Heat stress impacts dairy cattle across generations

MADISON, Wis.— Heat stress not only impacts lactating dairy cows, it also affects dry cows, as well as first-gestation heifers.

“Heat stress is not a regional issue; it has become a global issue,” said Jimena Laporta, associate professor of lactation physiology at the University of Wisconsin-Madison.

“It’s now affecting highly populated regions that were once considered a low risk such as the Midwest, Canada and some parts of northern Europe,” said Laporta during a webinar hosted by Hoard’s Dairyman. “By 2050, the U.S. is projected to have increases in temperatures of more than 2 degrees Celsius.”

There are well-documented effects of heat stress on lactating dairy cows, including reduced feed intake, lower milk production and negative impacts on fertility and health.

“Most of my research has focused on the non-lactating members of the herd, including dry cows, heifers and preweaned calves, which are not a priority for summer heat abatement,” Laporta said.

Multiple generation programming events are happening in a dry pregnant cow.

“The cow is regenerating her gland for her next lactation,” Laporta said.

“She is also pregnant, so there are programming effects on the calf,” she explained. “One generation we don’t typically talk about is the future granddaughters, but in the fetal ovary of the daughter, the fetal germline is developing in utero that will become the oocytes that will give rise to the second generation.”

Research shows dry dairy cows with heat stress produce less milk during their next lactation.

“If cows had heat stress during six to eight weeks of the dry period, there were carryover effects 40 weeks in milk,” Laporta said. “Those cows produced on average 5 kilos less milk than a cow that was offered heat abatement during the dry period.”

During the last trimester of gestation, a lot of dynamic processes are happening for the calf in utero.

“About 70% of the birthweight of the calf will be acquired during the last two months of gestation,” Laporta said.

“Cellular processes happening in utero are all temperature sensitive and the fetus is relying on the maternal mechanisms of thermal regulation,” she said. “Maternal heat stress is going to lead to fetal hypothermia and that will lead to effects on the calf that will persist even after they are born.”

Some of the recurring effects include reduced gestation length, reduced birth weight and shorter heifers.

“The calves have a reduced ability to absorb immunoglobulins from colostrum leading to a less robust immune system,” Laporta said. “These calves consume less milk and grain, and after one year of life these heifers are shorter and smaller, so the effect is lifelong.”

Calves heat stressed in utero have reduced survival to first calving, the university professor said.

“On average, 11% less are going to make it to first calving,” she said.

Laporta followed a group of heifers until puberty that were heat stressed in utero.

“We see they have lower circulating estrogen and at 12 months of age they have lower expression of estrogen receptors,” she said. “That’s leading to a 40% reduction in proliferating of the cells, so the glands are growing less quickly.”

The connection between the mother, daughter and granddaughter begins in utero.

“When a pregnant cow is carrying a female calf, the fetus is going to be developing the oocytes in her ovaries,” Laporta said. “The daughter is going to be born with a lifetime supply of eggs.”

“We wanted to test whether in utero hypothermia of the daughter affects the growth, survival and productive capacity of the second generation,” she said. “We see a similar pattern of early life losses and reduced productive life — 40% of the granddaughters left the herd before their first lactation.”

Laporta took mammary biopsies of the granddaughters of heat-stressed cows and found fewer proliferating cells that led to cows producing less milk in the next lactation.

“The granddaughters of heat-stressed dry cows produced on average 1.5 kilos less milk during their first lactation,” she said. “This is four to five years after the original exposure of the grandmother.”

Growing, pregnant heifers are not immune to heat stress effects, Laporta said, despite the fact that they’re not actively producing milk or they don’t have a dry period.

“There are a few things that make them more susceptible to heat stress — they are still growing and they have a smaller body size that is less effective in dissipating heat,” she said.

“They are also supporting fetal growth, they are growing their mammary gland and they are typically housed on pasture with limited access to shade.”

Since dairy cows are not very efficient at sweating, they use more respiration for dissipating heat.

“When we provide fans and soakers, we’re helping them remove heat from the skin through evaporation,” Laporta said. “We’re able to reduce the sweating rate by twofold.”

Research shows cooling heifers during the last 60 days of gestation results in improved milk yields.

“They produced 3.9 kilos per day more milk and we see improvements in fat and protein yield,” Laporta said. “We also saw an increase in immunoglobulin absorption up to 16% and higher circulating levels of immunoglobulins by 10 milligrams per deciliter.”

Heat is also stressful for preweaned calves which can result in poor growth and health outcomes.

“Calves are always trying to thermal regulate to maintain their core body temperature,” Laporta said.

“Our research shows that a temperature humidity index as low as 65 can trigger changes in preweaned animals,” she said. “Every unit increase above a temperature humidity index of 65, respiration rate will increase by two to five breaths per minute.”

Laporta encourages dairy producers to monitor their dry cows and youngstock during summer months for heat stress.

“Provide heat stress abatement whenever possible, especially in early life development to allow them to meet their full potential, establish long-term performance and minimize long-term generational effects,” she said.