by Sally Colby

Heat has negative implications for dairy cow comfort and productivity, but there’s also clear evidence that heifer calves born of heat-stressed dams have lower productivity.

Dr. Jimena Laporta, assistant professor in lactation physiology, University Wisconsin-Madison, addressed the issue of in utero heat stress during Cornell Dairy Day. Laporta said it’s been shown in various species that exposure to heat stress during this time can have long-term consequences on growth, health and future performance, even many years down the road.

“The dry period for the dairy cow coincides with a portion of the last trimester of gestation,” said Laporta. “This is a very sensitive period for fetal programming. The fetus is growing at a very fast rate – 60% of its birth weight is acquired during this short period of time.” Although organ formation is complete, heat stress can take a toll on organ maturation and cell differentiation.

The exposure to dry period heat stress in cows can lead to intrauterine growth restriction in heifers. Birth weights and survival rates will be lower and heifers will produce less milk in their first lactation.

Laporta described the short-term consequence of in utero heat stress on offspring: “One of the hallmarks is reduction in gestation length. That impacts the cow and the calf at the same time. In terms of the calf, there is less time for fetal development; the calf is born smaller. The cow will have a shorter dry period, and that means she will have less time for mammary development.” Laporta added that the average reduction in gestation length in heat-stressed animals is four to five days, which is ample time to impact both the cow and her calf.

Another issue with heat stress is alteration in both adaptive and innate immunity. “Successful passive transfer is important for neonatal calves, but in utero heat-stressed calves are less efficient in absorbing immunoglobulins from colostrum at birth compared to in utero cooled heifers,” said Laporta. “Consistent results in studies show 10% to 15% reduction in in utero heat-stressed heifers in absorbing IgG.” This means calves begin their first week of life with lower immunoglobulin concentration. Levels remain low for the first week of life and remain low through pre- and post-weaning. In utero heat-stressed calves have less robust immune systems and require more treatments in early calfhood.

Calves that experienced heat stress in utero have reduced milk and grain intake. Laporta explained a study that recorded calves’ daily milk replacer and grain intake. In utero heat- stressed calves ate less during the hottest time of the day (from noon – 7 p.m.) and didn’t compensate for lower intake during the cooler times of day.

“In utero heat stress will assert a strong and persistent influence in calf post-natal growth and weight gain,” said Laporta. “Heifers are born lighter on average – we see a 10% reduction in birthweight in heifers compared to in utero cooled heifers across 12 studies.” Average daily gain and weaning weights were also lower.

Heifer body size is impacted by heat stress. A recent study found calves that experienced heat stress in utero are shorter in stature and have smaller chest girth, shorter body length and a significant reduction in head circumference, which has implications for learning.

“It seems that in utero heat stress is programming operations in the thermo-regulatory capacity of calves,” said Laporta. “What we see consistently is that in utero heat-stressed heifers have elevated core body temperature throughout the pre-weaning period. They also have elevated sweating rates and longer hair.” Heifers that experienced heat stress in utero had smaller and fewer sweat glands, and the glands were located farther from the skin surface. In utero cooled heifers had larger sweat glands and shorter hairs that may allow them to thermoregulate more efficiently to evaporate sweat from the skin surface to maintain a lower core body temperature.

Heat stress in utero also impacts organ weights. Calves that experienced heat stress in utero are already smaller, and relative to body size, showed alterations in immune organs such as the thymus and spleen. Calves also had smaller livers and thyroid glands, which may have implications on future metabolism. Laporta also saw calves with smaller ovaries, which could have implications on future reproduction.

A heat stress period can impact mammary gland development even before a heifer is born. Laporta found that in utero heat-stressed heifers have smaller mammary glands, evidenced by less secretory tissue early in life.

“The milk output of an animal is driven by the number of secretory cells in the gland,” said Laporta. “What we think is happening is in utero heat stress is derailing the developmental progression of the mammary gland very early in life. Two years after heifers experienced in utero heat stress they have a 50% reduction in the area capable of producing milk. We also see a higher proportion of connective tissue between the alveoli, which is not bad, but during lactation we want to maximize secretory tissue and minimize connective tissue.”

Long-term consequences of in utero heat stress are significant. Laporta’s studies were conducted over 10 years with multiparous cows that were heat-stressed during their entire dry period. The study followed 400 dams, 80 daughters and 50 granddaughters. The survival percentage of daughters is notable: In utero heat-stressed daughters had lower survival to first calving. Survival to subsequent lactations was negatively impacted, and overall productive life was reduced by an average of five months.

The total lifespan of animals was reduced by almost 12 months. “These are significant and impactful differences,” said Laporta. “Heifers that make it to the first lactation produced less milk.” The survival of granddaughters whose dams were heat-stressed during late gestation had lower survivability and produced less milk, which makes it clear that late gestation heat stress affects at least two generations and manifests three or more years after the initial insult to the grandmother.

Economic losses due to heat stress in lactating cows has been estimated at $1.5 billion. Laporta references the work of Dr. Geoff Dahl at the University of Florida which shows an estimated annual economic loss due to heat stress exposure of dams (as dry cows) and the milk they are not able to produce in the next lactation at $800 million.

“The take-home is we need to manage heat stress in dry cows because it’s as important as it is for lactating cows,” said Laporta.