact, it looked like calves born to hot cows had lower ability to absorb IgG," Dahl said.

Questioning whether the difference was from the particular colostrum or the effect of keeping cool before birth, further research found cooling the calf in-utero increases her apparent efficiency of IgG absorption.

"Something about a hot calf was limiting her ability to absorb IgG from colostrum," he observed.

For one thing, researchers found calves from heated cows had accelerated gut closure, compared to calves from cooled dams.

"To nail it down, there was no effect of whether colostrum was from cooled or from heat-stressed cows on calf performance," Dahl summarized. "The effect is all on the calf and her ability to absorb."

The long view

A retrospective analysis of records of 150 calves was conducted to study the downstream effects of heating or cooling the dams.

It showed, again, that calves born to cooled dams had higher birth weights and that in-utero heat stress decreases the calf's body weight into puberty.

In-utero heat stress decreases calves' chances of survival through the first lactation, shown by more animals born to heat-stressed dams leaving the herd versus calves from cooled dams.

There is also an effect on reproduction. In-utero heat stress decreases reproductive performance, with cooled calves requiring fewer services and achieving pregnancy at an earlier age at pregnancy, by almost a month.

Another big item with in-uterine heat stress is reduced milk production.

"With all calves managed the same after birth, after calving we found a pretty dramatic negative effect on milk production of 10 pounds less a day," Dahl said.

In-utero heat stress does not affect the mature body weight of the animal.

"They just milk less," Dahl said. "We have essentially created a situation where calves cannot reach their genetic potential when they suffer heat stress in the dry period."

Cooling dry cows makes cents

Considering the economic impacts of dry cow cooling, Dahl believes it makes sense financially in almost all situations. In Wisconsin, where cows potentially experience heat stress about 20 percent of the year, cooling could have a dramatic effect on overall herd financial numbers.  

"We found it's probably a good idea to build a barn to accommodate heat stress, and retrofitting a barn with a cooling system, like fans or soakers, is even less of a question," Dahl said. "It makes sense to cool dry cows."

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