The summer heat is upon us. As many readers are aware, we can monitor and anticipate changes to the dairy herd when the temperature-humidity index (THI) begins to climb above the comfort zone of 68. Often, herd dry matter intake slides, and then the tank gets hit, typically after four consecutive hot days.

Research to better understand the physiology behind this common summer phenomenon has recently been conducted at Virginia Tech. “We replicated summer-type conditions in environmental chambers where we could prescriptively change the level of heat stress to mimic a hot day and slightly cooler evening,” described Benjamin Corl, an associate professor at Virginia Tech, on the June edition of the “Dairy Science Digest” podcast. “After just four days of heat, we observed significant heat stress symptoms, which ultimately resulted in a 9.5-pound per cow per day reduction in milk yield, so we collected mammary tissue samples to determine why,” he continued.

Microscopic tissue changes

Inside the mammary gland, epithelial cells are responsible for milk production. They secrete milk into the alveoli, which expand to hold the milk until milking, like a water balloon. The tissue samples from the heat-stressed cows and their pair-fed control comparison group were evaluated under a microscope to reveal:

• Less alveoli open space in heat-stressed cows, meaning they had a reduced capacity to hold milk
• A higher number of alveoli per area due to less filling
• Greater death loss of secretory epithelial cells

“Cell number heavily influences the yield of milk,” said Corl, “and a previous data set shows that there’s an 82% increase in the number of epithelial cells leaving the mammary gland in milk after 10 days of heat stress.”

Immune cell function

A summer spike in bulk tank somatic cell counts is common. While these counts are typically related to mastitis, somatic cells are comprised of many different types of cells, including epithelial and immune cells.

The most common immune cell in the mammary gland is a type of granulocyte, the neutrophil. During the 10-day heat stress challenge, the Virginia Tech team captured a 70% to 80% decline in granulocyte viability. This means these cells are present in the gland, but they are not alive or capable of attacking infection as it arises.

“These immune cells seem to be sensitive to heat,” explained Corl. “While not a direct causation, this lack of surveillance by the neutrophil cell limits the cow from being able to defeat infections, slowing its response. Most of the time, a cow responds to the infection, and we don’t even know.”

Gene expression changes

Another aspect studied in the small yet robust dataset of 16 cows was cell-level changes in the mammary gland in response to heat stress. How did heat stress affect the cellular DNA that cause changes in milk?

While there was not a significant change in the amount of DNA template used to guide milk casein production, there was a decline the activation of protein synthesis — the cells were less stimulated to make proteins of all types. This lower activity can contribute to reduced production.

Another sign of cell stress was evident. A protein that responds to the accumulation of damaged cell components was activated, indicating that cells were struggling to maintain normal functions in the face of heat stress. Life is just harder in the heat, for the herd and herdsman.

Cooling cows pays dividends

It’s easy to look at the udder as a singular structure and forget that it is made up of millions of cells that are experiencing a constant rate of turnover and rebuilding. The cellular damage observed in this study occurred after just four days of stress. But how long will it take to regenerate the losses, and how much energy is committed to that process that could be conserved if the cows were housed in a comfortable environment?

We tend to think short term and are thankful when the heat subsides and the tank starts to fill again. However, the herd might not recover its full potential following the damage caused by heat stress. The important long-term implications, even after the cow rebounds, could negatively impact the entire lactation and even the cow family’s potential later on, as summarized last month.

This episode is part two of a series on the importance of heat stress in your dairy herd on a monthly podcast, “Dairy Science Digest.” Listen in on your favorite podcast platform. The original research findings were summarized in a peer-reviewed open access Journal of Dairy Science at article found at www.journalofdairyscience.org.