For decades, dairy producers have recognized the struggle their herds experience through the summer months. Heat stress has been known to negatively impact milk yield as much as 40% if the heat and humidity dial stays high for several consecutive days and nights.

This topic is especially relevant to the Florida dairy industry, so researcher Jimena Laporta and her team sought to determine if having dry cows exposed to heat stress resulted in lasting impacts on the calf developing in utero.

Several research projects later, the answer is a resounding yes. Laporta joined the conversation on the most recent edition of the “Dairy Science Digest” podcast to discuss the years of research on this multi-generational project and what to expect from your heat-stressed offspring.

Just pause for a moment and imagine the conditions these unborn calves are exposed to through the summer. It would mimic a Crock-Pot – the fetus is incubating while it attempts to develop critical tissues such as the ovary, mammary gland, and adrenal gland, to name a few.

To better understand these impacts, Laporta’s group partnered with a commercial dairy in Florida to host the experiment. Their dry cow barn was divided in half, with some cows cooled with soakers and fans and the other half experiencing no cooling for the 60-day dry period.

Following heat stress, the team tracked the mother (F0) and daughter (F1) for milk production, growth, and survival parameters. They continued to follow the heat-stressed daughter through its first, second, and third lactations.

Following that animal’s first calving event, the team was able to extend their research question to include now a third generation, the F2 or granddaughters of the heat-stressed dam. A commitment of eight years was made to quantify if the “incubation effect” was transferred through the cow family through epigenetic changes.

Less capacity for milk production

As expected, the team did quantify a milk loss in the initial production of the dam. On average, over several replicated studies, the heat-stressed dry cow will produce 8 to 10 less pounds of milk than the ones cooled during the dry period. Remember, after calving, all cows were cooled. However, this project sought to focus in on the F1 and F2 generations’ production potential even when reared in a comfortable/non-heat stress scenario.

The impact on milk was huge. The reductions in milk yield are illustrated here in pounds per day:

“As a mammary biologist, I was especially interested in what might be happening inside the udder to cause these reduced yields over multiple generations,” Laporta said. “In addition to ultrasound and biopsies, we also euthanized a few animals to tackle this question.”

What they found inside of the mammary tissue of the F1 dam and F2 calf explained the reduction in production. Initially, the ultrasound verified reduced parenchyma tissue, the productive tissue of the mammary gland, and fat pad, the cushion in which the parenchyma will grow.

When dissected, the team confirmed those observations and further found the consolidation of the luminal space in this parenchyma as well as a 30% to 40% reduction in growing cells. This consolidation would limit the capacity of the udder to grow and hold milk being produced by the secretory cells at maturity. Additionally, estrogen receptors were limited, which also could impede the development and growth of secretory cells.

Adrenal changes

Another organ of interest to the researcher was the adrenal gland. Adrenal tissues are responsible for a whole host of biological functions, such as stress response and inflammation. Most commonly attributed to the release of cortisol and adrenaline, the research team hypothesized that this organ would be overloaded in the animals with heat stress in their lineage.

“We measured the areas of different zones of the adrenal via histology,” Laporta reported. “Specifically, at weaning, a time of stress, we saw an increase of the area of all zones and the size of the cells, in addition to the overall size of the organ.”

While its not yet well understood how these cellular differences directly affect the cow two years later in lactation, it could hold a key to understanding their response to stressors.

Ultimately, the most important take-home message is to understand the lasting implications of a relatively short period of time in the womb on years, or even generations, of production. The carryover effect can derail a cow family from maximizing its genetic potential. This data suggests that focusing on 45 to 60 days of cooling through the dry period will have lasting effects on the productivity of the herd through improved fetal programming that will span at least two generations of offspring.

These findings were summarized in a peer-reviewed, open access Journal of Dairy Science article found at To learn more, listen in to the monthly podcast, “Dairy Science Digest” on your favorite podcast platform.

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June 6, 2024
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